Hoffman TV - Improper Picture Height

Hi all. I recently bought a vintage Hoffman television from a house sale and it works great except for improper height of picture, which my Sylvania TV Repair Booklet says could be caused by improper voltages supplied by low power supply, resistors changing their values, or bad vert osc tube. I tried adjusting vert hold and vert linearity and the picture is maxxed at about 50-per-cent of the screen height, centered in the screen.

I am wondering if anyone else has had this problem and if it might just be a bad tube or other part?

Also, I have heard of people changing all the electrolytic condensers in old equipment as one of the first things they do. Is that really necessary or could I let it go? I really don't want to be installing new stuff in this set (yes I am aware of drilling out old condenser cases and hiding new ones inside).
Thanks in advance for any help!

[Big Dave]

Replace all electrolytic and paper caps. If your set uses black plastic caps, get rid of them too. if one of the lytics fails, it could do irreversable damage. Chances are, most of the tubes are good. Recap the set. The caps are the culprits.

Hey Dave

I'll try that over the weekend and see how that works out. Hopefuly that may clear up or at least partially fix the problem. I could see if some of the condensers have high leakage or are shorted it could be messing up the voltages. I guess I'd be looking in the wrong places if I just started popping tubes.

On a side note, I have heard of tubes lasting for decades while the condensers go bad. Yet I have a big box full of electrolytic condensers from the 50's-60's, which I have been collecting over the years. Most of the values are dead-on when I test them with my meter and have been using them in small projects with no problems. Did I just get lucky with these? Or maybe they just haven't failed yet?

[Big Dave]

They haven't failed yet. They will fail. You can hide the replacements under the chassis and leave the cans in place.

Hey again. I'm actually working on uit now. I have the chassis out and am cleaning the dust out of it and the cabinet right now. Should the ceramic condensers be replaced, too? I remember reading somewhere that although they are more reliable then electrolytic ones it's not by much.

I'm thinking I should also lift and test resistors while I'm at it. The thing is heavy enough that I may as well do as much as I can in one shot.

Oh by the way, the chassis is such that you can't see underneath it from the top, and in order to get underneath, everything - including the CRT - has to come out as one unit. o I guess I'm not so worried about authenticity anymore. (-: Of course, I'll leave the two large multi-sectional filter condensers and hide new ones directly underneath.

Here are some pics!









What a mess!!!

[peverett]

I see that this set has the dreaded "disintegrating plastic" yoke. I have 1 Hoffman and several RCAs and Zeniths with the same problem. I do not know what kind of plastic was used, but I wish that they had chosen something else.

Ah that must be why it rotates freely on the neck. No worries. On a positive note, it can actually make it easier to adjust, and when I am satisfied I'll just put a dab of hot glue, which I can always peel off if I need to readjust.

[Adam]

"Replace all electrolytic and paper caps." -- I second that, even if some of them are good now, they will just go bad later if you watch it for a while. The ceramic caps rarely ever go bad, I wouldn't worry too much about them.

What is that gear and chain on the back of the tuner? Does this set have power-tuning or a remote?

[yagosaga]

Originally Posted by Adam

"Replace all electrolytic and paper caps." -- I second that, even if some of them are good now, they will just go bad later if you watch it for a while.

This is not really a good recommendation. It s better to check the circuits and to find the failure. So you have the opportunity to learn something.
If the capacitors made of wax and paper, there is no need to replace them. You can take them out of the chassis and "cook" them in 110°-degrees-Celsius hot wax. The water will boil and evaporate. After that the old capacitors are like a new one. Perhaps they have lost 5% - 10% of capacity, but this is within tolerance.

[wa2ise]

Originally Posted by yagosaga

If the capacitors made of wax and paper, there is no need to replace them. You can take them out of the chassis and "cook" them in 110°-degrees-Celsius hot wax. The water will boil and evaporate. .

I don't know about that. The paper may have acid in it, and may have reacted with the moisture to degrade the foil and the paper. MY father had mentioned this cooking method, but that was 35 years ago. Might have worked back then, but I wouldn't trust it today.

[yagosaga]

Originally Posted by wa2ise

I don't know about that. The paper may have acid in it, and may have reacted with the moisture to degrade the foil and the paper. MY father had mentioned this cooking method, but that was 35 years ago. Might have worked back then, but I wouldn't trust it today.

Several people here have tested this method with success. Even the cooking of bad flyback transformers (those with open coils) brings new live to them. A friend of mine has done this with a flyback for the RCA ctc5. If you try it, you should test the capacitors with a high voltage capacitor tester at 500 volts.

[Adam]

Originally Posted by yagosaga

It s better to check the circuits and to find the failure. So you have the opportunity to learn something.

This is a good idea. It is not best to just go randomly replacing stuff. I do try to check the circuits to find specifically what is wrong when I'm fixing a TV, the information might be useful if something else goes bad later on.

But after I fix the problem, then I still go and change all the wax and paper caps even if they are good at the time. I'm hardly an authority on the subject, but I've just found on sets where I haven't replaced all these they tend to go bad after having watched the set for a while, and I just keep having to take it apart to change them later. I've never tried that method of restoring the originals though.

Originally Posted by yagosaga

This is not really a good recommendation. It s better to check the circuits and to find the failure. So you have the opportunity to learn something.

I agree! I want to learn as much as I can about this. I feel I'm pretty good with radios. I remember buying a big lot of transistor radios on eBay. I fixed most of them from being dead and resold them. The electrolytic condensers all had to be replaced and many transistors did, also (generic replacements often work). But ah,. I'm getting off-topic.

I did make a list of all the condensers I need. I figure I'll buy 10 of each from Mouser so that way I'll have some extras. I only need 1-3 of each.

(MFD @ WVDC)
0.001 @ 400
0.0022 @ 400
0.0047 @ 200
0.01 @ 600, 1600
0.039 @ 400
0.047 @ 200, 600
0.068 @ 400
0.1 @ 200, 400, 600
0.22 @ 200
0.47 @ 200
4 @ 600 (four 1's in parallel)
10 @ 350
40 @ 350 (four 10's in parallel)
100 @ 350

I am thinking of getting axial aluminum electrolytic for the ones to replace the large filter condensers. Are there any recommendations for what to replace the smaller ones with? I was thinking of going with polyester film because they have high voltages for relativeloy low cost (under a dollar each from Cornell Dubilier).

None of the resistors look suspect. But I was thinking of replacing the small carbon comp ones anyway. What's anyones' takes on this idea?

I'm not sure exactly what that is for. The front knob says "Fine Tuning" but they both seem to turn in unicen.

This is a VHF-only set by the way. It is interesting to note that the back panel has a layout for the UHF if it were present, and the chassis seems to suggest a possible installment of a UHF tuner on some models.