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 Zenith Model 6-S-128 repairs

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FrankB
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PostSubject: Re: Zenith Model 6-S-128 repairs   Zenith Model 6-S-128 repairs I_icon_minitimeDecember 30th 2016, 4:56 pm

Typically, an antenna of 30-50' is best, along with a good ground.
Great its working!
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PostSubject: Re: Zenith Model 6-S-128 repairs   Zenith Model 6-S-128 repairs I_icon_minitimeDecember 5th 2016, 5:50 pm

O
Hey Cliff and Frank B..........I fixed it!!!  (or......well I had help) I found a local guy through the California Radio Historical Society.  He invited me into his place and schooled me on where I was misreading the schematic.  Long story short, the 2uf Ecap needed to go between R9 and R4 just like the schematic said.....just like you were trying to tell me Cliff. Simply moving that 2uf Ecap solved everything. All systems are go!!! Very strong reception.  Any ideas on an antenna?  I am going to make a 6 ft single lead short long wire to connect to the A terminal.  May go with a doublet.  Your thoughts?  BTW thanks  
for all of your help with this Cliff.  I really appreciate the lessons.




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PostSubject: Re: Zenith Model 6-S-128 repairs   Zenith Model 6-S-128 repairs I_icon_minitimeDecember 1st 2016, 4:07 pm

If you disconnect all the transformer secondary wires, and let the transformer "coast" and it gets extremely hot or the dim bulb tester starts glowing a lot, the transformer is shorted. Typically I recommend a half an hr of running.
 his eliminates the transformer as a likely candidate. I have seen internal shorts also, but they are rare.
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PostSubject: Re: Zenith Model 6-S-128 repairs   Zenith Model 6-S-128 repairs I_icon_minitimeNovember 29th 2016, 2:12 pm

OK great!!!  Thanks Cliff!!!!
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Cliff Jones
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PostSubject: Re: Zenith Model 6-S-128 repairs   Zenith Model 6-S-128 repairs I_icon_minitimeNovember 29th 2016, 1:20 pm

So the methods to use:
Before plugging in power. Have a schematic.
 first are as follows, do a smell test for burned components.
 Second do a visual for damage.
Third check for correct tubes and in correct sockets
Fourth test resistors for correct values, may have changed due to age, or recent replacement that isn't correct color code. 
Fifth Check for miss-wiring.
Sixth test inductors and transformers
Seventh test capacitors. Check for polarity.
Eighth test tubes
Ninth look for modifications and restore to original.
Tenth use a variac and dim bulb tester. Bring up slowly increase voltage by 25 volts every 5 minutes and set last voltage at 115-117v.
Eleventh start measuring test point voltages if radio not functioning.
Twelfth do an alignment.

Mark and note all changes. This will help solve similar problems that you run across.

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PostSubject: Re: Zenith Model 6-S-128 repairs   Zenith Model 6-S-128 repairs I_icon_minitimeNovember 29th 2016, 9:41 am

Cliff I tested the resistance of both R9 (19k ohms) and R4 (11k ohms).  R9 showed a value of 22.7 set at 200k ohms.  R4 showed a value of 13.5 set at 200k.  These values fall beyond +/- 10%.  I ordered replacements.
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PostSubject: Re: Zenith Model 6-S-128 repairs   Zenith Model 6-S-128 repairs I_icon_minitimeNovember 29th 2016, 8:19 am

Good deal Cliff.  Thank you!!!
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Cliff Jones
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PostSubject: Re: Zenith Model 6-S-128 repairs   Zenith Model 6-S-128 repairs I_icon_minitimeNovember 29th 2016, 7:58 am

I am going to rename this topic, so if other people have this model, Internet browsers will find it faster

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Cliff Jones
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PostSubject: Re: Zenith Model 6-S-128 repairs   Zenith Model 6-S-128 repairs I_icon_minitimeNovember 28th 2016, 10:39 pm

If a resistor were completely smoked, then yes because the physical body would melt away by thermal runaway, or blow up with no further electrical conduction, but in the process of doing this electricity may continue to arc over, causing further damage,  or with less effect to decrease resistance to a lower value.
Yes you need to test them, because of the heat generated, would change resistance.

I sure that when resistance changes it can either allow more or less current than amount designed for in that particular circuit.
That, depending on the resistance that is present, would also change the voltage drop or gain. And limit current through a circuit.
Most resistors are not designed to block a signal, but act as a voltage divider, or to regulate voltage within certain bounds.

You need to test the candohm as that's where the B+ is divided, and because of a short elsewhere  it may have changed also, but a candohm resistor has a metal heat sink that draws away heat to operate as specified, but again excess current can change its resistance.
That's what electronic theory does to help you understand problems and steps to use to repair.
I ask! do you have any background in electronics. The reason I ask is based on the questions you ask.

Understanding this will help me give you support, as I may be explaining things you already know, and therefore giving complicated answers, when not needed.

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PostSubject: Re: Zenith Model 6-S-128 repairs   Zenith Model 6-S-128 repairs I_icon_minitimeNovember 28th 2016, 9:29 pm

Again thank you Cliff.  Yes I made the test jigs for the Cx scale.  Works perfectly. I tested a couple of the old paper caps left over from my recap and they were fine. I do need to test the R9 and R4 and see where they are. 
Q1.  If the R9 was completely smoked then it would have infinite resistance right? 

Q2.  Would that be enough to lose the sound in the speaker field?
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PostSubject: Re: Zenith Model 6-S-128 repairs   Zenith Model 6-S-128 repairs I_icon_minitimeNovember 28th 2016, 8:04 pm

Yes make up your own test jig, for caps.

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PostSubject: Re: Zenith Model 6-S-128 repairs   Zenith Model 6-S-128 repairs I_icon_minitimeNovember 28th 2016, 7:28 pm

Good readings within tolerance. 
At this point I'm trying to cover so many methods as it comes to mind, they will not be organized.


Any component more than 10% (resistors)to 20% (caps) off of reading is suspect and needs to be replaced.  However capacitors can have quite a leeway if there filter caps in the power supply up to as high as ( - 10% to + 50% depending on manufacture. For paper caps From 20-50%
You can take a chance and only replace the out of tolerance ones. But that's up to you. Or just run the radio.
After you've tested all the paper caps, then you need to concentrate on seeing if there are miswired parts. 
Then Test (resistors including the candohm ones, inductive devices- transformers-both primary and secondaries) speaker voice coil. 

A hint -if the radio no longer starts smoking, then you can approach several methods of further testing.
First one 
Test voltages, even power outlet.
Most older radios were designed to run at 115-117 v.a.c.
Look at a tube manual and see what voltages are expected at the different pins. 
Also note the plate resistance, this will give you an idea of load (resistor) resistance or transformer resistance. Should be close.

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PostSubject: Re: Zenith Model 6-S-128 repairs   Zenith Model 6-S-128 repairs I_icon_minitimeNovember 28th 2016, 5:17 pm

The 16 uf Ecap C21 shows a value of 14.0 uf as it sits in the chassis with the negative lead cut. C18 on the speaker field shows a value of .047 uf.
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PostSubject: Re: Zenith Model 6-S-128 repairs   Zenith Model 6-S-128 repairs I_icon_minitimeNovember 28th 2016, 4:42 pm

Thanks Cliff!!!  I tried the Cx slots with two new caps that I had not used in my recap.  Their both radial safety film caps X2 and Y2.  Both are .05 uf.  X2 showed a value of.047 uf and Y2 showed a value of .044.  Should I test the other caps in the radio now Cliff? Not sure if I cut them out that their leads are long enough to get them in Cx.  May have to rig up something.
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PostSubject: Re: Zenith Model 6-S-128 repairs   Zenith Model 6-S-128 repairs I_icon_minitimeNovember 27th 2016, 1:02 pm

By the way you should have a adapter for measuring capacitors, you meter has a special socket just for capacitors. It's marked something like cX or CX . Two horizontal slots. Regular test leads won't work, unless they have a symbol of capitance on one of test jacks. Doesn't look like yours does, but you should have a small circuit card with a small divider slot and two test leads with tiny alligator clips attached.

If not you can insert the cap leads, one into each slot, the reason for that, if you were using regular test leads, the capacitance between the leads would effect the measurement.
There are some meters that compensate for that, but THIER more expensive.

Go to our topics on test equipment, look for our link on How to use a multimeter
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this is the best I have found.

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PostSubject: Re: Zenith Model 6-S-128 repairs   Zenith Model 6-S-128 repairs I_icon_minitimeNovember 27th 2016, 1:00 pm

Thank you Cliff!!!  Thanks again for your patience and for taking so much to to work with me.
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PostSubject: Re: Zenith Model 6-S-128 repairs   Zenith Model 6-S-128 repairs I_icon_minitimeNovember 27th 2016, 12:02 pm

Ok take it down to the next step 2ufd. See what you get.
Also take your test leads and short them togather, what does your display show while doing this?

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PostSubject: Re: Zenith Model 6-S-128 repairs   Zenith Model 6-S-128 repairs I_icon_minitimeNovember 27th 2016, 11:59 am

What's wrong with your tube tester? But that's another topic. Let's stick to the radio itself, or we will get confused.
I have asked a couple of the moderators to help out, I should like their experience and expertise at this point. It's always better to have more than one point of view.

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PostSubject: Re: Zenith Model 6-S-128 repairs   Zenith Model 6-S-128 repairs I_icon_minitimeNovember 27th 2016, 11:56 am

Sorry Cliff for the confusion. Here in the picture I have the negative lead to the capacitor clipped.  I attached the negative from my meter to the negative side of the cap using an alligator clip.  I am touching the positive lead of my meter to the positive side of the cap. Note the reading on the multi meter is 0 with the meter set at 200 uf.  Not sure what I am doing wrong here.
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PostSubject: Re: Zenith Model 6-S-128 repairs   Zenith Model 6-S-128 repairs I_icon_minitimeNovember 27th 2016, 11:22 am

No, I don't think your using your meter correctly. You don't read capacitance value on the resistance ranges.
I am confused on your statements.

You need to measure its capacitance, not its resistance.

If you make a statement then be more detailed, such as "I used the meter using the capacitance range and found that it's reading indicated zero capacitance in all ranges, or only the 2ufd range."

Turn your selector, down to the white band at the bottom of your meter. That is your capacitance portion of the meter, then Select the 200ufd position. So when you stated zero capacitance, is this how you tested the capacitor?
Note:
Always start on the highest section in the range and if it doesn't read then go to the next lower range. This is always the safest way to start using any measurement you take, whether voltage , current, resistance  or capacitance until you know what range to use through experience.


Measuring resistance of a capacitor only shows if a capacitor is leaking, the lower the resistance, the more it leaks. The best way to check for good or bad capacitors is by using a capacitor checker, most of those used high voltage which makes the test more valid. With solid state test equipment for measuring capacitors don't use high voltage to test for leakage.

If you get the radio to play, remember like you said, capacitors that old have ended THIER useful life at this point. You may get lucky and they will continue to function. A lot of people think the same thing, and later have to go back and repair again, but if you don't replace, you may end up no being able to find unobtainium parts in the future. Better be safe then sorry, so replace ALL PAPER AND ELECTRLYTICS. Mica capacitors normally are ok and don't need to be replaced.

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PostSubject: Re: Zenith Model 6-S-128 repairs   Zenith Model 6-S-128 repairs I_icon_minitimeNovember 26th 2016, 9:13 pm

Ok  C18 paper cap at speaker R is .4 with the meter set at 200k ohms.  Zero capacitance. Are these values expected for an 80 year old paper/wax cap?
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PostSubject: Re: Zenith Model 6-S-128 repairs   Zenith Model 6-S-128 repairs I_icon_minitimeNovember 26th 2016, 7:42 pm

Now test that capacitor, while the lead is disconnected.

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PostSubject: Re: Zenith Model 6-S-128 repairs   Zenith Model 6-S-128 repairs I_icon_minitimeNovember 26th 2016, 5:05 pm

OK great.  .56 with meter set at 2k ohms. But I hope I clipped the correct lead.
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PostSubject: Re: Zenith Model 6-S-128 repairs   Zenith Model 6-S-128 repairs I_icon_minitimeNovember 26th 2016, 10:23 am

"
Not at this point. Disconect one lead of the speaker field and measure its resistance. It should measure around 1000 Ohms.

You did say that it no longer smokes, that's a good sign that there are no shorts.
You also said that you didn't hear any sound.
So two items to check are the speaker voice coil, which should be ok and the audio transformer primary winding for shorts and opens. 
The speaker has the field coil also, so you have the speaker field to test, while the radio is on use a screwdriver on the back of the speaker field to see if it's magnetic, if it's not the field is open.
If it's ok then test the primary of the audio transformer. If it's ok then check the secondary resistance.
If the speaker voice  coil is bad then hook up a speaker with a magnet  even a transistor speaker, try that.

If those are unproductive solutions, then comes the task of signal tracing and replacing caps that you haven't replaced. 
Do you have a signal tracer? 
If not , here's a test you can perform, get a transistor radio turn it on and you turn on the subject radio.
Tune test radio up and down, if the transistor radio starts to squeal. That shows that the oscillator section is working.
Then if yes start at the voice coil for sound, then go to the input grid of the audio output tube.
If no noise then do the same on each previous stage. All the way back to the antenna.
It could be a cold solder joint on a tube, an open or shorted wrong value resistor, or cap
Also an I.F. Could be open or shorted,  a bad trimmer pad, or even a shorted tuner capacitors plate(s)

If those don't provide a solution then you have to start checking voltages.

Just remember to do your tests systematically and write down what tests were done and each results obtained. So if you have to backtrack you will be more proficient next time and see what solutions resolved the problem.

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PostSubject: Re: Zenith Model 6-S-128 repairs   Zenith Model 6-S-128 repairs I_icon_minitimeNovember 26th 2016, 8:04 am

YES!!!!!  So what do you think at this point Cliff?  I must have a shorted speaker field right?  Should I replace the R4 R9 resistors and the 16 uF Ecap? (the one that has zero capacitance)  I have a Hickcock tube tester I-177 but it needs work too. Maybe I should get new stock tubes to replace the old stock. Your thoughts?
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PostSubject: Re: Zenith Model 6-S-128 repairs   Zenith Model 6-S-128 repairs I_icon_minitimeNovember 25th 2016, 7:50 pm

I added to my last comment additional info, so re-read previous.

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PostSubject: Re: Zenith Model 6-S-128 repairs   Zenith Model 6-S-128 repairs I_icon_minitimeNovember 25th 2016, 7:39 pm

Zero.  Maybe I shorted it when the strip started smoking
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PostSubject: Re: Zenith Model 6-S-128 repairs   Zenith Model 6-S-128 repairs I_icon_minitimeNovember 25th 2016, 6:44 pm

P
oh my! I thought the center solder tab had no connections to start and was just used as a tiepoint for C 25(s) 
The two resistors behind the terminal strip are (R-9 & R-4 ? ) 19 Megohms and 11 Megohms in respect.
Correct??
So that center lug ( terminal 2) should have C-25 (2Mfd +) connected
On terminal 3 (right) should be C-21 (16Mfd +) connected
On R-11 ( candohm ) tab 1 should be connected to (C-21 - ) also the transformer center-tap high voltage should be also, and R-8. (This is common negative)
Back on (C-21 +) should go to the filament connection of the rectifier tube, R-9, and speaker field.
(C-25 + ) should be connected to the other end of speaker field and tab 6 of candohm.
It looks like the mica cap is connected to terminal strip, tab 1 and should be OK.
------
yes as long as it climbs, it shows that the cap is taking a charge. Can you use the capacitor section, of your multimeter, to check its value? Just switch it to the 200uf position. 
Be sure and bleed the capacitor, because when you checked for resistance you put a charge on the cap. If it were charged with power supply type voltages then bleed them down with a resistor of say 5-10 Kohms, that way it won't put a stress on the cap. Leave the resistor connected across the resistor for at least 1 minute just to err on the side of safety.

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PostSubject: Re: Zenith Model 6-S-128 repairs   Zenith Model 6-S-128 repairs I_icon_minitimeNovember 25th 2016, 5:04 pm

Also I found a mistake in what I told you about the terminal strip that was smoking.  There are two resistors connected to the back of the terminal strip not one.  Not sure if it matters.
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PostSubject: Re: Zenith Model 6-S-128 repairs   Zenith Model 6-S-128 repairs I_icon_minitimeNovember 25th 2016, 4:24 pm

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?
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