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  #31  
Old 04-14-2009, 11:59 PM
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Smoke test

Well, I'm glad I went the variac route. I did use the watt-miser. I used it on the primary of the variac so I was only able to monitor total power and not the voltage and current separately.

Things were coming up nicely. While letting it sit at 90 VAC smoke suddenly wafting up. A 100 ohm plate resistor on the 4th video IF stage was overheating. Since I was watching the bottom of the chassis when this happened it took me a bit to understand what was going on. One end of the resistor goes to +170V and the other connects to the IF transformer primary which connects to the plate of a 6AG5. When I measured the IF transformer side of the resistor it showed zero ohms to ground. I thought the IF was shorted. I pulled the 6AG5 and immediately saw what the problem was. It had gone up to air.

The pins on this tube had been severely bent at some point in its life. It tested good so I kept it, but there must have been a small crack in the glass. As it heated up it eventually cracked open.

Up until this point everything was fine and the voltages were coming up properly and nothing was overheating. The power consumption was still below spec.

I'll start again tomorrow. It's late now.

John
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  #32  
Old 04-17-2009, 12:13 AM
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First Picture

I powered the TV up tonight. I checked out the FM receiver first and it sounds great! I need to make a loop antenna for the AM. It can't work without it. I tried anyway and all I heard was my neighbor's SSB transmission.

I then checked out the TV. I was surprised to get a raster let alone an image. I was very happy the the HV section was working because there were quite a few bad components that I had to replace.

I had to readjust the Horizontal frequency coil by about 15 turns to get it to get this close.

There is no audio on TV. But that's a pretty small section to look at so I don't expect it to be too difficult to fix.

Anyway, the image is below. All the major components are working so I should be able to fix the remaining problems.

Actually the main problem probably is that I wanted to try to keep the selenium rectifiers. They seem to test good, turning on at 6 volts and having zero reverse current at 100 volts. The low reverse current was pretty unusual.

They seemed fine until I plugged the HOT, VOT, and RF HV Osc tubes in. Then there was a 40 volt drop and the seleniums started to get warm. I was going to use NOS ones that I have, but they have a high reverse current.

As much as I like playing with old technology like selenium rectifiers, I'll have to give up on them.

Going to 1N4007's will bring the -90VDC up to -130VDC and may solve some of the apparent problems. That's not related to the sound though.

I may have to do a complete horizontal alignment since I replaced all the caps in that section.

I was very happy to see it do this much!

John
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File Type: jpg First_Picture.jpg (142.4 KB, 45 views)

Last edited by jeyurkon; 11-14-2009 at 07:02 PM.
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  #33  
Old 04-17-2009, 09:06 PM
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Looking good! Very good for the first power up. I've never had a terrible experience with seleniums; they just seem to be "weak" on some sets. (maybe most sets!) I agree, it is fun to get it working the "old way". I was thinking, you might just be the guy to try rebuilding a selenium!
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  #34  
Old 04-18-2009, 08:21 AM
kvflyer kvflyer is offline
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Yes, it looks good. I can understand your desire to keep the selenium rectifiers. But, they will be a pain from this day forward. If you are looking for originality, you can hide a 1N4007 behind that selenium I am sure. Just a thought...
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  #35  
Old 04-18-2009, 11:50 AM
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Quote:
Originally Posted by bgadow View Post
Looking good! Very good for the first power up. I've never had a terrible experience with seleniums; they just seem to be "weak" on some sets. (maybe most sets!) I agree, it is fun to get it working the "old way". I was thinking, you might just be the guy to try rebuilding a selenium!
Actually there are at least still two manufacturers of selenium rectifiers.

I might try putting together a supply with the NOS seleniums and a dummy load to see how they behave. I might have too high expectations for how I think they should test.

But, for now I'm going to use the 1N4007s. and an additional 110 ohms with each.

I'm glad I fused the seleniums in case they died.

John
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  #36  
Old 04-18-2009, 08:08 PM
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Found the problem

Well, using the 1N4007s brought the -HV up. I was a little too conservative with the resistors. I'm about 5 volts low, but that shouldn't be a problem.

I saw no change in the performance. The AM radio is working to a degree now without the loop, but that's not related to the -HV since it doesn't use it. It's just that there's a strong AM station in the daytime.

I'm amazed that the set had a picture at all. While trying to figure out why the video was so weak and the AGC wasn't kicking in I discovered that the primary on the first video IF transformer is open. I only had a picture because of the signal bleeding through.

It's probably hopeless to ask, but does anyone have a Sylvania 125-0001 IF transformer? I don't think Merit made a replacement.

I may have to rewind it which would be difficult. It appears to be coated with glyptal which is extremely difficult to remove. On the other hand, the coil shows no signs of having been over heated. It might just be a cold solder joint to one of the terminations.

I suspect that once I get a decent signal through the video IF stages my horizontal problem will take care of itself.

Oh, the CRT HV is 500 volts low. I can only get it up to 9KV. I should be able to get 11KV max and set it to 9.5KV. I think it's because I have a weak 6Y6-G in there. The one in the audio section is better, but it doesn't fit under the coffee can. Which is funny, because it's the Sylvania tube. The weak 6Y6-G that fits is a Philco. I have a 6Y6-GT I can try. I didn't because Wallace's recommends using a coated 6Y6.

John

Last edited by jeyurkon; 04-18-2009 at 08:12 PM. Reason: Forgot 6Y6-G comments
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  #37  
Old 04-18-2009, 11:25 PM
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Video IF

Here's a photo of the video IF transformer that I need to repair or replace.

John
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File Type: jpg Video_IF_Transformer.jpg (65.2 KB, 35 views)

Last edited by jeyurkon; 11-14-2009 at 07:03 PM.
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  #38  
Old 04-19-2009, 01:24 PM
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Video IF repair

I'm torn between rewinding the coil and repairing it. It has 16 turns and is bifilar wound. Should be easy enough to rewind, but just as easy to fix. Both ends are open. One end I can just reattach to the terminal, the other end I would need to splice.

I had to soak it in lacquer thinner. That didn't dissolve the alkyd coating but softened it enough to scrape off.

Any suggestions as to which route I should take?

There are only two components that could have caused the failure. Other than someone twisting the form while trying to adjust it.

One would be the tube which tested o.k. At least not shorted. The other is a 0.68 pF 500V axial ceramic. It's too small a value for me to test, but the leakage at 500V is less than 100 nA. There's a 0.005uF disc ceramic on the secondary that has 1.2uA leakage at 500V. That didn't cause any problems though. I'll replace it anyhow since it's really ugly looking.

The color code on the 0.68pF ceramic is Blue-Gray-Silver-Nothing. I'm not familiar with this coding. Anyone have a chart for this type of ceramic cap?

John
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File Type: jpg Video_IF_Stripped.jpg (61.5 KB, 27 views)

Last edited by jeyurkon; 11-14-2009 at 07:04 PM.
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  #39  
Old 04-21-2009, 12:33 AM
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IF rewinding

I decided to rewind the video IF transformer. You can see the progress below. It would have been easier if my eyes weren't the same age as the TV.

After installing it, it has a strong video signal. I have have a horizontal problem. I'll go through the horizontal alignment procedure and if that doesn't fix it I'll have to do some debugging.

I'll need to realign the video IF also. But, I need to do some work on my Hickok 610A first.

John
Attached Images
File Type: jpg video_IF_Bare.jpg (136.6 KB, 25 views)
File Type: jpg video_IF_wound.jpg (100.0 KB, 32 views)
File Type: jpg video_IF_finished.jpg (140.6 KB, 23 views)

Last edited by jeyurkon; 11-14-2009 at 07:06 PM.
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  #40  
Old 04-21-2009, 03:58 PM
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John Folsom John Folsom is offline
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John,

Nice job on the IF transformer. Here is a link to capacitor color codes, hope it will help:
http://www.tpub.com/neets/book2/3g.htm
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  #41  
Old 04-21-2009, 10:38 PM
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jeyurkon jeyurkon is offline
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Quote:
Originally Posted by John Folsom View Post
John,

Nice job on the IF transformer. Here is a link to capacitor color codes, hope it will help:
http://www.tpub.com/neets/book2/3g.htm
Thanks for the compliment and the link. The axial ceramic cap color code table is close. But, like all of the others I've looked at they don't include silver in the multiplier. I guess I should just assume it follows form since that gives the schematic value.

John
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  #42  
Old 04-21-2009, 10:58 PM
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Horizontal progress

I think I've found the problem with the horizontal circuit. The symptom is that it's only stable with the blanking bar down the center of the screen.

I don't fully understand the circuit. It's a sine wave type horizontal AFC.

But suffice it to say that the output of the sync clipper is fed to the horizontal discriminator through a 100pF capacitor. It was supposed to be an axial ceramic. It's actually a Micamold.

All the bad things I've heard about micamolds are justified. If a capacitor can be evil, this one is. The discriminator circuit requires it to not have any leakage. It has about 16 uA at rated voltage. But it gets weirder. The capacitor testing gives up trying to measure the value if the cap's been resting for a long time. After measuring the leakage, the capacitance tests out at 300 pF. It then gradually ramps up to 3,000 pF after a few minutes of testing.

At least this one is easy to get to. The other one that I'll replace is used to turn the line cord into an FM antenna. Given that it would probably burn my house down I'm inclined to replace it.

I wish the 13 others weren't so difficult to get to; I'd love to replace them but since they seem to be working I guess I'll replace them when their time comes.

John
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  #43  
Old 04-24-2009, 12:33 AM
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Smile Horizontal Fixed

Finally!

I changed the micamold and found two caps I had switched. A 0.005 and a 0.010 uF. There's one micamold left that is for the horizontal ringing coil. It's hard to get to, but I should change it.

Here are some photos. Is this the typical linearity that you should expect for a set of this vintage?

John
Attached Images
File Type: jpg Test Pattern.jpg (114.5 KB, 54 views)
File Type: jpg Picture_1.jpg (99.4 KB, 51 views)
File Type: jpg Picture_2.jpg (68.9 KB, 44 views)

Last edited by jeyurkon; 11-14-2009 at 07:07 PM. Reason: changed peaking to ringing
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  #44  
Old 04-24-2009, 09:52 AM
dreyfoos dreyfoos is offline
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John, those photos represent outstanding work on your part. Congratulations on getting that old babe back to as good as, or even better than, new.

As to linearity, looks to me like you're down to the point where any deviation may only be in the eye of the beholder.

Three cheers for a job well done! And thanks for sharing all the details with us.

Roger
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  #45  
Old 04-24-2009, 12:41 PM
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Awesome!
-Tony
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