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 Post subject: Zenith 25HC45 (1970s) AC capacitor
PostPosted: Dec Mon 04, 2017 3:42 am 
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Location: San Jose, CA USA
Gentlemen,

I usually do not accept color sets, let alone 1970s solid state color sets, for repair, but in a weak moment I agreed to "take a look at it."

After some usage, the set blows its circuit breaker. Upon pulling the chassis out, I noticed this large AC capacitor looking ready to explode. I've temporarily replaced it with seven 0.5 uF 630 VDC tubular caps in parallel, which works temporarily, but are likely not rated to handle the current in the long run.

I don't have the schematic for this set (I could download the Sams, but might not need to). Wondering what the function of this cap is? Is it a cap for a ferroresonant power transformer (like early DuMonts)?

I know several of you were active servicemen in the 1970s and likely recognize this right away. Do any of you have one in your stash of stuff?

Attachment:
Zenith TV capacitor.jpg
Zenith TV capacitor.jpg [ 69.8 KiB | Viewed 334 times ]

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 Post subject: Re: Zenith 25HC45 (1970s) AC capacitor
PostPosted: Dec Mon 04, 2017 4:04 pm 
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Location: Pewaukee, WI
It is for the ferroresonant power transformer. I've got a spare chassis I could borrow one off of if you can't find another. I'd prefer to keep mine though.

If you need a copy of the schematic I can probably help.


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 Post subject: Re: Zenith 25HC45 (1970s) AC capacitor
PostPosted: Dec Mon 04, 2017 4:27 pm 
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Location: Detroit, MI USA
If there's a color dot on it, you need to replace with another having the identical marking as they were hand selected to match each specific transformer. If not, you have to use the same part number and exact value.

Shouldn't be too difficult to locate one, that was a fairly common failure. I think I used the last spare one I had several years ago.

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 Post subject: Re: Zenith 25HC45 (1970s) AC capacitor
PostPosted: Dec Mon 04, 2017 5:15 pm 
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Thanks for the clarification of the function of this capacitor. This one doesn't have a color dot on it.

After some searching, I found this capacitor, which appears to have basically identical specs: https://www.amazon.com/piece-Ceiling-Me ... B00DHOYFYS . Since it's a motor run capcitor, it should be able to handle continuous AC current. Any concern with trying this type of cap?

Somewhat to my surprise, my cluster of seven film caps ran for over an hour last night with no obvious problem. I won't leave that in there, however, since it is quite far from being the correct part.

If the capacitor is not sufficiently well matched to the transformer, would that be evident as an error in some voltage that I could measure?

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 Post subject: Re: Zenith 25HC45 (1970s) AC capacitor
PostPosted: Dec Mon 04, 2017 6:09 pm 
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Joined: Apr Sat 06, 2013 2:18 pm
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Location: New Hampshire
Any cap from E line to K line upright chassii should be the same. They built
millions of them. Just match the 22-####-## part number.
Try VK also there are a lot of solid state collectors over there & everyone
loves these......

73
Zeno 8)
LFOD !


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 Post subject: Re: Zenith 25HC45 (1970s) AC capacitor
PostPosted: Dec Mon 04, 2017 6:13 pm 
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Location: San Jose, CA USA
I placed an ad at Videokarma and got this suggestion:

https://www.tedss.com/2020038517

This company is quite expensive to deal with, with a $25 min order, but I went ahead and found some "filler" items and bought it. Nice that it appears to be the exact replacement.

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 Post subject: Re: Zenith 25HC45 (1970s) AC capacitor
PostPosted: Dec Tue 05, 2017 8:01 am 
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Location: Powell River BC Canada
If the capacitor overheated, the transformer might be partially shorted.

If you get it going with another capacitor, it might finish off transformer.

Ferro resonant ballast replacements always came as a pair.

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 Post subject: Re: Zenith 25HC45 (1970s) AC capacitor
PostPosted: Dec Tue 05, 2017 9:00 am 
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Thanks for the tip. I'll check the temperature of the transformer over an extended time to see if there is evidence of overheating.

The old cap shows a measureable leakage resistance even at ohmmeter applied voltage, so I think the cap is leaky. Hopefully that's the only problem.

As I mentioned above, simply using seven 0.5 uF caps in parallel as a substitute, the set ran fine for about an hour. I checked the temperature of the caps (they were just slightly warm), but I didn't think to check the temperature of the transformer, although if it was roasting hot I would have noticed it just by proximity to where the caps were positioned.

I did check the current draw of the whole set. With the bad cap, it would draw 4 amps before blowing its breaker. With my sub cap, it drew only 1 amp steady state, so that is another good sign.

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 Post subject: Re: Zenith 25HC45 (1970s) AC capacitor
PostPosted: Dec Tue 05, 2017 7:08 pm 
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Transformer seems to be running cool, so no problem there.

Another question: This set has seen a lot of transport in recent years, and would benefit from a adjustment of purity and convergence. Since I generally don't work on these newer color sets, I'm not familiar with the purity and convergence adjustments. Looking at the picture below, I think I know what some of the things are, but less certain about others:

Attachment:
Zenith CRT rear.jpg
Zenith CRT rear.jpg [ 54.81 KiB | Viewed 188 times ]


1. Static convergence: Small rotatable magnets at ends of dynamic convergence coils?
2. Purity: Pretty sure the two rings at the rear are for purity adjustments. Any particular procedure works best?
3. There is a white gear mechanism just ahead of the purity rings ("behind" the purity rings in this picture from the rear). What is this for?

I could find out what these are by trial and error, but I might as well save some frustration by learning what you gentlemen already know.

Degausser seems to be working fine at set turn-on.

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 Post subject: Re: Zenith 25HC45 (1970s) AC capacitor
PostPosted: Dec Tue 05, 2017 7:36 pm 
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Tom Albrecht wrote:
Transformer seems to be running cool, so no problem there.

Another question: This set has seen a lot of transport in recent years, and would benefit from a adjustment of purity and convergence. Since I generally don't work on these newer color sets, I'm not familiar with the purity and convergence adjustments. Looking at the picture below, I think I know what some of the things are, but less certain about others:

Attachment:
Zenith CRT rear.jpg


1. Static convergence: Small rotatable magnets at ends of dynamic convergence coils?
2. Purity: Pretty sure the two rings at the rear are for purity adjustments. Any particular procedure works best?
3. There is a white gear mechanism just ahead of the purity rings ("behind" the purity rings in this picture from the rear). What is this for?

I could find out what these are by trial and error, but I might as well save some frustration by learning what you gentlemen already know.

Degausser seems to be working fine at set turn-on.


Responding to your picture:
Yes.
Blue Lateral static conv.
Yes.

Purity adjustment: with a red field test pattern (or blue and green G2s turned down), loosten deflection yoke and slide back towards purity yoke till red spot appears, center red spot with purity rings, slide deflection yoke forward till raster is pure red, secure yoke, done.


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 Post subject: Re: Zenith 25HC45 (1970s) AC capacitor
PostPosted: Dec Tue 05, 2017 8:03 pm 
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Location: San Jose, CA USA
Ah yes -- I forgot that we need both a lateral and vertical adjustment for blue static convergence. Now it's clear what the extra mechanism is for.

Nice concise explanation of purity adjustment. That will definitely save me a ton of time.

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 Post subject: Re: Zenith 25HC45 (1970s) AC capacitor
PostPosted: Dec Tue 05, 2017 11:17 pm 
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When you slide the yoke forward, it the screen does not want to become pure red over the whole screen you may have to fudge the position of the purity rings a little.

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 Post subject: Re: Zenith 25HC45 (1970s) AC capacitor
PostPosted: Dec Sun 10, 2017 3:34 am 
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Got the replacement cap in from TEDSS.com, and it works fine (as did my little bunch of yellow caps). Purity adjustment went fine, and I also spent some time dialing in the static and dynamic convergence. Convergence is now gorgeous. Purity problem wasn't really a purity problem. There is something in the video that is changing the lumninance a little from left to right, causing the color intensity to appear diminished on the right. Couldn't manage to track that down, and it's pretty subtle, so I'll leave it, unless one of you immediately recognizes that symptom and has an idea about it.

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 Post subject: Re: Zenith 25HC45 (1970s) AC capacitor
PostPosted: Dec Sun 10, 2017 4:03 am 
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Location: pensacola fl
hi. Most likely that shading of intensity is an electrolytic that filters the B+ to the video output transistors collectors. I do not remember the value but it was around 10UF at 350V. If you do not have the cartoon on the set follow the three video output collectors through their resistor and the little peaking coil they will common to the capacitor that I referenced. You can replace it with a larger value if that is what you have. The voltage comes from a diode off the flyback. That diode is a high speed type but probably not bad.
Hope I have led you in the right direction.


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 Post subject: Re: Zenith 25HC45 (1970s) AC capacitor
PostPosted: Dec Sun 10, 2017 5:02 am 
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Audioman,

I am impressed! You nailed it. A 10 uF 300 V section of the main FP filter cap was connected to the video output B+. My ESR tester showed it was open circuit, and replacing that fixed the problem.

Also found that one of two main 300 uF 150 V filter caps was dead. Surprising how little problem that caused. Just a hint of 60 Hz waviness in the picture (but probably less than an old tube black and white set with all good filters).

Since two of four sections in the main FP were out, I replaced the whole unit, so it should be solid going forward.

Prior to posting above, I was on the right track looking for problems with electrolytics on the video and chroma cards. Didn't find any bad ones with my ESR tester, but replaced them all anyway (only 10 caps or so), with no improvement. For whatever reason, I hadn't thought the main FP cap would be the problem (since it didn't look like a classic power supply problem), so I hadn't checked that one yet.

Anyway, now the set works like brand new, and I'm reminded that a well-tuned-up 1970s color set can really look quite good!

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 Post subject: Re: Zenith 25HC45 (1970s) AC capacitor
PostPosted: Dec Sun 10, 2017 2:37 pm 
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Joined: Apr Sat 06, 2013 2:18 pm
Posts: 308
Location: New Hampshire
That cap causing the video problem is a common fail across all brands. One
of the most common things but I never changed on these sets & I fixed a
LOT of them !
Last thing to watch is the white resistor packs on the 9-88 & 9-89 modules.
They almost always have cold joints.

73
Zeno 8)
LFOD !


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