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 Post subject: Zenith Z940 repair
PostPosted: Apr Wed 21, 2021 12:32 am 
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Location: Davenport, Iowa
I'm already moving onto another one. I got a Zenith Z940 made in 1965. I saw there were already a handful of threads on this model (or very similar). Looks like I have my work curt out for me. There was one that said to be very mindful of ripple current on these large power caps, which is good to know.

The stereo does turn on and it does put out sound, but it sounds awful. It needs a lot of work. The turntable does spin but makes an awful grinding noise because the idler is so hardened. I've already sent that in to VM to get it reworked. The changer mechanism is stuck in a loop, too, but that's a familiar problem with lubrication. The knobs and switches all need Deoxit.

This is the first one I've worked on from the pre-circuit board era. It's wild how they just soldered like 5-6 components or wires to one post or something. This isn't very workable. Suggestions for cleaning this up a bit and making it easier to work on? Some kind of small bread board or something? I really don't care about the internal aesthetics of this thing. I favor serviceability over appearance. Any pointers on how best to approach this mess would be appreciated.


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 Post subject: Re: Zenith Z940 repair
PostPosted: Apr Wed 21, 2021 1:11 am 
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Joined: Jan Mon 16, 2012 4:15 pm
Posts: 8332
Location: Near Brandon, Iowa
"Point to point" wiring, which is what Zenith stuck with as an assembly process long after the rest of the industry had migrated to PC boards, is perfectly serviceable, just not as obviously orderly as a PCB layout. I think that you will be metaphorically shooting yourself in the foot with a shotgun if you try to "upgrade" the assembly method. Not to mention having subsequent owners (if any) make and burn wax dolls of you.

There should be a Sams Photofact folder for this unit. It will have photographs of the original chassis wiring, showing the location of every discrete component.

I'm guessing that this unit is solid state. If it is, it may have two problems: (1) aged-out and bad electrolytic coupling caps (maybe a dozen of them between both channels); and (2) leaky germanium transistors. I'd concentrate on addressing Potential Problem 1 first; if the sound is still distorted or "whiny" after the coupling and filter electrolytics have all been replaced, the Leaky Transistor scenario can next be tackled.


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 Post subject: Re: Zenith Z940 repair
PostPosted: Apr Wed 21, 2021 1:27 am 
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Location: Mountains of Mourne. Ireland.
Covered in Sams Photofact Set-1012 Folder-8

Image
Large image: https://i.ebayimg.com/thumbs/images/g/W ... -l1600.jpg
$ https://www.ebay.com/itm/401250380784

The Photofact cover image is a representative image.

ref: https://www.samswebsite.com/en/photofac ... sid/1012_8

------------

There's a guy selling pdf's for ten bucks.
I wouldn't purchase from him.

Greg.


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 Post subject: Re: Zenith Z940 repair
PostPosted: Apr Wed 21, 2021 3:33 am 
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Joined: Dec Mon 28, 2020 11:52 pm
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Location: Davenport, Iowa
lorenz200w wrote:
"Point to point" wiring, which is what Zenith stuck with as an assembly process long after the rest of the industry had migrated to PC boards, is perfectly serviceable, just not as obviously orderly as a PCB layout. I think that you will be metaphorically shooting yourself in the foot with a shotgun if you try to "upgrade" the assembly method. Not to mention having subsequent owners (if any) make and burn wax dolls of you.

There should be a Sams Photofact folder for this unit. It will have photographs of the original chassis wiring, showing the location of every discrete component.

I'm guessing that this unit is solid state. If it is, it may have two problems: (1) aged-out and bad electrolytic coupling caps (maybe a dozen of them between both channels); and (2) leaky germanium transistors. I'd concentrate on addressing Potential Problem 1 first; if the sound is still distorted or "whiny" after the coupling and filter electrolytics have all been replaced, the Leaky Transistor scenario can next be tackled.


Yes, it is solid state.

The main thing I was referring to are the two big "aluminum can" style electrolytic capacitors that almost certainly need to be replaced. Those are going to be the toughest. Those cans each have eight (8!) posts underneath them, and a majority of those 16 posts have multiple things soldered to them. I highly, highly doubt I will find a direct replacement for these two big aluminum caps with 8 posts each. Therefore, the electronics are going to end up looking quite a bit different for the next owner no matter what.

I already got the Sams photofact as a PDF for this one. The two caps I'm referring to are C1 and C2 on the schematic...but C2 is actually 3 caps in one (there is C2A, C2B and C2C on the schematic). So 3 caps will very likely be replacing that one. As I said, it's going to end up looking a lot different. There is one other cap (C8) that is a "2 in 1" according to the schematic.

The other 12 electrolytic caps on this one are singles. Those might not be too bad to replace. I'm just not used to working with a glob of solder that is connecting 3 or 4 wires together, trying to pull 1 wire out of that, and put 1 new wire into the same glob. Is there a better way to approach those kind of joints?
(that's sort of what I was trying to ask in the original post, but didn't word it very well)


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 Post subject: Re: Zenith Z940 repair
PostPosted: Apr Wed 21, 2021 3:57 pm 
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Location: Near Brandon, Iowa
You're correct that you are not likely to find drop-in replacement multicaps from one of the "normal" suppliers. You could commission the build of a custom multicap from Hayseed Hamfest but that will be somewhat more expensive than using discrete caps.

One alternative is to use a Zona razor saw or vet obstetric wire to slice off the old cap can just above the base flange. Pitch the old cap body. Then clean off the debris from the base and ensure that none of the positive contact posts are touching anything on the topside. You can then epoxy a plastic cover plate over the base to prevent shock hazard. Or just fill with epoxy and let harden.

What remains on the wired side of the chassis are the cap terminal posts, but they are now no longer electrically connected to the old cap. You can use these posts are tie points for the new axial-leaded caps that you will install. No need to move existing wiring; just add the new cap lead to the connection point.

Dealing with coupling electrolytics can be done the "morally upright way" (unsoldering the old component and installing the new one exactly in the same manner as the original)... this is labor-intensive at best and requires a fair amount of skill. An alternative (quick and dirty) approach is to cut off the old cap's leads close to the body of the part, leaving a length of lead still soldered to the terminals. Form the end of this "floating lead" into a J shape and do the same with one lead of the new cap. Hook the two J leads together, crimp tightly, and solder. Repeat for the other lead. Ensure that the lead lengths aren't excessive. Bam! You're done!


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 Post subject: Re: Zenith Z940 repair
PostPosted: Apr Wed 21, 2021 4:17 pm 
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Location: Mountains of Mourne. Ireland.
Removing and restuffing/rebuilding capacitor cans.
viewtopic.php?p=3368747#p3368747


Making butt connections is a crowded chassis...
Dennis Wessviewtopic.php?f=10&t=79721

Another quig tool...
viewtopic.php?f=8&t=303863

Greg.


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 Post subject: Re: Zenith Z940 repair
PostPosted: Apr Wed 21, 2021 5:35 pm 
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Joined: Aug Wed 24, 2011 4:35 am
Posts: 5821
Location: Sunnyvale CA
pioneer07 wrote:
This is the first one I've worked on from the pre-circuit board era. It's wild how they just soldered like 5-6 components or wires to one post or something. This isn't very workable. Suggestions for cleaning this up a bit and making it easier to work on? Some kind of small bread board or something? I really don't care about the internal aesthetics of this thing. I favor serviceability over appearance. Any pointers on how best to approach this mess would be appreciated.


I have occasionally built subassemblies on perf board or similar to clean up some particularly egregious point-to-point wiring sins. Usually that is not necessary, you might want to add terminal strips to clean it up, but a wholesale reengineering to PWB or something like that, I wouldn't recommend. And unless you design/fabricate your own PWB, a perf board version is not likely to be a lot neater or easier to service, everything has to be wired together underneath, and many times that is harder to deal with than the original.

It's a skill to learn to work on these types of assemblies, but it is a viable way to do it. It just takes some patience.

Brett


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 Post subject: Re: Zenith Z940 repair
PostPosted: Apr Wed 21, 2021 11:56 pm 
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Joined: Dec Mon 28, 2020 11:52 pm
Posts: 419
Location: Davenport, Iowa
Thanks for all the info. One thing I'm struggling with a bit is identifying all the capacitors even with the Sams manual. Here is a screenshot from the Sams and then an actual photo that I took. The one I am struggling to identify is C7. I think it's this gray thing, but it has no markings on it and it's a weird shape for a capacitor, and has weird ends. There's nothing else in that area though that could really be C7. Is that really a capacitor?

I am confident that small brown disk one is C93 because it says "330" on it, and that is what it's supposed to be (330uF).

Attachment:
Capture.JPG
Capture.JPG [ 284.59 KiB | Viewed 2587 times ]


Attachment:
Capture2.JPG
Capture2.JPG [ 223.49 KiB | Viewed 2587 times ]




Also, I was looking at capacitors on Mouser today and I'm not sure what I need for ripple current. Some caps have it listed and some do not. Any suggestions there, especially for those C1 and C2 replacements?


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 Post subject: Re: Zenith Z940 repair
PostPosted: Apr Thu 22, 2021 12:15 am 
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Location: Mountains of Mourne. Ireland.
Components can be confirmed by what they are connected to on the schematic.
I'm not sure why you are trying to identify all those capacitors.

You only need to replace the Electrolytics.
Those Film and Ceramic capacitors will be good for another 100 years.

C1 and C2?
I don't have the schematic.

If you want a shopping list, post a scan of the Electrolytics from the parts list.
Like this→ viewtopic.php?p=3256775#p3256775


Greg.


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 Post subject: Re: Zenith Z940 repair
PostPosted: Apr Thu 22, 2021 9:05 pm 
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Joined: Dec Mon 28, 2020 11:52 pm
Posts: 419
Location: Davenport, Iowa
egg wrote:
Components can be confirmed by what they are connected to on the schematic.
I'm not sure why you are trying to identify all those capacitors.

You only need to replace the Electrolytics.
Those Film and Ceramic capacitors will be good for another 100 years.

C1 and C2?
I don't have the schematic.

If you want a shopping list, post a scan of the Electrolytics from the parts list.
Like this→ viewtopic.php?p=3256775#p3256775


Greg.


Yeah I am only really trying to identify the Electrolytics. The only reason I identified a couple non-electrolytics is because it's unclear to me in that one area which one is which. The image in the Sams is really bad in that area. I only create these images for my own reference. It makes it easier to keep things straight when I go to do the work.

I already have the list of Electrolytics that I need, see below. So can someone recommend an actual part or two to replace C1 and some of the ones in C2? I am not finding much information on ripple current ratings on all of the ones on Mouser for example, and evidently (according to a previous thread on a model like this) that is a big deal on these. C1 and C2 need to be rated to like 2A ripple current according to this thread. I'm not finding anything that big, but I'm also not finding much info at all on ripple current ratings in general.

I'm also curious what the square symbol, half-circle symbol and triangle symbol mean next to some of these numbers. The Sams doesn't have a decoder for that.

Attachment:
Electrolytics.JPG
Electrolytics.JPG [ 355.39 KiB | Viewed 2562 times ]


As for C7....here is where it is on the schematic. It must be that gray thing that I identified but it just looks very odd. One side goes to the bass knob, and there is a wire from that solder joint to the bass knob. The other side goes to a transistor + two resistors. So that must be it. Again, it just looks very strange for a capacitor.


Attachments:
C7.JPG
C7.JPG [ 168.92 KiB | Viewed 2562 times ]
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 Post subject: Re: Zenith Z940 repair
PostPosted: Apr Thu 22, 2021 9:09 pm 
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Joined: Dec Mon 28, 2020 11:52 pm
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Location: Davenport, Iowa
Oh yeah, and even though the Sams says several of these are 5uF 12V caps, the markings on them say they are 5uF 15V, not that it probably matters much.

But at Mouser at least, the closest I can find is 5uF and 25V.


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 Post subject: Re: Zenith Z940 repair
PostPosted: Apr Thu 22, 2021 10:17 pm 
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Joined: Jan Mon 16, 2012 4:15 pm
Posts: 8332
Location: Near Brandon, Iowa
pioneer07 wrote:


I'm also curious what the square symbol, half-circle symbol and triangle symbol mean next to some of these numbers. The Sams doesn't have a decoder for that.


These symbols are cut into the phenolic bases of a multicap. They appear right next to the positive terminals on the cap base and identify which terminal goes with which section. For example, C2 is a 3-section multicap. The three sections are identified by the symbols. The symbol key is also printed or stamped into the body of the cap housing.

C7 is a discrete coupling electrolytic cap which is a prime candidate for replacement.


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 Post subject: Re: Zenith Z940 repair
PostPosted: Apr Thu 22, 2021 11:04 pm 
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Some of the old μF (microfarad) capacitance values are no longer commonly found, so choose the next nearest value, up or down.

Examples:
47µF (microfarad) is a common replacement value today for the old 50µF
4.7µF is a common replacement value today for the old 5µF

10µF is a common found value.

The WVDC (working voltage) of a capacitor is how much "juice" that it can handle
this can be increased.

For all those small Voltage ratings use 25V or 35V or 50V rated replacements.


DigiKey part numbers for the 1000µF and 500µF
can be seen here→ viewtopic.php?p=3321329#p3321329

Use 330µF for the old 300µF

------------

C10 and C13 are 3µF NP (non polar) aka bi-polar capacitors. As the name implies, they can be connected any way round.


:) Greg.
Modern day electrolytic capacitors have become so much smaller since the 1960s - 1970s
viewtopic.php?p=3353646#p3353646

Ask questions or post your BOM (bill of materials) if you are unsure about anything.


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 Post subject: Re: Zenith Z940 repair
PostPosted: Apr Thu 22, 2021 11:14 pm 
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You can "make" an axial capacitor from a radial.
just be sure to use some spaghetti or heat-shrink tubing on the bent over leg.
Image

It's best to use Electrolytics with a temperature rating of 105˚
85˚ caps are good enough for transistor radios, and casino monitors. lol

------------
For my money, Panasonic... 105°C
FC series
FM series
FR series are all good capacitors.
------------

Before and after soldering...
viewtopic.php?p=3097733#p3097733


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 Post subject: Re: Zenith Z940 repair
PostPosted: Apr Fri 23, 2021 7:45 pm 
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Joined: Dec Mon 28, 2020 11:52 pm
Posts: 419
Location: Davenport, Iowa
lorenz200w wrote:
pioneer07 wrote:


I'm also curious what the square symbol, half-circle symbol and triangle symbol mean next to some of these numbers. The Sams doesn't have a decoder for that.


These symbols are cut into the phenolic bases of a multicap. They appear right next to the positive terminals on the cap base and identify which terminal goes with which section. For example, C2 is a 3-section multicap. The three sections are identified by the symbols. The symbol key is also printed or stamped into the body of the cap housing.

C7 is a discrete coupling electrolytic cap which is a prime candidate for replacement.


Thank you!


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 Post subject: Re: Zenith Z940 repair
PostPosted: Apr Fri 23, 2021 7:47 pm 
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Joined: Dec Mon 28, 2020 11:52 pm
Posts: 419
Location: Davenport, Iowa
egg wrote:
You can "make" an axial capacitor from a radial.
just be sure to use some spaghetti or heat-shrink tubing on the bent over leg.
Image

It's best to use Electrolytics with a temperature rating of 105˚
85˚ caps are good enough for transistor radios, and casino monitors. lol

------------
For my money, Panasonic... 105°C
FC series
FM series
FR series are all good capacitors.
------------

Before and after soldering...
viewtopic.php?p=3097733#p3097733



Thanks! Yes I had already picked up that I should use the higher temp rating from one of those old threads on the similar Zenith model

But all this is really useful info thanks


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 Post subject: Re: Zenith Z940 repair
PostPosted: Apr Fri 23, 2021 8:13 pm 
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To keep it simple... Yes, run with 105° rated capacitors.
Theoretically they will have a longer life in normal use.

The fundamental enemy to electronics is heat!
Greg.


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 Post subject: Re: Zenith Z940 repair
PostPosted: Apr Thu 29, 2021 10:17 pm 
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Location: Davenport, Iowa
So I sawed off those two big electrolytic caps C1 and C2 (C2 is the 3 in 1). This is what they looked like right after I sawed them off and pulled out the dielectric material:
Attachment:
C1.JPG
C1.JPG [ 116.57 KiB | Viewed 2418 times ]

Attachment:
C2.JPG
C2.JPG [ 106.62 KiB | Viewed 2418 times ]


Those metal posts align with the solder joints underneath, so I made the assumption that I could connect my new capacitors to these metal posts. I aligned the positive side with the symbol underneath (triangle, square or semicircle) and the negative side to the post with no symbol next to it. Was that an incorrect assumption? Because when I fired it up, I blew the fuse. Good news is I did not blow a capacitor. Do I need to dig deeper into this capacitor shell than this to make a good connection? Not sure why this didn't work.

Here are the caps original values and what I used to replace them:
C1 Original = 1000uF 50V replaced with 1000uF 100V
C2A original = 500uF 50V replaced with 470uF 100V
C2B original = 500uF 50V replaced with 470uF 100V
C2C original = 300uF 50V replaced with 330uF 50V

Related: anyone know the best place to get an N-type 2A 125V fuse? The Sams says it is Littlefuse part number 333002. It's one of those that you push down, then twist to pull it out. It's a weird size. It's 6.4mm by 36.5mm long.


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 Post subject: Re: Zenith Z940 repair
PostPosted: Apr Thu 29, 2021 10:30 pm 
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Joined: Dec Mon 28, 2020 11:52 pm
Posts: 419
Location: Davenport, Iowa
I found 2 sites that reference Littlefuse part number 333002 for that fuse, but they both say it's a 2A 250V fuse instead of a 2A 125V fuse. So I'm reluctant to order. This is one of the sites that has that part number (it's 10th from the bottom):

https://www.elliottelectronicsupply.com ... fuses.html


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 Post subject: Re: Zenith Z940 repair
PostPosted: Apr Thu 29, 2021 11:30 pm 
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Joined: Jan Mon 16, 2012 4:15 pm
Posts: 8332
Location: Near Brandon, Iowa
pioneer07 wrote:


...I made the assumption that I could connect my new capacitors to these metal posts. I aligned the positive side with the symbol underneath (triangle, square or semicircle) and the negative side to the post with no symbol next to it. Was that an incorrect assumption?


Yes, bad assumption. The negative common connection is not brought out to any of the metal posts- it is connected to the aluminum can. It is impractical to solder to aluminum. That's why I suggested installing the new caps under the chassis using the old cap's terminals as tie points. You will have access to the old cap's negative terminal solder connection that way.

Lack of a symbol is a symbol itself. But it is just the positive terminal of another section of the multicap.

IF somehow you managed to connect one of the caps such that the negative lead was at a higher DC potential than the positive lead, that cap is very likely ruined. Don't try to re-use it by correctly connecting it.


Last edited by lorenz200w on Apr Fri 30, 2021 1:45 am, edited 1 time in total.

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