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 Zenith 5-S-127

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wildcat445
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PostSubject: Re: Zenith 5-S-127   Zenith 5-S-127 I_icon_minitimeAugust 29th 2019, 8:59 am

I'm going to consider Rod's suggestion of using the coils from AES to at least make the BC band work.  My wife is not going to give two hoots about shortwave.  The other consideration is this chassis is basically junk like it is.  And I am not concerned about originality.  And there is at least one person on ARF looking for a 5516 chassis for some time with no success.  I might even figure out a way to put an audio input into this radio, since I am reasonably certain the band switch is working.  I'll have to figure out how much of the wave traps and padders  I can do without.  I may revisit this thread after I decide whether to put any more work into this radio or not.  I'll have to buy a new dial in any case if I plan to display the radio.  And a dial belt.

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PostSubject: Re: Zenith 5-S-127   Zenith 5-S-127 I_icon_minitimeAugust 28th 2019, 8:04 pm

If you have a spare 5516 chassis anywhere, I would be in the market.  And, you're right about the new step in triage.  This is the first time I have run across this condition.  The cabinet and chassis give no indication of damp storage.  It had a nice layer of dust on it, typical of dry storage around here.  If it was damp, there would be rust.  I believe my extra step in triage will kick in when I get a radio that won't work, but should work.  Or if I consistently get funky readings, or changing readings.  Thanks for your help.

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chas
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PostSubject: Re: Zenith 5-S-127   Zenith 5-S-127 I_icon_minitimeAugust 27th 2019, 6:33 pm

Quote :
I don't know what you mean to "ring" out the band switch.

It is a term used when I was working at Raytheon Sub-Signal. Huge test board 3, 4x8 pegboard panels covered with neons and terminals for continuity/discontinuity. The tester holding a wired wand hitting each test point an sounding a buzzer or neon lamp. The tested device was wired by dozens of Cannon connectors. For each row/terminal of ringing, a note was made in the log some 2" thick...

Hmm, sorry to hear the the coils are ruined...
I'll keep your need in mind.

The coils are multi-band, FWIR AES is not an option...

I guess there a new step in your triage of a radio Neutral

Very Best!

Chas
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PostSubject: Re: Zenith 5-S-127   Zenith 5-S-127 I_icon_minitimeAugust 27th 2019, 5:51 pm

I have a Halli SX-110 that I use for my bench radio.  I haven't had to redo the strings on it yet, thank goodness.  Awhile back, I restrung the dial on a Stromberg-Carlson 1121.  A fairly complex setup.  I got some springs from Harbor Freight that are the berries, I think, for tuning dial string.  Their 1/4" X 13/32" size seems to work best.  They are a tad stiffer than regular dial cord springs, but not enough to bend anything.  These springs seem to give a little more "oomph" on the cord so I don't have as much trouble.  I use the metal roof on my garage as an antenna for the Hallicrafters.


I hadn't thought about those replacement coils at AES.  That might work until I can find a chassis and can get everything working again.  I might take one of the positions on the band switch and put some kind of audio input there.  Thanks, Rod.

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Rod Clay
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PostSubject: Re: Zenith 5-S-127   Zenith 5-S-127 I_icon_minitimeAugust 27th 2019, 5:41 pm

Well, I am sorry to hear all that about the RF coils. But it does explain what you have had to deal with. I have had radios come in, get them going and have an IF transformer go bad due to corrosion that is not even visible. Some radios have come in with non-working oscillator coils, BFO coils, and corroded or broken bandswitch wafers.

OK on wanting to pick up another good 5516 chassis. I have had to do that before. Sometimes though I have ended up bringing the non-working chassis back to life. That happened with the second Hallicrafters SX-130 receiver I bought. The set, sold as "working and tested" barely functioned as received. It also was also intermittent. I set it aside but later managed to get it reasonably aligned and cured the intermittent by replacing only (4) resistors. (1) in the 1st IF stage and (3) in the power supply.

In the meantime, I suggest you consider replacing the Antenna and Oscillator Coils to get the Broadcast Band working again. Antique Electronic Supply has new Universal Antenna, RF, and Oscillator coils. I got an Antenna coil for the S-120 from them. They don't come with mounting clips anymore but I got some by buying a few NOS TV type width or linearity coils at fairly low prices just to get the clips. The new coils are small, lightweight and can be adjusted for proper alignment. They could just be wired in with some heavy gauge wire. There are also NOS coils out there by Miller and others that could be used to get one or both of the short wave bands working again. You would have to rewire the "front end" of the radio to make it work but it could be done.

Here, I am having trouble redoing the Bandspread dial cord on my Hallicrafters SX-110. The dial drum for the bandspread is quite small so there isn't much room in there for the dial spring. I got some NOS .020" dial cord that matches the original but after restringing it twice with no improvement I concluded I need a new, better spring. The original spring was stretched out of shape and this may have caused the dial to stop working. The second time around, I used a different spring but no good. This spring was smaller and probably not stiff enough. I am looking at buying some new ones.

I think the problem is getting enough initial tension on the cord before tying it to the spring. I don't know how to handle this. I didn't have much better luck on redoing the Bandspread dial on the SX-99 receiver. It works but has an unacceptable amount of backlash in the tuning. I am not sure if another attempt at restringing would help. Another factor affecting the tension is that the Bandspread dial cords are so long compared with the Main Tuning setups. I am thinking now that factory made replacement dial cords were the way to go when they were available from Hallicrafters, GC and others. What to do now I don't know.


Last edited by Rod Clay on August 27th 2019, 8:05 pm; edited 4 times in total
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wildcat445
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PostSubject: Re: Zenith 5-S-127   Zenith 5-S-127 I_icon_minitimeAugust 27th 2019, 2:52 pm

In a moment you are going to hear the REST of the story........

I know what the problem is with this radio.  It explains why readings would change daily.  It explains when I got something working, something else would quit, then what I got working would quit.  NOW it all makes sense.  The outcome is not good, but I know what the problem is.

Green crud.  This radio is infested with the green crud.  The antenna coil and oscillator coils are eaten up with the stuff.  The inside of the coils that you can see from the bottom are not that bad.  The chassis is clean and does not have a lot of rust and corrosion.  I took the cans off the coils and was taken aback by how bad they were.  This project is now dead.  I want to find a good Zenith 5516 chassis out of a radio with a bad cabinet or something.  A working chassis.  This is kind of too bad, it's a cute little radio.  When I got the voltage back up on the oscillator anode by changing that resistor, that did it.  That opened up the oscillator coil.  This chassis will yield lots of good parts.  I'll go ahead and get a new dial and drive belt for it while they are still available.  I'll refinish the cabinet and the wife can display it.  I'll run across another chassis and we'll have the parts to put it back together again.  Thanks, everybody, for all your help.

If anybody has a good 5516 chassis, I'd be in the market.

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PostSubject: Re: Zenith 5-S-127   Zenith 5-S-127 I_icon_minitimeAugust 27th 2019, 1:45 pm

Now I'm back to positive voltage on the oscillator grid.  I changed the anode resistor.  The spec was "11M" which I took to be 11K, 11 thousand ohms.  I replaced it temporarily with an 18K, which is the closest I had.  I have 220 volts on the oscillator anode, and that is now too high.  The primary of the antenna coil was zero ohms.  I got my lighted magnifying glass out and found a couple wires touching.  That cleared the short.  I had 35 ohms then.  Then it disappeared, back to short.  I have found and fixed several things.  That has either done nothing or it has made matters worse.  All I did this session was to replace that resistor and look in my magnifying glass at wiring, then moved those two wires.  Something is causing a short in the primary of the antenna coil.  I have not found the RF choke like I thought.  Apparently it is inside the antenna coil.  Now resistance across the oscillator coil is zero ohms.  I checked the band switch to see if it cut those trimmers in and out and it does.

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PostSubject: Re: Zenith 5-S-127   Zenith 5-S-127 I_icon_minitimeAugust 27th 2019, 11:12 am

I don't know what you mean to "ring" out the band switch. 

I'm not going to vindicate the band switch just yet.  The other day, I could get the coils to come in and out of circuit with the band switch, yesterday they wouldn't.  Last time I checked voltage on the oscillator anode, it was 130.  Yesterday it was 75.  How can it change like that?  I was taking resistance measurements and found the antenna terminal had zero ohms continuity to chassis.  That does not seem right.  There is no indication of that on the schematic.  The only places there is ground on the antenna circuit is the RF choke and the band switch.  I may have found that RF choke.  I'll comment more on that later.  I appreciate the resistance specs on the coils.  That will be my next step.  I'll keep fumbling with this thing.  I'll get it eventually.

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chas
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PostSubject: Re: Zenith 5-S-127   Zenith 5-S-127 I_icon_minitimeAugust 26th 2019, 11:53 am

Zenith 5-S-127 Zenith11
I have posted a snip of the Zenith schematic, if it is not the correct one I will delete it if I can...

All the coils in the RF/OSC sections should not be over 30 ohms or so. If the L/C circuit labeled 1 (the trap) is missing the radio should still work. FWIR the trap was to reduce interference from Maritime communications but that system does not exist anymore. The trap may have been left out for Chassis expected to be out of range of that type of interference. Others may say the trap was to prevent the IF from feeding back, O.K. if it is the radio would howl like mad but still receive until the trap is tuned to IF...

Please note your reference to a resistance to a particular 6A8 grid... Where were the two meter connections?

It think the band switch is shown in BC position.

FWIR C21 is inside one of the coils...

If coils are O.K. for each band switch position, "ring" through the band switch to all coils and their taps. Sanity check is to do a discontinuity too.

Ring out the AVC circuit, be sure it has a very high resistance to ground and finds its way to the grid of the 6A8 and the 6K7.

If all seems O.K. and it still does not work, I assume all B+ and fils are O.K. I have one more trick. An all else fails method that may work the miracle...

Chas
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wildcat445
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PostSubject: Re: Zenith 5-S-127   Zenith 5-S-127 I_icon_minitimeAugust 26th 2019, 10:13 am

It's a good thing I did not post my latest theory on what ails this piece of junk.......

That theory hit a brick wall like all the rest have.  I'm back to having no idea what is going on.  I have 44,000 ohms resistance between the input of the band switch and the grid of the oscillator.  I still have a strong oscillator.  That seems like a lot.  Another thing that is odd, is I can take readings and they'll change in an hour.  I'm using three instruments that I deem dependable.  I'm beginning to wonder if somebody has been in here and has changed something that is not per the schematic.  I sure don't see anything.  I have cleaned the band switch until I'm ashamed of myself.  With alcohol like somebody recommended.  I'm still not sure I'm getting thru it what I should.  I have never found the RF choke that is in the antenna circuit.  I am as sure as I can be on this thing that the problem is ahead of the grid on the 6A8.  Other than that, I have done everything I know to do and everything anyone else has suggested.  I have consulted two old radio books of known reputation and still am barking up a stump.

My theory was that maybe something was going on in the IF stage.  That did not prove to be accurate.

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PostSubject: Re: Zenith 5-S-127   Zenith 5-S-127 I_icon_minitimeAugust 25th 2019, 9:06 pm

I made an interesting discovery this afternoon.  I'd like to keep it under my hat until I've had a chance to confirm what I believe I found.  I realize this sounds over-dramatic, but I've made a big enough fool of myself over this silly little radio.........

I appreciate your hanging in there with this project, Rod.

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PostSubject: Re: Zenith 5-S-127   Zenith 5-S-127 I_icon_minitimeAugust 25th 2019, 3:27 pm

Yes. I have noticed that RF choke that is part of the antenna input network. It comes into play (in my judgement) when an external dipole antenna is hooked up for short wave reception, other wise it gets grounded out with the jumper from Z to G is in place. If the RF choke has been removed by someone in the past and the wiring to the bandswitch has been disturbed, then the antenna input system won't work properly. Check this out. The set could have been modified by some repairman for long wire antenna use only but I'd rather you look it over first before I get into that.

I see you tried the trick of putting your antenna lead right on the mixer stator section of the tuning capacitor. That will usually bring in some stations if the Local Oscillator and IF stages and all the rest are functioning even if the bandswitch and antenna coil are faulty. Just for grins try putting the antenna lead directly on the grid cap of the 6A8.

I have a new Broadcast Band Antenna coil I could send you if you would like to try it. It is of the adjustable iron/ferrite core type. I put one just like it into a Hallicrafters S-120 receiver recently. It snaps into a 5/16" hole. It would need some adjustment after wiring it in.

Here is something that happened to me that I have posted about before:

I have been working on a Hallicrafters SX-99 receiver that came in with a lot of corrosion problems although externally the set looked pretty good. It had obviously been stored in a poor environment for many years. The bandswitch was basically inoperable as received. I found after freeing it up (it was frozen) that the 6SA7 Local Oscillator stage was functioning on the Broadcast Band (Band 1 of 4). I started from there. The Antenna and RF sections of the bandswitch were essentially dead (not passing any signal). I found (2) things: 1. That the bandswitch contacts were covered with a layer of insulating crud. I scrapped off what I could with a small screwdriver. 2. A few of the contact fingers were sticking out in the air and not making contact with the switch as they should. These I carefully pushed back into place with my screwdriver. And I sprayed the whole thing repeatedly with contact cleaner while working the switch back and forth. The set works reliably on bands 1, 2, and 3 now. Band 4 is still hard to get to come in.

73, Rod  WB6FBF
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PostSubject: Re: Zenith 5-S-127   Zenith 5-S-127 I_icon_minitimeAugust 25th 2019, 8:51 am

Let's concentrate on what improvements I've made on this radio.  The oscillator is working.  I have it somewhat aligned, it should work like it is.  I can get a signal thru from antenna terminal to speaker.  I also realize my initial assessment of this radio was not my best work.

Speaking of work, although my tenure in this hobby may be lengthy, I consider my electronic troubleshooting and repair skills hobbyist grade, at best.  My theory is "Even a blind squirrel finds an acorn occasionally".  I can't and don't speak electronic jargon and have little interest in the intimate details of electronics.  I fix things for the item, not to be an electronics guru. 

That said, I believe my next investigative path should be ahead of the tuning capacitor.  I can get a signal from the antenna thru to the speaker now, when I connect it to the tuning cap directly.  Not a station, per se, but I get noise.  And that is good enough for now.  Still nothing when I connect the antenna to the antenna terminal.  We can reasonably assume that the oscillator section of the tuning cap is not shorted, since the oscillator voltage varies with turning the tuning cap.  The "detector" section of the tuning cap does not show shorted.  It measures infinite ohms.  I don't remember working on a radio where the tuning cap was causing a problem.  So please bear with me here. 

I'm still not 100% convinced the antenna coil is above suspicion.  And I have not attempted to adjust C1, the antenna coil trimmer.  I'm also beginning to wonder about the wisdom of injecting a modulated RF signal at the antenna then concluding all is well when you can hear the note in the speaker.  I had an Arvin 444-A that did this one time, but had a bad antenna coil.  All the wires on the antenna coil on the Zenith are black.  They twist and turn.  I'm looking into a black hole.  What I have checked measures good.  The schematic shows an RF choke to ground off the antenna coil, #2.  It is alleged to be separate from the antenna coil.  I have not found that choke.  There do not appear to be any caps in that area that don't belong there.

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PostSubject: Re: Zenith 5-S-127   Zenith 5-S-127 I_icon_minitimeAugust 25th 2019, 7:38 am

The little jumper wire is there.  It is mentioned in the service literature.

I'm beginning to think the tube is not the issue.  If all voltages were good EXCEPT for the oscillator grid, then a defective tube might be considered.  I have good, negative voltage on the oscillator grid, and it varies as the tuning cap is turned. And, to have negative voltage on the oscillator grid, the tuning cap, oscillator coil, voltages, tube would all have to be good.  How about the band switch?  It seems to be working.  The easiest place to check band switch operation is on the various band trimmers found on the oscillator coil.  These little trimmers are switched in and out of the circuit as I move the band switch.  I have jumpered all the "extra" components like RF chokes and wave trap trimmers in an effort to see if some of them are the cause.  Not so far they aren't.  I even got out my old "Elements of Radio Servicing" book.  It talks about what to do if you have no voltage on the oscillator grid, but it says nothing about what to do if you have voltage on the oscillator grid, but still, for all practical intents and purposes, have a dead oscillator as far as how the radio works is concerned.  This radio is basically in alignment, at least good enough to work somewhat, I'd think.  I have a working oscillator.  I can get a signal from the antenna to the speaker.  It's dead as it ever was.  I'm willing to start desoldering components and checking them individually.  But which ones?  Now what?

And another thing.  From the time I started working on this until now, certain things have changed.  I can swear that this radio somewhat worked then.  It seems like it is deader now than it was.  What kind of sense does that make?  I'm overlooking something, I know I am.  Both of my elmers are gone, and I miss them to this day.

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PostSubject: Re: Zenith 5-S-127   Zenith 5-S-127 I_icon_minitimeAugust 24th 2019, 8:59 pm

That is pretty amazing. If I was you I'd probably like to take a break from it for a while.

It's true that tube type circuitry can withstand wide variations in component values but even then they have their limits on what they can stand. Well, I'll give the matter some more thought and get back to you.

Just one more comment and it is regarding the Antenna Input connections: This is something I have wanted to ask about before. The (3) terminals marked A, Z, and G. There should be a jumper wire between Z and G. The only reason I can think of why Zenith did it that way was to allow for the provision of connecting a dipole antenna between A and Z for the short wave bands or possibly a loop antenna for the Broadcast Band. For these types of antennas the jumper wire from Z to G would be removed. I just thought I'd bring this up in case the jumper wire is missing. Without it, reception might well be near zero.

Rod
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PostSubject: Re: Zenith 5-S-127   Zenith 5-S-127 I_icon_minitimeAugust 24th 2019, 8:33 pm

I have changed the filter caps and all the coupling caps.  I've had old radios with resistors drifted wildly up or down with no effect on performance.  Since I can get a signal from the generator all the way to the speaker, I believe that tells me the antenna coil, oscillator coil, band switch the IF cans are all okay.  The oscillator is running.  Connecting an antenna to this chassis does nothing.  A signal tracer does not detect any activity whatsoever.  I really don't know what my next step should be.  Thanks, Rod.  I appreciate your help.



Chas, I have three VTVM's, several VOM's, a digital volt meter, a couple signal generators, a signal tracer.  I have a scope, but don't really like using it.  I have 30,000 tubes for spares and subs.  I've been in this hobby for several years, close to 40 to be exact.  This is one of those projects that only comes along infrequently, thank goodness.  I am almost finished with an every nut, bolt, screw, connection and component restoration of a Magnavox Stereo Theater.  I had everything out of that cabinet I could get out, then refinished the cabinet, then put it all back together and it works, so far.  I still have the TV to sort.  This little five-tube Zenith has my number.  I have done all I know to do, and everything someone else has suggested.  I don't know what to check next.  I'll get my box of 6A8's and start plugging them in to see if one will work.  I have gone back over my wiring checking for a mistake and have found nothing.

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PostSubject: Re: Zenith 5-S-127   Zenith 5-S-127 I_icon_minitimeAugust 24th 2019, 8:28 pm

The pentagrid converter tube can fail to generate sufficient electron for the oscillator "portion" to work. Most tube testers test for both oscillator and mixer functions.

Try several 6A8's known to work in another radio that uses them.

A pocket transistor radio can be used to detect a working oscillator. Tune the Zenith to 1mhz, tune a pocket radio to 1455 or so. Bring the loopstick end of the pocket radio as close to the oscillator variable condenser as possible, rock the tuning, a nulling or quieting signal should swoop into the pocket radio if the oscillator is working

You can inject a substitute oscillator signal into the Zenith at the tuning condenser oscillator section coupling with a .0001 cap. Leave the modulator on the generator off. Set the generator frequency 455 higher than the tuned frequency on the dial.  You will still need to have an antenna and ground connected. If the station received has a hetrodyne whistle, or growl that means the local oscillator is working

FWIW This is really a two tube radio... Do, check the continuity of all the coils and as the circuits pass through the band switch. Use the VOM, not the DVM, not the VTVM. Any circuit on any band that doesn't not make it fix.

If the band switch has been sprayed with any kind of contact cleaner including De-Oxit, remove it by using 91% alcohol and careful acid brush. Back up the chassis with a clean rag around the switch. Clean the wafers including the rotary wafer. Apply tiniest spot of grease on the ball detent and tiny drop of fine oil on shaft.

Sometimes the tube sockets have been sprayed with contact cleaner, clean them in the same way, the alcohol treatment.

Find a dead octal tube that has all its pins, take very coarse sandpaper and sand the pins until they are rough all over. Insert this "socket cleaner" into the 6A8 socket gently, in/out repeat dozens of times. The rough brass will burnish the socket of tin oxides in the very important contact area.

If for some unknown reason, contact cleaner found its way onto mica compression trimmers they will have to be cleaned too, use the same alcohol treatment. DO NOT USE compressed air on trimmers! The air blast will cause the mica to vibrate and shatter into little bits!

If you had a rubber bulb solder sucker, clean out solder bits and use it to puff air through the trimmer to dry the alcohol. Rubber bulb devices can be found at photography stores or a pharmacy.

I have been away from this topic, it would be helpful to know what you have for other old school radio service instruments outside of a VTVM, VOM...

'O'scope, signal injector/tracer? RF probes for any?

Chas
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PostSubject: Re: Zenith 5-S-127   Zenith 5-S-127 I_icon_minitimeAugust 24th 2019, 6:19 pm

I suggest patience. Getting a signal thru the radio is a start. Doing an alignment can help with troubleshooting as you spot things, while doing the alignment, that need further checking. 

Some carbon resistor values are prone to going down while others will go up or even open. It depends on their place in the circuit and how much stress they have on them. Sometimes it just appears due to the effects of aging as with R5 the AVC resistor.

Yes. Replace those resistors that are way out of tolerance. Any carbon resistor that has B+ on it is subject to change over time. By all means replace R3 the Oscillator plate dropping resistor. That might help things a bit.

Did you mean R8 in the power supply or R8 in the 1st audio plate circuit? Getting R8 back to 480K (470K replacement) in the plate of the 6Q7 might actually increase the gain of the first audio stage. If you haven't replaced R2, the oscillator grid leak resistor, do it as soon as possible. That should help too.

Since the B+ still seems to be high (300V) that is an indication that the audio output stage is not fully conducting (drawing the proper amount of plate current) like it should. There may be too much grid bias on the audio output stage (6F6) if the bias setting resistors R8 and R10 have changed value considerably and they probably have so check them and replace as needed. Note that they are connected across the speaker field winding which is positioned in the negative lead coming from the power transformer. This set has a combination of fixed grid bias and cathode bias on the 6F6 output stage so it is important to make sure the values there are correct.

The value of R5 is somewhat less important right now but changing it to a fresh 1 Meg will be a help down the line. The values of R5 and C21 do affect the time constant (response time) of the AVC circuit but it is not so critical in a Broadcast set receiver as compared to a communications receiver. And if C21 is leaky it tends to upset and short out the AVC circuit resulting in no negative  grid bias getting to the control grids of the Converter and IF tubes (6A8 and 6K7).

I agree with Chas regarding the mica capacitors. Don't replace them unless tests confirm one or more appears leaky or shorted. Shorting of micas in a radio receiver is highly unlikely although it does happen in radio transmitters where higher voltages and more stress on them takes place.

As for the tubular paper capacitors: Yes. It would be a good idea to replace all of them. Particularly in the audio stages a leaking capacitor will upset the tube's control grid bias-not a good thing. I am thinking here about C12 especially. Good luck.

73, Rod  WB6FBF

P.S. To answer your most recent question about the 6A8 tube mixer-oscillator action. The answer is no. Even with somewhat lower voltage at the oscillator plate, the local oscillator signal will still mix with the incoming radio frequency signal as this is a function of the internal tube design. What low voltage at the oscillator plate is likely to do is to reduce the output voltage of the oscillator stage. The output voltage is internally coupled to the mixer in what is referred to as "electron coupling".
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PostSubject: Re: Zenith 5-S-127   Zenith 5-S-127 I_icon_minitimeAugust 24th 2019, 5:15 pm

wildcat445 wrote:
I still have a dead radio.  It may just be a little LESS dead......

I checked several times, with several instruments, the voltages on the 6A8.  I used a digital meter, a VOM, and a VTVM. 

Pin       Reading        Spec
1             0                 0
2             Filament
3 plate     260             240
4              85               80
5 osc grid  -5 to-11      -1
6osc adode 130          166
7 filament
8                4              4

The voltage to grid 2 the oscillator anode is low by 36 volts or so.  It is obvious by looking at the oscillator grid voltages that the oscillator is running.  Would the low voltage on the oscillator anode, grid 2 be enough to drive the mixer to cutoff?  R3, a 1/2 watt resistor rated at 11K ohms actually measures 72,900 ohms, a tad high.  There is almost 300 volts on one side of this resistor and 130 on the other side.  I also found R8, a 480K ohm resistor measured 240K and R5, the 990K AVC resistor measured 250K.  I did not cut one end loose, but all three instruments agreed on the reading.  I don't typically see resistors head toward a short, usually they drift high.  That can't be good if the AVC resistor is really that far off.  My original intention was to shotgun the caps, resistors and micas.  I decided to take a directed tests approach and learn something along the way.  What I have learned is that I should have used the shotgun approach and been done with it.  I am no further along to having a working radio than I was when I pulled it from the cabinet.  What am I missing?

Oh, before I forget to mention it, I tried your suggestion of 1400kc and got a signal all the way thru from the antenna to speaker.  That is progress.  I realigned the IF's and did the RF alignment three times.  Still no working radio and I have no idea why.  When I connect the antenna, I get a click when I connect it to the antenna terminal, but no talking.  This, too, is progress.....


A 6A8 pentagrid converter tube has eight elements.  Filament, cathode, five grids, and a plate.  The cathode and grids 1&2 serve as a triode oscillator.  Grids 3 to 5 and the plate serve as a tetrode amplifier.  Since there is no plate, per se, in the oscillator, the output of the oscillator rather leaks into the grid of the mixer.  If the voltage on grid 2, the oscillator anode, was low, would that prevent the oscillator signal from reaching the mixer? 
ADMIN NOTE: Double post so one was deleted!

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PostSubject: Re: Zenith 5-S-127   Zenith 5-S-127 I_icon_minitimeAugust 24th 2019, 5:01 pm

I still have a dead radio.  It may just be a little LESS dead......

I checked several times, with several instruments, the voltages on the 6A8.  I used a digital meter, a VOM, and a VTVM. 

Pin       Reading        Spec
1             0                 0
2             Filament
3 plate     260             240
4              85               80
5 osc grid  -5 to-11      -1
6osc adode 130          166
7 filament
8                4              4

The voltage to grid 2 the oscillator anode is low by 36 volts or so.  It is obvious by looking at the oscillator grid voltages that the oscillator is running.  Would the low voltage on the oscillator anode, grid 2 be enough to drive the mixer to cutoff?  R3, a 1/2 watt resistor rated at 11K ohms actually measures 72,900 ohms, a tad high.  There is almost 300 volts on one side of this resistor and 130 on the other side.  I also found R8, a 480K ohm resistor measured 240K and R5, the 990K AVC resistor measured 250K.  I did not cut one end loose, but all three instruments agreed on the reading.  I don't typically see resistors head toward a short, usually they drift high.  That can't be good if the AVC resistor is really that far off.  My original intention was to shotgun the caps, resistors and micas.  I decided to take a directed tests approach and learn something along the way.  What I have learned is that I should have used the shotgun approach and been done with it.  I am no further along to having a working radio than I was when I pulled it from the cabinet.  What am I missing?

Oh, before I forget to mention it, I tried your suggestion of 1400kc and got a signal all the way thru from the antenna to speaker.  That is progress.  I realigned the IF's and did the RF alignment three times.  Still no working radio and I have no idea why.  When I connect the antenna, I get a click when I connect it to the antenna terminal, but no talking.

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PostSubject: Re: Zenith 5-S-127   Zenith 5-S-127 I_icon_minitimeAugust 23rd 2019, 10:23 pm

Yes. Set your signal generator to 1400 KC. Set the band switch to the Broadcast band. Turn the output on your signal generator to maximum and leave it there until you find the signal on your radio. Then you can reduce the signal level of the generator as needed. This type of alignment is called RF alignment as opposed to IF alignment. IF alignment comes first then the RF alignment. Sometimes I put off the RF alignment till later if the radio is functioning reasonably well.

Next. Retune the set to 600 KC and adjust C4 for maximum output of the radio at that point. This adjustment also helps improve the dial calibration. I hope you can find these dial calibration points even though the radio's dial belt is missing or broken.

You can set aside adjusting the wave trap for now. Do that adjustment last. After you get the Rf alignment done and the radio set functioning, the wave trap adjustment will consist of injecting a 456 KC signal at the antenna input then adjusting the wavetrap trimmer for minimum output from the radio.

Actually, there is no need for the wavetrap on this radio any more. It could be disconnected or bypassed. Explanation: No one is transmitting on 455 KC now. There used to be some ship to ship and ship to shore CW radio communications going on at 450 to 460 KC years ago but not now. I do remember hearing CW radio operators coming in on AM Broadcast Band radios back in the 1960s and 1970s when I lived in the Bay Area of Northern California.

Wavetraps can still be helpful if the radio has an IF frequency in the Broadcast Band. IF frequencies of 1500-1600 KC are vulnerable to broadcast band interference. I have a National Co. radio with a 1560 KC IF that pickups up BC stations operating at 1560 KC, especially at night. This type of interference is hard to keep out. A couple of Hallicrafters radios I have use 1650 KC for the IF. These radios haven't had a problem with BC interference.

Another use for wavetraps was to reduce interference in the BC Band from strong stations that tended to ride in or overpower your favorite station of choice. You would adjust the wavetrap to null out the interfering station as much as possible. Not as much of a problem these days.
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PostSubject: Re: Zenith 5-S-127   Zenith 5-S-127 I_icon_minitimeAugust 23rd 2019, 9:22 pm

Let me make sure I'm not chasing my tail first.  I need to recheck some things before I go much further.  If I read the instructions right, they want me to set the wave trap for minimum at the IF frequency.  If C1 should be adjusted at 1400kc, won't that stop the 456 from getting thru? 

Duh.  It finally dawned on me what you are trying to tell me.  I need to make my signal injection at 1400Kc modulated instead of the IF frequency.  I'll try that.  I appreciate your help, Rod.  Thank you.

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PostSubject: Re: Zenith 5-S-127   Zenith 5-S-127 I_icon_minitimeAugust 23rd 2019, 11:51 am

Yes. Try adjusting C1 for maximum output while the radio is tuned to the upper end of the Broadcast Band-at 1400 KC or higher. 1400 KC was the usual alignment frequency used for adjusting a trimmer on the Broadcast band (600 KC is the frequency to use for adjusting the padder capacitor C4).

I hope you will be able to get a signal through (1400 to 1600 KC) by cranking up the output of your signal generator to maximum, if necessary, to get some output from the radio.

Put the signal generator on modulation if you like to hear and monitor the AC output voltage at the speaker voice coil.

Some techs prefer to monitor the output at the detector with a VTVM. In this case you'll be measuring the negative DC output (AVC) voltage. This would be at the junction of R5 and R6 (the volume control) to ground (chassis). R5 (990 K Ohms) is the AVC line resistor. The voltage could also be measured at the lower end of R5 (see schematic). As you align the receiver you should see this voltage peak (become more negative).

Grounding the lower end of R5 to the chassis during alignment is sometimes recommended to prevent any AVC voltage generated from reducing the gain (sensitivity) of the receiver. Modulation of the signal generator is not required when measuring the Detector DC output voltage for alignment purposes although of course you won't be able to hear the signal from the radio.

If C1 doesn't have much affect at 1400 KC don't worry-it may come into play on one of the Short Wave Bands.
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PostSubject: Re: Zenith 5-S-127   Zenith 5-S-127 I_icon_minitimeAugust 23rd 2019, 6:28 am

Rod, you identified the trimmer I was wondering about.  Couldn't see any purpose for it under the chassis.  That is the trimmer for the antenna coil.  I reckon I adjust it for maximum output?  I don't see any mention of it in the service literature.

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PostSubject: Re: Zenith 5-S-127   Zenith 5-S-127 I_icon_minitimeAugust 22nd 2019, 10:09 pm

Rod I thank you for all your trouble researching this for me.  I have not found C1 yet, the trimmer for the antenna coil.  I can't get a signal thru from the antenna terminal yet.  I have a few theories I'd like to investigate on this thing.  One thing I may have overlooked.  I need to make notes this time so I don't have to rely on memory.  The alignment instructions make no mention of C1.  I have done an If alignment, at least a preliminary one.  I can get a modulated RF signal at IF thru the radio from the grid of the converter tube thru to the speaker.  So that part is okay for now.  I need to make sure the oscillator is still running, check some tube voltages on the converter tube, then figure out why I can't seem to get anything from the antenna terminal to the grid of the converter tube.  After I figure this out, I'll be better able to respond to the information you asked for.  Thank you again and I appreciate your patience.

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PostSubject: Re: Zenith 5-S-127   Zenith 5-S-127 I_icon_minitimeAugust 22nd 2019, 5:21 pm

Hi Wildcat445,

A summary and a few comments from the postings and a check of the schematic diagram in Rider's shows:

The radio is not picking up any stations right? It has a Broadcast Band and (2) Short Wave bands.

You have checked for continuity on all the coils and their various connections?

It has a single, tapped Antenna coil and a single, tapped Oscillator coil.

It has some adjustable mica capacitors in the Antenna and Local Oscillator section:  (1) trimmer (C1) on the antenna coil and (2) trimmers (C17) and a padder capacitor (C4) on the oscillator coil (for aligning the Broadcast band). (C4) is adjusted with the receiver tuned to the low end of the Broadcast Band (600 KC on the radio's dial) to get the oscillator on frequency at that point. If you are checking for the radio's oscillator signal on another receiver, you should hear it coming in at 1056 KC (600 KC + 456 KC). If you don't hear it at 1056 KC, adjust C4 until you do. C5 and C6 are padders for the low end of the (2) Short Wave bands but are not adjustable.

You have done an IF alignment at 456 KC?

You have attempted an RF alignment with your signal generator set to 600 KC and 1400 KC with the Bandswitch set to the Broadcast Band. On Bands (2) and (3) with the signal generator set at appropriate high and low frequencies for the two Short Wave bands (frequencies not given in Rider's)?

If after getting the Local Oscillator on the correct frequency for the Broadcast Band at 600 Kc retune the set to 1400 KC and try to find and peak up a signal at 1400 KC from your signal generator. You may have tune the radio back and forth a bit to find the signal. Try adjusting the trimmer (C1) to get maximum output (it is the only trimmer on the Antenna Coil far as I can tell).

If you are successful in this to this point then you can experiment with the two C7 trimmers. Try turning them back and forth a bit to see what they do while tuned to 1400 KC on the broadcast Band. One of them may affect the alignment of the Oscillator stage at 1400 KC. If you don't see any change at 1400 KC then they may only come into play on the short wave bands. It may turn out that changes in the two C7 trimmers may interact and affect all (3) band's alignment. This could also be true of trimming up C1 on the Antenna Coil. There may be interaction here with the (2) short wave bands. This is something the factory alignment technician would have been familiar with and could adjust out for the best results for all (3) bands.

Hope this helps.

73, Rod  WB6FBF
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PostSubject: Re: Zenith 5-S-127   Zenith 5-S-127 I_icon_minitimeAugust 22nd 2019, 9:38 am

All the wave traps and other Zenith-only nonsense aside, I tried injecting the antenna right at the antenna portion of the tuning capacitor.  Still nothing.  I should have gotten elevated noise if nothing else.  No response whatsoever.  So, this finding preliminarily relieves the band switch and all the components ahead of the tuning cap of suspicion.  My problem HAS to be in the converter stage at this point.  The output in the speaker sounds to me like a dead oscillator.  I'll recheck it, but I had negative voltage on the oscillator grid, pin 5 of the 6A8.  That does not necessarily indicate a working oscillator stage.  What if there is no or low voltage on pin 6, the oscillator anode, grid 2?  The oscillator might still "run" but its output would not be sent to the mixer stage.  I can inject a signal at IF into the converter tube grid, and I get the note in the speaker.  That will only indicate I have a working mixer stage, right?  So, my problem may still be in the oscillator stage somewhere.  And I need to recheck the mixer stage.  If I have incorrect voltage somewhere in the mixer stage, the output of the oscillator may not be passing to or thru the mixer stage.  I have used jumper wires and have bypassed some of the "Zenith-only nonsense" to wire this radio like most others are.  At least until I get some kind of signal thru it.  Then I can go back and return the radio to how it was designed and troubleshoot those components one at a time.  I don't know anything else to do at this point.

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PostSubject: Re: Zenith 5-S-127   Zenith 5-S-127 I_icon_minitimeAugust 21st 2019, 5:47 pm

I want to go back over some of the suggestions posted above to make sure I have not overlooked something.  In the interim, I had the idea of how to check the wave trap trimmer, the broadcast band trimmer, stuff like that.  What if I injected a signal on one side of those trimmers, the connected a signal tracer to the other side of the trimmer, then adjusted for maximum output.  The wave trap adjustment has a coil paralleled with it.  This combination gives a 6 ohms reading.  So that should be good.  I have continuity from the antenna terminals to one side of the antenna coil.  Since it is drawn like a transformer, I won't have, and don't have, continuity across the transformer.  I have continuity from the other side of that antenna transformer to the band switch.  Then I have continuity from the band switch to the broadcast band trimmer.  The bandswitch changes this trimmer in and out of the circuit when I manipulate the band switch, so that should be okay.  I had continuity across the oscillator coil and also had negative voltage at the oscillator grid.  I will recheck these.  I will also try using another radio to see if I hear the oscillator in it.  I have a kluge drive on the Zenith dial at this point, so I'll manipulate the other radio against the Zenith dial.  I have continuity to all the above, but still can't get a signal at IF thru the radio from the antenna terminals.  I can do that just fine from the grid of the 6A8 converter tube.  I checked for rivets causing a problem and don't find any rivets at all that supply ground or connection to chassis.  I will record and report exact measurements across the various coils I have not mentioned above.  I really believe I have lost ground to when I was working with this thing earlier this spring.  There is something strange with this radio.

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PostSubject: Re: Zenith 5-S-127   Zenith 5-S-127 I_icon_minitimeAugust 21st 2019, 2:57 pm

The oscillator is running.  I get from -5 to -15 volts, depending on dial position.

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PostSubject: Re: Zenith 5-S-127   Zenith 5-S-127 I_icon_minitimeAugust 21st 2019, 2:55 pm

Have you tried the second radio method of checking the oscillator?  You take a portable radio, digital is best and tune it to 455 above the problem radio.  You should hear a swoosh or similar sound as you tune the problem radio around that frequency.  For example tune the portable to say 1455 and tune the problem radio around 1000, you should hear a change as you move through that frequency.
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