Zenith 5-S-127

First topic message reminder :

First of all, let me say how much I detest Zenith radios.  Secondly, I have this one thru very little effort on my own.  Thirdly, the wife thinks it is cute, so I'm stuck messing with the silly thing. 

This little five tube radio is kicking my butt.  I should be getting ready for winter, but, no, I'm fooling with this thing.  What I have done so far is this.  Replaced all but three capacitors.  There are two [email=.05uf@200].05uf@200[/email] volt caps paralleled with 400 ohm resistors that are hard to get to.  Then there is a [email=.1uf@600].1uf@600[/email] volt cap on the tone control that is weird.  It looks like it is grounded in the middle, but it only shows the one cap on the schematic.  Part of the reason I hate messing with a damned old Zenith.  Their proprietary component nonsense.  All but three of the dogbone resistors I have checked have drifted to about twice their rated value.  Yesterday, I had most everything working in this radio but the oscillator.  Today, I can't get an IF signal thru this thing anywhere.  B+ is high, at around 300 volts, spec is 240 at 112 volts input.  I'm running it on my Variac, so input is 112volts.  I have voltage at all the tubes, approximately where they should be with the exception of the oscillator grid, which shows positive rather than negative voltage.  So the oscillator is not running.  Yesterday, I could get an IF signal thru from the converter plate to the speaker.  Now all I get is a growl when I connect the signal generator.  I tested the generator on another radio and it works fine.  The schematic for this contraption is in Riders volume 7, page 7 of Zenith.  Has to be something simple.  I almost had a radio yesterday.  The resistors are high, but I should be able to get the thing to work, perhaps not perfectly.  The B+ being high tells me something is not conducting current, thus not loading the power supply.  This radio needs one of everything and I wanted to keep it simple.  I guess I'm in too deep now to stop.  The dial is broken, there is no dial drive belt, the grommets for the tuner and chassis are all shot.  This stuff is repopped, but I wanted to see what I had before I bought a bunch of parts and put lipstick on a pig.  I can get noise thru the radio.  I can tap the grid caps with a screwdriver and get a click in the speaker.  I just can't get a signal thru.  What am I overlooking?

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.

I'm back.  I'm still working on this thing.  I work on it awhile, get aggravated, shove it to the end of the bench.  Work some more, repeat.  This little five-tube radio is kicking my butt. 

For all intents and purposes, this thing is still dead, graveyard dead.  Absolutely no reception, no stations, no hash, nothing.  Once in awhile, if I'm lucky, I can touch the center of the volume control and get some response.  Maybe a growl, maybe a squeal, sometimes strong, sometimes weak.  I do consistently have good B+.  A tad high, maybe 10%, but my line voltage is running 120 volts here lately.  I have changed every capacitor, I have checked resistors and have replaced those drifted high.  The oscillator was dead, now it works.  The grid leak resistor had drifted from 49K to over 170K.  Replacing that resistor let the oscillator run.  I have subbed tubes from a radio that worked well, no change.  I seem, most of the time, to have continuity thru the band switch.  I can't get a signal, at IF, from the signal generator thru the radio from the antenna terminals.  The coils all check good as best I can tell.  This thing should work.  The alignment instructions as complex and vague and this is a Zenith.  Zenith radios give me fits unless they work when I get them.  There are trimmers and padders and all manner of doodads that the service documents do not mention.  I have also determined that someone has turned screws and adjustments sometime in the past.  I'm beginning to think there is a trimmer or padder that is so out of adjustment it has stopped the signal.  I have turned and turned what I feel safe to mess with.  Absolutely no sign of life.  I can get a signal at IF thru the radio from the grid of the 6A8 on.  The trouble is ahead of the grid of the 6A8, I'm almost sure.  I'm also concerned about there being a bad ground or common negative connection somewhere in this radio.  When I find what I'm overlooking, I'll have one of those learning occasions we all have from time to time.  I have no idea where to look.  I won't give up.  I've been working on a bakelite chassis Zenith for the better part of 20 years.  I'm actually making a little progress with it.  I hope this one does not take 20 years.  It needs a dial belt and a new dial and I want to get those while they are still available.  I won't buy those items until I get the radio working.  I made pads for under the tuner and under the chassis in the cabinet out of cork.  The repop ones are more than I wanted to spend.  These seem to work well, the dial now fits the cabinet.

Do any of you guys have a method of checking those trimmers and padders individually and with a signal from the generator?  I need to check them and satisfy myself they are not massively out of adjustment like the IF transformers were.  I should be able to put a signal thru each trimmer and observe the result, but I can't get my head around how best to accomplish this.

Dunno, send the O.P. a private message. Maybe gave up on the radio?

It is a good idea to stop if stymied. Come back another day with renewed spirit and fresh ideas...

The picnic tables came out fine and still look as good as the day I sealed them. That Helmsman is a good product...

Start a new topic if you posted because you have this radio...

Chas

So what happened to this thread?

Ran short of paint, in between coats.

Quote :

I don't intend to show any disrespect, Chas.  I don't understand what you mean by my "picking apart" your answers.  I am trying to make sure I understand what you are telling me.  If I repeat what you just posted, it is to make sure I understand.  I appreciate your guiding me thru this radio.

I only meant that I didn't have time to do a separately quoted answer that is easy to understand. You will have to pick through my mega-paragraph

I wanted to make sure that a repair was not made that would be wrong...

I'm rushing through this one...


I'll check in again this evening or tomorrow morning, I have to get out to Lowes and get a couple more quarts of Min-Wax Helmsman satin. Sealing a pair of 6' picnic benches, The hot sun is wiping me out but the bench look sweet... This break (I'm cooling down) will help, one more coat on the bottom this late afternoon...

Start sanding the second one on Wednesday after Florence rains through

You will be fine...

Oh, this Zenith and other similar 5 tubers like RCA are prone to have images in the shortwave. One has to be careful where the oscillator is set to to get tracking. The radio even if aligned correctly will have images when it receives. To control that one electrically shortens the antenna or makes it resonant or makes use of the split second dial to log. That is why the tuner needs to be mechanically perfect and the dial plate align horizontally, the ZERO for the split second and the tuner fully meshed is how to set up. Good grommets shimmed if needed.

Renovated Radios will have the correct offset tuner grommets in new molded urethane.

http://www.renovatedradios.com/

When you get a radio with a full, RF stage mixer and separate oscillator, all the bells and whistles, alignment its tedious, but when done radio will have repeatable settings referencing the log scale so short wave programing can be found from on line schedules...

He he

YMMV

Chas I'm off...

I may have mis-spoken about the gimmick on the tuner.  I looked again and it is not soldered.  It is wrapped as you mentioned.  It appears to be okay, then.  I had never seen this before.  I'd like to soldier thru this radio.  I like to get the really junky sets going again.  Anybody can do the easy ones, right?  I believe it will be an attractive radio, once all the broken things are fixed.  This chassis may give me some insight as to why my 5-S-56 is such a lousy performer. 

I don't intend to show any disrespect, Chas.  I don't understand what you mean by my "picking apart" your answers.  I am trying to make sure I understand what you are telling me.  If I repeat what you just posted, it is to make sure I understand.  I appreciate your guiding me thru this radio.

I have gone thru a Magnavox Concert Grand and am currently restoring a Stereo Theater.  I know the "Magnavox way," as well as the "Philco way."  I have not learned the secret to Zenith radios.  Their rubber wiring and flimsy power supplies are famous gripes among radio collectors.  I guess I got a burr under my blanket about Zenith when I got the bakelite-chassis console.  I have had that chassis on my bench for probably close to 15 years and I still don't have it where it will work.  The chassis is about a half inch deep and loaded with crowded components and rubber wiring.  I don't believe I have the filter caps wired correctly, and I can't see in the chassis well enough to fix it.  Perhaps, after I get this little radio and my 5-S-56 going better, we can dig into the bakelite chassis.  I'd like to get it going just so I could say I owned one.  They certainly are unique!

Egads! Wildcat!!

You can pick apart my answers...

The oscillator and antenna circuits are NOT connected together, what you see, that should be on the top of the tuning condenser is a short wire from the oscillator that should not be connected but wrapped, about three turns around the antenna wire... This is a low capacity gimmick about 3pf to keep "birdies" out of the shortwave band...

If it has been disconnected and soldered remove it and fix. Use the same wire size/type, FWIR #22 solid plastic covered with overbraid.. Twist around the open end about three turns, no more...

Easy-peasy to check an oscillator coil. First go blind to the band switch... Look at the coil on the schematic, identify the ends, don't bother with the taps, measure the ends it should be a complete circuit at low ohms. If n,t then find a tap from one end and measure at the band switch, repeat to find the open which generally will be a wire/joint from the switch.

On rare occasions and certain brands of radios, a celluloid layer insulates a primary from the secondary, the celluloid de-composes and corrodes the copper wire despite the enamel. That means a re-wind, really, not hard, just labor intensive, great work for a rainy day...

I think the 49k resistor is the problem as is that connected oscillator/antenna at the tuner...

That "leaky" bypass with a 400 ohm resistor is of not consequence, replace later, the 400 ohms has a far far greater effect...

Do not re-dress any of the under chassis hookup wiring, this can cause a problem once the gain of the radio is up...

I would like you to suffer through this radio, the learning curve will be gratifying and the hands-on knowledge will be beneficial...

Gotta a lot of outdoor work to do today. Try to catch-up tonight...

GL

Chas

My thinking at this point is the "49M" resistor that reads nealy 110,000 ohms is likely causing the oscillator not to run.  Also, there is a [email=.05@2].05uf[/email] at 400 volt cap, paralleled with a 400 ohm resistor in the cathode circuit that may be leaky.  I will hold off on replacing the mica in the oscillator circuit for the moment.  There are two trimmers in the oscillator coil circuit, that, as you rightly pointed out, are not on the alignment sheet.  This chassis, model 5516 apparently shares the same alignment sheet with chassis model 5517.  5517 differs in the fact it has a phono circuit.  I made the measurements you suggested and got the readings you said would be normal.  I struggle with the schematics of function switches and fairly complex oscillator coils like this radio has.  I hope I'm checking the right spots, because all I can tell is the coil seems okay.  This oscillator should really be running.  Is the "49M" resistor, the grid leak resistor, value critical for oscillator operation?  I believe this is the first totally dead oscillator I have had to deal with.  I'm not sure I understand all I know about it.  I certainly appreciate your help with this.


A curious feature of this chassis is that the oscillator tuning capacitor and the antenna tuning capacitor are actually connected together and fed into the grid of the mixer.  I have never seen this before and wonder how that is supposed to work?  I first thought if was a kluge of some kind, but this arrangement is plainly shown on the service literature.  My tuner drive arrangement does not have the wheel commonly found on Zenith tuner drives.  The "Z" pointer and the red "second hand" are easier to remove than the ones on my Grunow 1191, for what that may be worth.

wildcat445 wrote:

....replace the mica caps in the oscillator circuit just for grins,

Leave that for last, you won't have any grins if the oscillator quits, the radio will not track or drifts. Even then, I have found I need only to shift the values to get the radio to track. FWIR there are a couple of adjustable mica padder caps, these are not on the Zenith alignment schedule. Check the circuit carefully for which band they are for, they may need to be adjusted, LAST and are shifting the center of the band if there is a padder for the low end. If there is no padder for the low end, that is it, depending on the band... Micas in the oscillator, mixer, RF seldom go bad, but coils will drift with age, then other circuit constants must change to get alignment/tracking. An OEM mica may have temperature compensation

wildcat445 wrote:

My plan is to replace most, if not all, the resistors in the chassis....

That is a very good idea, I have been finding that old carbon resistors are unreliable. Consider, what is the value of the resistor when current is flowing? Does that value shift when it warms up? Is the resistor noisy? Messing about measuring a vintage resistor for those parameters is, well, a fools errand. Costing .25 cents why bother? New 2-watt resistors can be "faked" to look like dog bones is one wishes...

wildcat445 wrote:

..It is spring loaded and without a dial drive belt, the tuner wants to stay in the fully-meshed position....

Ah, the spring is too strong, it can be reset... Be sure all the friction in the variable cap is reduced to a minimum, de-grease that front ball bearing, the contact fingers and the rear bearing apply your favorite lube. Unload the spring, without the gear train, the cap should drop close on its own. Now set-up the return spring to balance the tuner rotor with it set 1/2 open, this is the maximum the rotor will weigh. Clean the anti-backlash gear and set so it pinches the teeth of the next gear in progression, such that it performs its function removing all dead play from the mesh. That should be all the parts involved, until a belt is installed. BTW there may or not be a friction clutch on the main tuning shaft. Often, that has been messed with, there is a wrap around flat spring that holds a nut in place, a screw passes through that nut and bears down on a phenolic friction pellet. The drive pulley does not have a threaded set screw. What Zenith anticipated could happen is the tuner shaft is spun, the flywheel carrying it, when the end of the tuner is reached the clutch slips so not to apply additional torque to the tuner gear train and eventual heavy wear and damage.

If a "O" ring is used, there will be a certain "elastic" quality to the tuner drive... That is why the flat belt drive, however, rubberized flat belts are not around any more. These that are are 60 years old, hard and dry...

Though opposite from Zenith design, arrange the idler pulley, it must rotate, often stuck with dried oil, such that it bears on the outside of the belt or "O" ring. That forces more of the belt, "O" ring around the driver pulley for less if any slippage. Remember, it is the clutch that is supposed to slip..

Also, there has to be a fiber bushing at the far end of the tuner shaft if this chassis has a shaft that passes through to the rear. There may be a fiber bushing even if it has a short shaft. What happens with metal bushings at the front and far end of the tuner shaft, a tiny bit of electrostatic is developed while tuning from the belt or "O" ring that electrical signal circulates through the chassis and the shaft. In parts of a band that the signals are low the AVC goes positive and the IF amp gets very sensitive. Thus, there will be a lot of noise from the tuning shaft.

Not all Zeniths are built like the parts I discussed. there are changes from model to model. If there is no indication of a clutch or there are steel bushings to support the tuning shaft, it is what it is. It was all those things I found is some of the models and they were not high end either.

Get that tuner drive in good order and a Zenith is a fun radio to go band cruising. True to the words, "Twirling the knobs"

Oh, be sure to use all glass tubes where specified, with the correct shields, metal tubes, even if a pin 1 ground has been made, may cause a problem...

YMMV

Chas

I tinkered some more with this little radio.  R2, part of the grid leak circuit, rated at "49M" which is 49K ohms, measures a tad under 110K, so it is drifted twice its value.  I need to replace C2 which is paralleled with R1, a 400 ohm resistor.  These are in a tight spot, so I need to do some disassembly to get to them.  The same setup exists on the 6K7 tube as well, in a similar tight spot.  R3, rated at 11K ohms, reads a tad over 23,000 ohms.  I cleaned the function switch thoroughly.  Whatever I did brought B+ down to 255 Volts.  If I inject a 456 k-hertz modulated tone from the signal generator to the grid of the 6A8, I can hear hash from the speaker like the radio wants to work, but can't quite bring in a station.  I believe I'm getting closer.  I need to be able to control the tuner now.  It is spring loaded and without a dial drive belt, the tuner wants to stay in the fully-meshed position.  The tuner is not shorted.  My plan is to replace most, if not all, the resistors in the chassis, replace the mica caps in the oscillator circuit just for grins, then replace the grommets for the tuner and get the drive belt situation sorted.

Thank you for posting the above.  I have decided to order enough resistors to replace all in the radio.  I'm going to replace the mica caps in the oscillator circuit.  I will secure the riveted grounds as you suggested.  Also clean the band switch again.  Make the resistance checks you suggested.  There are a couple other things I'd like to clean up.  Then we'll see what we have at that point.

wildcat445 wrote:

First of all, let me say how much I detest Zenith radios

Hmm could that be better expressed as any radio that is not a "tough dog"?

Reference this schematic:
http://www.nostalgiaair.org/pagesbymodel/888/M0024888.pdf

So far you have established that a "signal" (modulated 456khz ) is getting through the IF from the plate of the 6A8...

B+ is still high, (somewhere).

Overall condition of the radio is "poor".

If I know a Zenith it's this, they used un-plated steel rivets to fasten sockets terminal strips and the odd ground lug. What happens is corrosion takes place and the "good" connection fails.

Oh! easy-peasy, I'll measure and get zero ohms, sure   Not with any garden variety service ohmmeter. The resistance may be low at DC but at RF it can be something else. This is especially troublesome, how to detect a fault condition if one can't measure cause and effect? In actuality, there are so few of these riveted grounds as a matter of good practice, either, drill out the rivet and replace with hardware with serrated washers or solder the connector side of the rivet to the chassis.

Do look at the field coil, R8, R10 combination for the correct resistance when cold and not leaking to frame. Be sure C12 at 6F6 is good, no leaks... Be sure the output transformer is correct and not leaking to the frame. Be sure C19 did not get grounded to chassis.

Re-clean the band switch this time with an auto electrical contact cleaner that leaves no residue, use an acid brush for gentile scrubbing action. THEN clean with isopropanol alcohol, the 91% from the drug department. This action will remove all greases and stray rosin. The alcohol will chill the switch and condensation may form, followup with a warming with a heat gun. Use an electric motor lubricant or mineral oil on a toothpick to the switch bearings, spot of cup grease or petroleum jelly on the ball detent. Manipulate the switch at least 50 times. The "dry" contacts should burnish themselves. If the switch proves noisy when the radio is working, use the toothpick and De-oxit to the contacts, do not let the De-oxit get onto the phenolic.

Put the ohmmeter on the junction of R5/C21 and the chassis. It should read infinite resistance, it the measurement hangs at any value other than infinity find out and fix... Oh, run the band switch up/down when testing...

Keep the connection at R5/C21 and bring the negative lead of the ohmmeter to the grid (cap) of the 6A8, the reading should be no more than 10 ohms, likely less. Again, swing the band switch. Do the same to the 6K7, disregard the band switch. looking for low ohms.
Trace the detector circuit if so inclined, R5, 6, 7, 11 and C10 just for snicks and giggles...

Now check the continuities of the oscillator coil and the input circuit of the antenna. The coils should all be around 10 ohms more or less any high resistance is an open coil.

Note the tone control places B+ on that control, not good if the control has been dosed with control lubricant. The tone circuit can be rewired by 1/2 the value of C15, .047. Use of foil/poly cap is recommended, also for C14, noise pules will eventually spoil a common metalized cap.

Assume all is good, and the radio still refuses, consider there may be a LOT of accumulated moisture in the oscillator or antenna input coil.

Remove the covers of these coils and play a heat gun on them CAREFULLY until the bees wax turns translucent to almost clear, if it should go clear it will run off and that is not desirable, at the translucent stage it will be at 140 or so F. Try to maintain that for a few minutes then let cool. Try the result.

I do note the radio uses one of those "Braided" lead bypass caps on the AVC line, C21. You can use a modern cap as they have less inductance.

I read the board on a daily basis, post what is happening.

Chas

Okay, now that I've quit whining and started thinking, I may have made a bit of progress.  Apparently, a dirty volume control/tone control situation killed my signal generator output.  I have the signal generator tone back from the converter plate to the speaker.  The IF trimmers were massively mis-aligned.  I'm beginning to wonder if "happy hands at home" have been tinkering.  Perhaps the oscillator is as mis aligned as the IF's were.  It really should be working.  There are some drifted components, but I should get something out of the radio.

First of all, let me say how much I detest Zenith radios.  Secondly, I have this one thru very little effort on my own.  Thirdly, the wife thinks it is cute, so I'm stuck messing with the silly thing. 

This little five tube radio is kicking my butt.  I should be getting ready for winter, but, no, I'm fooling with this thing.  What I have done so far is this.  Replaced all but three capacitors.  There are two [email=.05uf@200].05uf@200[/email] volt caps paralleled with 400 ohm resistors that are hard to get to.  Then there is a [email=.1uf@600].1uf@600[/email] volt cap on the tone control that is weird.  It looks like it is grounded in the middle, but it only shows the one cap on the schematic.  Part of the reason I hate messing with a damned old Zenith.  Their proprietary component nonsense.  All but three of the dogbone resistors I have checked have drifted to about twice their rated value.  Yesterday, I had most everything working in this radio but the oscillator.  Today, I can't get an IF signal thru this thing anywhere.  B+ is high, at around 300 volts, spec is 240 at 112 volts input.  I'm running it on my Variac, so input is 112volts.  I have voltage at all the tubes, approximately where they should be with the exception of the oscillator grid, which shows positive rather than negative voltage.  So the oscillator is not running.  Yesterday, I could get an IF signal thru from the converter plate to the speaker.  Now all I get is a growl when I connect the signal generator.  I tested the generator on another radio and it works fine.  The schematic for this contraption is in Riders volume 7, page 7 of Zenith.  Has to be something simple.  I almost had a radio yesterday.  The resistors are high, but I should be able to get the thing to work, perhaps not perfectly.  The B+ being high tells me something is not conducting current, thus not loading the power supply.  This radio needs one of everything and I wanted to keep it simple.  I guess I'm in too deep now to stop.  The dial is broken, there is no dial drive belt, the grommets for the tuner and chassis are all shot.  This stuff is repopped, but I wanted to see what I had before I bought a bunch of parts and put lipstick on a pig.  I can get noise thru the radio.  I can tap the grid caps with a screwdriver and get a click in the speaker.  I just can't get a signal thru.  What am I overlooking?