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!

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?

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!

http://www.erniewerbel.com/temp/000_1181.JPG

http://www.erniewerbel.com/temp/000_1182.JPG

http://www.erniewerbel.com/temp/000_1183.JPG

http://www.erniewerbel.com/temp/000_1184.JPG

What a mess!!!

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.

"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?

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.

No remote. Just a dial.

Oh I just remembered. There is a position for UHF on the dial. It is just a dot, no number. I wonder if I could hook up an external UHF tuner to this somehow?

Ah well. Let's fix the display problem first. (-:

Yes, and aftermarket "set-top" UHF tuner is easy to add and they are plentiful. Or you could just feed it from an old vcr.

Vertical problems like this are extremely common-one of the most common problems in my experience. Usually its due to bad paper caps. If that doesn't cure it look for a bad resistor somewhere in the circuit.

This looks like a reasonably well built tv. It should be a good performer when it's all wrapped up. Glad you joined us-keep us posted!

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.

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.

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.

There is a terminal lug strip labeled for testing voltages "A", "B", and "C". Are there any standard voltages these shouldbe or does it depend on the manufacturer?

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?

What I usually do with the resistors, is not to replace them all, (while they do burn out from time to time, not nearly as much as the capacitors) but if you get the schematic it will tell you what the resistance from all the pins on the tubes should be usually to ground. While, this won't always allow you to check specific resistors, it will tell you if there might be a bad one in any particular area. I usually don't go pulling any resistors in any particular area unless this is off my more than 10% or so, and I am already sure it isn't off because of a shorted capacitor or something.

But then I don't always know what I'm doing, I'm not really a TV repair man, I just like to fix TVs in my spare time. I usually try to avoid giving technical advice unless I'm absolutely sure what I'm saying, but it has been really slow here this week and I want to write something just to give people something new to look at. :banana:

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.

I never bother with resistors unless there is a problem. The most suspect would be those with higher resistance ratings, especially stuff like 1 meg or more. Even if a resistor has drifted a little in value, it won't drift so far out like a capacitor would.

I still say condenser, because that's what it is. It condenses, it doesn't capacit. I also say cycles-per-second, not Hertz. But that's just me. (-:

Well I ordered the condensers from Mouser over teh weekend. Hopefully they'll be coming any day this week and I can start fixing the set! I ordered Cornell Dublier Polyester Film ones to replace the paper and plastic ones, and radial and axial electrolytics for the large filter condensers.

Received the condensers today! I will start replacing the old ones. SInce the chassis is for the most part solid, I won't worry about underside looks. For the paper condenser on the top-mounted lug strip, I will hollow out the old cardboard tube and install the modern one, and seal it with wax and hot glue. I'll post some pics when I do that. As for the large input condensers, I'll leave those mounted on the chassis but I will disconnect them and mount the modern ones underneath. I have read of cutting them open and potting new ones inside, but I don't want to ruin them. It'll be fine to hide modern ones underneath... there's enough room.

http://www.erniewerbel.com/temp/000_1216.JPG

Here is some progress on the rework in the input section. Note that I don't have 40 uFd condensers, so I needed to make an array of four 10's... in two places.

This is becoming messier than it was to start with! Of course, replacing single paper condensers with ceramic ones reduces clutter dramatically. I find myself double-checking my double-checking to make sure I am using the right components.

It's just so hard to beleive that the new components of the same rating and value are about 1/20th of the size of the original. I can't help but worry if I am installing a wrong part! :-)

I usually use a 47mfd to replace a 40, but what you are doing will work fine, too. I know that I have double checked myself sometimes when those new parts just looked too small! Especially with lower value paper caps. A .001 in paper isn't much smaller than a .01 but some of those new ones are tiny.

Looking forward to the finished product!

I cleaned the tubes... this 6DT5 (vert opt) looks burnt even after cleaning... I wonder if it could be the culprit after all? Not that replacing the condensers was a bad idea. I don't have a tube tester so I need someone's expert advice. Is it me, or does the tube on the left look like it's on its way out?

http://www.erniewerbel.com/temp/Picture 159.jpg

Ok I put the set back together with most of the condensers replaced (couldn't get some values but only about three).

I GOT FULL VERTICAL SIZE! WOOT!

But now there is another problem. Set is only snowy with a thin brighter line down the center, top to bottom. I'm thinking it could have something to do witht he yoke, which spins freely on the neck.

I also might not have reinserted the yoke plug right in the socket. I speculate this because although I am getting full vertical, perhaps it's not being directed where it should. Let's see what the Sylvania book says.

Even though this is a Hoffman TV, my Sylvania book, which I got for only $0.99 on eBay, is very useful in troubleshooting. "Servicing TV receivers" by Sylvania Electric Products Inc. First Edition 1950. It feaures test patterns and schematics of what to look for, and troubleshooting tips. I highly recommend a book like this.