Zenith Model 6-S-128 repairs

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Hello members!!  I bought my first fixer upper and it plays with a loud hum so I figured I would start to recap it beginning with the wet electrolytic capacitor. I was hoping someone could explain to me how to replace these two 10-10 mfd 450V electrolytic capacitors.  The original 16 mfd 450V wet electrolytic capacitor was left in place for aesthetics but the wires were cut and replaced with one or both of these Mallorys by somebody else. I have a 16 mfd 450V axial Ecap to replace it with.  If anybody could help me with understanding this I would really appreciate it. Also, in the schematic, if you could point out the wires that I labeled in the photo it would really help me to better read and understand the schematic.  Thanks to anyone willing to help!!!!
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Thanks Cliff!!.  I will snip the negative lead of each cap to test resistance.  If I get a reading then the cap is bad right?  Except on the electrolytic caps.  When I tested the electrolytic cap 16 uF. it showed a steady climbing reading starting at like 1.3 with the meter set at 200 M ohms.  Is this a good Ecap or a bad one?

On the checking of tubes, you can check for filament continuity, and interelement shorts.
But it will not check for gaseous tubes or elements that only short when heated.
Other than the heater, you also can't tell if elements are open internally (which is normally not found anyway) you cannot test for emissions quality. So a tube can be tested with a multimeter and found to be  ok, but the tube may be so weak that it will not function in circuit.

That's the reason you may consider obtaining a Tube tester if your budget, and objectives are to restore many radios.

As a follow up, electrolytics use a chemical paste or liquid. Whereas paper caps use impregnated paper usually wax. It acts as an insulator between the foil layers.
Electrolytics form a charge that separate the positive from the negative charges, forming an insulation barrier. If this polarity is reversed by applying the incorrect connection, then it becomes a short circuit and causes other components to fail because of excess current from the shorted capacitor.

When capacitors are in a circuit, they can be used with resistors or coils to charge up a capacitor and then discharge through resistors designed as cycle, which can used in an filter or an oscillator circuit, the filter to smooth out variations in voltage fluctuations, by discharging voltage back to sagging voltage and bringing the supplying voltage back up. When designed as an oscillator component it's constant charging and discharging can be amplified through a tube and it becomes an oscillator. 

Capacitors use this timing function to only pass certain frequencies, and are used in R.F.,  OSCILLATOR, I.F., and audio circuitry. That's why electrolytics are used to filter out hum in power supplies, being electrolytics they can store larger currents for filtering. Instead of microamps or milliamperes of current, electrolytics can store amps of current for filtering action.

Answer to:
1. No it's resistance makes no difference (It does on the other hand if it's leaking)
Capacitors are designed to block DC. Voltage. So when you measure resistance in a resistor, you are seeing DC passing through the multimeter and through the capacitor, which means it's not blocking DC. as it should. Capacitors act as an insulator to DC. When a capacitor is failing that is the only time you should see resistance.

2. You shouldn't be able to read resistance in a good cap (electrolytics being the exception)It should read infinite resistance. Zero resistance means a dead short. Infinite means the resistance is too high to read on a multimeter.
You really need to get into electronics theory to understand caps.

3. Yes in a DC. Circuit
Note: capacitors store and block DC. but capacitors also pass A.C. (such shorting to ground yet not shorting the DC.) Capacitors also pass AC signals to the next circuit while blocking DC to the next circuit.
Capacitors have many functions and electronic theory again is essential to understanding this.


Your red circled cap is C-19. It's the one you marked in your next photo down.
I do see your meter can measure capacitance, using that function is the best way to test for good and bad caps.
Don't ask how that works, as again it's all related to theory and signal generation in the newer multimeters.

Thanks for the video Cliff. I do not have an oscilloscope only my multi meter.  So here's my questions for you regarding these film caps.
1. If the cap was wired correctly by chance,  shouldn't it  have a different resistance measurement than if it was wired wrong?
2. What should the resistance of any cap be ideally?
3. Does a cap store the correct voltage if it wired correctly?

From the top.
A. This is the area in question.  Perhaps I am misreading the schematic. Anyway I haven't de-soldered any resistor or cap.  I simply cut the leads off of the caps, bent the end of the leads into a hook and then hooked in my cap leads and soldered them.
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B. My multi meter is digital readout but has a center dial that turns to change from resistance to voltage etc
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C.

According to the schematic there is no cap between R2 & R8
Tell me what the cap number is that you replaced from the schematic legend. ( C- ? )
Do you have an anolog multimeter?
If so you need to check the capacitors, by using the resistance scales. It isn't used to measure resistance, but to see how fast or slow the capacitor is charging or leaking voltage provided by the meters supplied voltage while in the resistance mode. You have to disconnect at least one lead of the capacitor to do this test. If the needle moves fast then slows down towards the end, that means the capacitor holds a charge, but if it goes full scale immediately then the capacitor is shorted.

Anytime you measure a resistor, or capacitor it must have at least one end disconnected from the circuit. If not you get erroneously measured results.

You need to measure each resistor in the candohm by disconnecting each lead, especially both ends, because there are other paths in the radio that would give incorrect readings. Write those values down. R11= (250 Ohms tab 1 to 2) then (56 Ohms tab 2 to tab 3) then (86 Ohms tab 3 to tab 4) then (5,860 Ohms tab 4 to tab 5) then (11,000 Ohms tab 5 to tab 6) then reconnect the wire to tab 4 and measure to ground, it should show continuity ( 0 Ohms )

Before I forget non polarized capacitors can replace paper capacitors, BUT you have to note if there is a black line on one end that circles the paper cap. The reason for that is to designate that the outer foil of the cap goes to the negative or ground side of the circuit, the reason is to shield the cap from spurious or noisy signals. So see which lead on the new nonpolarized cap has the outer shield on it and mark that end with a permenant marker in the same manner. A better explanation follows on YouTube, see my next post.

Hi Cliff.  I can't see the reply that I sent this AM so I guess I did not send it.  Here goes again.  Happy Thanksgiving and thank you for all of your help!!!
From the top of your last post:
A. I test all the tubes with my multi meter set on sound for a continuity test.  They all check out with no shorts.  Not sure if this satisfies your request.
B. It did not smoke with the tubes removed.
1.The polarity of my two new Ecaps is correct.  The new film caps have no polarity according to the manufacturer.
2. Ok I checked the resistance on the cap across the speaker field.  It was a cap I missed.  .005 uF.
It checked out at .4 with the meter set at 200k ohms. I will order the correct cap and replace it.
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3. R9 checked out at .24 with the meter set at 200 k
4. R9 is connected to Terminal 1 and Terminal 3 of the terminal strip.  I re-soldered Terminal 2 and Terminal 3 but didn't touch the R9 resistor connections to the terminal.  When I did this and powered it up with tubes in, it did not smoke but I have no sound from the speakers.
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5.  See the picture below again Cliff.
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PS All tubes are correct and fit well into there sockets.

PS
When you remove the tubes, check and make sure they were in the correct socket. Also make sure that each one is the proper tube number.
They could have been incorrect in the number or socket.

Make sure you remove all tubes. Can you test them for shorts?
Power up the radio with tubes removed, to see if it starts smoking.
If so then something is miswired.


Yes R9 is changing value due to heat from too much voltage or current.
Ok, your going to have to do several things.
First, make resistance tests on your new caps. You could have a bad one, yes even new ones can be bad.
1. Sometimes you can still get the polarity wrong after checking it twice. Did you check polarity?
2. Do resistance check on the speaker field, it should read just about 1K if not close then it's shorted.
3. Is the R9 (19K) still correct in its resistance?
4. Is R9 connect to the terminal strip above it, as shown in your pictures?
5. Tell me how many wires are connected each of the solder terminals on that terminal and name what each one is connected to. Such as terminal 1 (left) terminal 2 (center) terminal 3 (right)
So terminal 1 goes to positive wire of c25 , 2Mfd. And positive wire of c25 of 8Mfd. Etc.
Do a resistance check of all 3 terminals and write down. Make sure radio isn't plugged in during resistance checks. And use a jumper wire to short the caps to bleed off residual voltage in the caps, one second on each cap is all that is needed.

Number your answers so I can see what you did. To correspond to each numbered step requested by Me.
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You may have to just take a resistance check on each positive side of each cap in the radio.
But just keep that in mind. Resistors are less obvious unless they are either open or shorted.
If you find no problem then each and every wire has to be confirmed that it isn't a designed ground wire.
If you find one that shows a ground that shouldn't, then you have to explore any thing that it's connect to, you may even find a shorted wire touching part of the chassis or another grounding point.
Capacitors have three functions, filter, block or bypass.
So looking at its function is important.

Hi Cliff.  Back from LA.  Finished the recap. No hum at all.  Smoke seems to be coming from either the big brown resistor (brown with white cap and black dot) or from the terminal strip that it mounts to. The wire on the middle lug of that strip got so hot that it de-soldered and came of with the slightest touch. The resistor should be 19 M ohms which makes it the R9 resistor on the schematic. When I tested it with my multimeter set on 200K ohms, the reading said right around 23.5. Then after I let it smoke some more while I took pictures, it went up to 34.6.  Does this make any sense?
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Be sure to make notes and diagrams, of the exact position of each wire. This is critical, especially because of the oscillator. Do not replace wires that are not exactly the same length and position. Otherwise you will have problems in alignment, that includes parts.

Thanks again Cliff for taking the time to help me.  Going to LA and back Mon-Tues.  I'll get back with you then.

Ok thanks again Cliff.  The area of smoke is in the area of the oscillator and short wave/broadcast trimmers.  I think I will start there.

I meant to say replace those caps.

Measure the resistance of of the speaker field coil, it could have a shorted wire to the core.
Check the resistance to the rectifier from ground to each plate. They should be equal, if not then follow the lower resistance path.

Did you notice if there was a blue glow from the rectifier?
If so it would indicate an overload in the power supply DC side.
Capacitors are the prime culprits, look for burned resistors also.

Use your meter to ferrite out, shorts, misswiring, even opens, if the current path has redirected, and no longer has the load, from resistors and caps, it will possibly increase the voltage to above what parts can handle.

I think if it ran with dim bulb then you have caps breaking down under higher voltages. Yes, again check polarities, and resistors in that area. Then insert tubes and run with dim bulb tester for at least a half hour, starting with the 25 watt bulb, watch for smoke. Then the same with 40 watt bulb.

I do have a multi meter so I could test specific areas if want me to.

Thanks Cliff!!! No it did not smoke during the dim bulb test. Should I put the tubes back take it off dim bulb and see if I can locate the source?  Should I start by replacing the caps nearest that area?

You posted during my postin, so you have answered my question on that. Did it smoke wile doing the dim bulb test?

You are going to have to check every cap and resistor for value. You could have a shorted tube. Did you test every tube? If it took a couple of minutes to start smoking, then it sounds like components breaking Down.
You could use a multimeter and check for current . Do some reading on radio troubleshooting. 
There are so many variables in radio faults, burned resistors that open, short, reduced value, or increased value and are out of tolerance. Bad caps, open or partial shorts, wrong wiring.

I would go through step by step, starting with power supply and tubes, replace all paper caps, then resistor value checks, trace all wires with manual hand over hand and note connection points.
Take a schematic and use a red pencil to mark each wire by tracing over the lines, or use a highliter marker. Mark each connection on the schematic after you confirm its correctness. The same with each component. 

What is on the topside of the chassis at the location of the smoke? Can you take a picture of the top of the chassis? Marking the location of the smoking.

I did a dim bulb test Cliff. Here's my setup and results.
I removed the tubes and mini lights. I tested bulbs from 100 Watts downward and the only bulb to light was the 40 Watt, though very very dimly.  The 25 Watt bulb glowed dimly.  [You must be registered and logged in to see this image.]

Oh I did replace one paper cap. C19 .03 uF 200 V with .05 uF 630 V.  I misread the schematic list of all caps.  I thought the .03 uF was .05 uF. All of my caps are film caps to replace the old paper caps. Besides the one mentioned above, all the other replacement caps are the correct uF value and all are 630 V caps.

Thanks Cliff.  Here's where I am at.  I replaced the cord, wired the fuse in line with the wire going to the switch. I replaced both Ecaps.  I put the tubes back in and turned it on. (I should have done a dim bulb test first but I did not) No hum at all but after about two minutes I noticed smoke.  I couldn't tell where it was coming from but it appeared in the general area that I point to near the top of the picture. I haven't replaced any of the other caps and don't want to proceed until I figure out what is going on. Any ideas? 
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Yes!

Ok great.  Would something like this 250 V inline fuse holder work?
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Put the fuse before the switch, in series. Use a polarized plug, in other words one blade is wider than the other, make sure that you wire the smaller blade to the wire that goes to the fuse then to the switch.
This is my recommendation the smaller blade on the plug is made so that in case there is a short circuit it will not handle as much current as the wider blade. The wider blade will handle more current and is always connected to the return wire. If it were reversed then if there were a short current then it would continue to flow damaging the circuit more.
Do not ground the fuse holder, if the fuse holder has mounting holes then drill a hole in the chassis and use either a rivet(s) or screws to mount the holder.to the chassis. Make sure the is a safe clearance from other parts on the chassis.

If you don't want to mount the holder that way, then mount the fuse holder with contact cement or epoxy, let it dry then wire it into the circuit. Again do-not ground the fuse holder to the chassis electrically. It must be insulated from the chassis and/or electrical ground.

Ok thank you Cliff. Does is matter which of the two 120 V leads going into the transformer I put this fuse on? Do I have to worry about ground or does the holder itself ground to the chasis when I install it?
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Perfect choice. Yes.
The voltage is 250v designed to prevent arcing. So when the fuse opens up from a short it won't continue to arc. So it will handle any voltage up to 250V.

If it were a 12V fuse the gap wouldn't be enough to prevent arcing. A 125 V would be dicy because there all rated for D.C not A.C.

Thank you Cliff.  So it's a .75 amp fuse but at what voltage? 250 V? Slo burn? Most everything I'm looking at looks like this:

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Not sure.

75W / 120V = I
Use 120 that's the normal line voltage now.
I + 10% overload =

.6875A round up to .75A

Since it's a tubed power supply go for slow blow fuse 250V rating