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 Zenith Model 6-S-128 repairs

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kps



Join date : 2016-11-10

PostSubject: Zenith Model 6-S-128 repairs   2016-11-11, 4:16 pm

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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Cliff Jones
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PostSubject: Re: Zenith Model 6-S-128 repairs   2016-11-12, 12:48 pm

The hanging metal clip (grounding collar) goes to the connection for the negative twisted wires that have no connection.

C21 (+ ) wire goes between R9 and field coil
And C25 (+)goes between R 9 and R 4.

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Last edited by Cliff Jones on 2016-11-12, 6:06 pm; edited 3 times in total
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kps



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PostSubject: Re: Zenith Model 6-S-128 repairs   2016-11-12, 3:31 pm

Thank you Cliff!!!
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Cliff Jones
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PostSubject: Re: Zenith Model 6-S-128 repairs   2016-11-12, 6:07 pm

Check polarities, and use a dim bulb tester to be safe.

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kps



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PostSubject: Re: Zenith Model 6-S-128 repairs   2016-11-14, 8:03 am

Thanks again Cliff!!  So these two pictures show my next step. I've removed both Ecaps and labeled the wiring correctly based on my understanding of what you've told me.  I haven't soldered anything yet.   Does this look correct to you?
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Cliff Jones
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PostSubject: Re: Zenith Model 6-S-128 repairs   2016-11-14, 2:00 pm

study (((Rather than call the candohm position 1 or position 2, redefine as tabs, as there are 6 tabs and 5 resistors in the candohm. )))) 
Makes it easier if connections are designated by numbers.

------------------------------------------------------
The tie point for both  C-25, 10 mfd. (?). By the way, in my riders, one value of C25 is (2) two mfd. that should have its positive lead connected between R4 & R9. Usually the first cap is smaller in value.


That Cap(s) (negative ) leads should connect at the middle lug as your top picture shows -where the twisted bare wires on that one cap were. Just to make it easier!






Along with the negative lead from the candohm. Which should be the lug on the candohm 250 ohm and 56-ohm common lead. Tab 2



The other C-25 value is 8 mfd. ( the 10 mfd is ok for only this side being Up to +/- 20 to 50% tolerance )

The 16 mfd. cap + connection goes between R9 and speaker field coil. The negative lead goes to the rectifier hi voltage transformer center tap wire. 
............

So ( Tab 1 ) is what the HV rectifier center-tap and C 21 ( - ) negative lead should be wired to.
The pictures help but I cannot see each surrounding wire in focus and its exact connection.  Therefore these more detailed instructions should help. I had to refer to my manuals to see the clearer schematic. 
So first tab then 250 ohm then 2nd tab then 56 ohm, then 3rd tab, etc.
Hope this helps you.

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kps



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PostSubject: Re: Zenith Model 6-S-128 repairs   2016-11-15, 8:38 am

Thanks so much again Cliff.  Ok I think I have it as you've described. How does it look?
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Cliff Jones
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PostSubject: Re: Zenith Model 6-S-128 repairs   2016-11-15, 11:51 am

Where's your dual caps? C25? 2mfd and 8 mfd?

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kps



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PostSubject: Re: Zenith Model 6-S-128 repairs   2016-11-15, 3:45 pm

OK the 8-2 dual cap is going to be replaced by an 8 and a 2 tied together.  There was only two wires in the old cap.The dark one (black) comes from Tab 2 Candohm and the light one comes from the Power tube which I've labeled. Is this correct?
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Cliff Jones
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PostSubject: Re: Zenith Model 6-S-128 repairs   2016-11-15, 8:26 pm

The old cap with two wires doesn't make sense. I could see 3 wires (2 being positive and one wire negative, being the negatives being tied togather internally then tied to the external negative wire on the cap, because it would go to the center lug which is the negative)

The caps you are going to use must have smaller cap ( 2 mfd ) wired to the (+) lead that connects the lead between R-4 and R-9. And it's negative lead to the center lug of that lug strip where you put the negative wire from tab 2.

The larger cap ( 8 mfd ) also has its negative lead to the same center lug as the smaller cap. However it's ( + ) lead goes to tab ( 6 ). Directly or through another wire.   ( there are so many wires that it would be hard to detail all the tie points, R-7, speaker field on the left side of schematic, the primary of the audio output transformer and more.) 
But that should set things straight as far as the cap wiring.

You should check all resistors, and replace all paper caps, the mica caps should stay normally.

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kps



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PostSubject: Re: Zenith Model 6-S-128 repairs   2016-11-15, 9:21 pm

Yes  The connections that you described all exist. Positive to R4-R9 and Negative to Tab 2. Hopefully this makes more sense.
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Cliff Jones
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PostSubject: Re: Zenith Model 6-S-128 repairs   2016-11-15, 10:49 pm

cheers

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kps



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PostSubject: Re: Zenith Model 6-S-128 repairs   2016-11-16, 7:08 am

Thanks again Cliff for all of your help!!!  Any feedback on how to tie the two different Ecaps together would be greatly appreciated.  I have a 2 uF axial and an 8 uF axial and just the two lead wires to work with. Should they be side by side so that the negative and positive leads of each cap tie together?
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Cliff Jones
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PostSubject: Re: Zenith Model 6-S-128 repairs   2016-11-16, 9:07 am

Yes yup, take the two negatives and twist togather and use a sleeve to cover the bare wire than solder to the center lug. Then use sleeves over the (+) leads and solder to connections as per schematic.

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kps



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PostSubject: Re: Zenith Model 6-S-128 repairs   2016-11-17, 6:43 am

OK got it!!! I have two other questions for you Cliff.  1) Should I install X2 and Y2 safety capacitors?
2) Should I place a fuse on one of the hot leads heading into the transformer?
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Cliff Jones
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PostSubject: Re: Zenith Model 6-S-128 repairs   2016-11-17, 11:01 am

1. It's a question of priorities, if you have a lot of spikes or power outages, or fluctuating voltage, then yes.

2. I would definitely insert a fuse and holder for any transformer powered set. Better than looking
For a replacement transformer.

Radios need protection in both cases, and if you put being safe, that's  good insurance in guareenting long life for you collection, and if you sell that's a plus to the buyers.

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kps



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PostSubject: Re: Zenith Model 6-S-128 repairs   2016-11-17, 3:26 pm

Fantastic Cliff.  I think I'll do the fuse and wait on the X2 and Y2.  How many amps do you think Cliff? Current consumption is 75 watts.  
75 watts/112 line volts =.6 amps
.6 amps X 135% overload = .9 amps.  So would a 1 amp fuse work?
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Cliff Jones
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PostSubject: Re: Zenith Model 6-S-128 repairs   2016-11-17, 4:27 pm

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

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kps



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PostSubject: Re: Zenith Model 6-S-128 repairs   2016-11-17, 11:33 pm

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.
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Cliff Jones
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PostSubject: Re: Zenith Model 6-S-128 repairs   2016-11-17, 11:51 pm

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.

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kps



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PostSubject: Re: Zenith Model 6-S-128 repairs   2016-11-18, 7:02 am

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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Cliff Jones
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PostSubject: Re: Zenith Model 6-S-128 repairs   2016-11-18, 10:44 am

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.

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kps



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PostSubject: Re: Zenith Model 6-S-128 repairs   2016-11-18, 12:29 pm

Ok great.  Would something like this 250 V inline fuse holder work?
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Cliff Jones
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PostSubject: Re: Zenith Model 6-S-128 repairs   2016-11-18, 3:03 pm

Yes!

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kps



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PostSubject: Re: Zenith Model 6-S-128 repairs   2016-11-19, 1:30 pm

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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kps



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PostSubject: Re: Zenith Model 6-S-128 repairs   2016-11-20, 9:56 am

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.
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kps



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PostSubject: Re: Zenith Model 6-S-128 repairs   2016-11-20, 12:17 pm

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.]
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PostSubject: Re: Zenith Model 6-S-128 repairs   2016-11-20, 12:40 pm

Look for smoke damage in that specific, to pinpoint what is causing smoke, use a bright flashlight and magnifier. You need to remove the rectifier tube, then power up. If you still get smoke then you have a shorted, or possibly reversed polarity cap. Look carefully at every component for overheating. You are going to have to do a wiring check for every wire and component, it could be the transformer is shorted. One indication is maybe why the caps were being replace and the former owner gave up.

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.

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PostSubject: Re: Zenith Model 6-S-128 repairs   2016-11-20, 12:42 pm

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

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kps



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PostSubject: Re: Zenith Model 6-S-128 repairs   2016-11-20, 1:00 pm

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?
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