Back to the VT-71H

[Electronic M]

The thing that makes adding composite video dangerous is if you forget to plug the TV into a power isolation transformer your signal source device will be destroyed...

[TV-collector]

Originally Posted by Chris K

I built Bob's design for the replacement ballast but the CL-90 hasn't arrived yet so I bypassed it with a jumper and brought the TV up slowly. I'll solder it in once it comes. The image on the picture of the screen is not what it looks like. The bright area in the picture is spread out in a 3-4 configuration across the entire screen face. There's no video signal through the IF as before but there's audio.

My next question is does the electrostatic tube and focus in any way affect the installing of jacks for composite video? I'm not giving up on solving the IF issue but I'd like to know if that's an option.

Hi,
I follow your poor story since a while.
Do you have any signal tracer?
They make it easy to find trouble sections in radios.
Try one (see safety instructions when working on life chassis) to check the video amp
and IF-section.
Do you have a clean sound from a "TV station", means a DVD or VCR?

Regards,
TV-collector

[Chris K]

Originally Posted by Yamamaya42

Looking at things closer, V6 has to be working for there to be sound, as the sound pickup is through C35, but perhaps problems with coils L32/L33 or C72, it should be easy enough to trace with a scope, cause you KNOW, it has to be getting as far as the video amp for sound, just need to see where it may be getting lost from pin 5 to CRT point C.

Back to this PIA TV inspired by Bob and his recent "Green CRT" video. I noticed the Sams and the Motorola service differ as to the direction of the 1N34 diode in the video detector. The service data has my chassis, VT-71H in it and the diode direction is the opposite configuration from how I installed it. I switched it around and it made no difference. I tried a different JP4 CRT to see if maybe there is a cathode issue with what I've been using and the "image", a full screen and bright raster with no video information getting to it, is the same. The sound coming off pin 6 of the video amp 6AU6 is clear and buzz free with plenty of volume. There's a new 0.047 uf cap replacement for the 0.05uf C-72. I've checked the 2 coils between the video signal off the plate and the CRT and their continuity and they are fine. Just for grins, I replaced them one at a time with the same coils off a parts chassis that tested fine with no change. Scope is down right now so I can't check signals but if I have audio getting through the detector, everything before it should be fine as far as the video signal correct? The only thing I have not confirmed are working are the 8 and 15 pf caps in the detector but they're the type that are supposed to never go bad. I am at a complete loss with this missing video signal. It's a real mystery. When I get my new, USB oscilloscope, I'll be able to do more accurate tests but until then, I'm totally stumped.

[Electronic M]

One bad but real possibility is that given the CRT is cathode fed it's video if a heater cathode short occurs it'll swamp out the video.

[Chris K]

Originally Posted by Electronic M

One bad but real possibility is that given the CRT is cathode fed it's video if a heater cathode short occurs it'll swamp out the video.

I've swapped out the 7JP4 with another good one I have and the picture is the same.

[timmy]

What about the crt plug bad connection at the cathode wire within the plug itself.

[Chris K]

Originally Posted by timmy

What about the crt plug bad connection at the cathode wire within the plug itself.

I have continuity all the way from the output side of the 0.05uf cap (C72 in the Sams) to the tabs in the CRT socket #2

[Chris K]

I also replaced C31, the 0.1uf cap that feeds pin 1 of the video amp with a tested new cap and I re-flowed the solder connections at all points.

[timmy]

Well you could take the schematic and start at the video amp and work back from there with double checking all connections and components I’m sure you will come up with something we all make mistakes I have been in the same situation as you are with this set. It’s time consuming but it will be worth it especially if you have sound all that’s left is the video. Even one leg of a resistor placed in the wrong spot can be a headache it’s happened to me and all of us with these sets.

[Chris K]

Originally Posted by timmy

Well you could take the schematic and start at the video amp and work back from there with double checking all connections and components I’m sure you will come up with something we all make mistakes I have been in the same situation as you are with this set. It’s time consuming but it will be worth it especially if you have sound all that’s left is the video. Even one leg of a resistor placed in the wrong spot can be a headache it’s happened to me and all of us with these sets.

OK but I'm confused. All of the information from the RF/IF comes out of the detector at one point. According to another expert in this thread, if I have sound, I should have video. The signal for both audio and video go into the video amp through pin 1. Video comes out of a grid at pin 5 and audio comes out from a grid on pin 6. Audio information then feeds into the limiter, ratio detector, audio amp etc... Video goes to the CRT cathode through a 0.05uf cap that has been replaced with a new 0.047 uf cap. I've only been looking at the video route after pin 5. Should I still go looking backwards?

[timmy]

Originally Posted by Chris K

OK but I'm confused. All of the information from the RF/IF comes out of the detector at one point. According to another expert in this thread, if I have sound, I should have video. The signal for both audio and video go into the video amp through pin 1. Video comes out of a grid at pin 5 and audio comes out from a grid on pin 6. Audio information then feeds into the limiter, ratio detector, audio amp etc... Video goes to the CRT cathode through a 0.05uf cap that has been replaced with a new 0.047 uf cap. I've only been looking at the video route after pin 5. Should I still go looking backwards?

Well yes it’s said that video rides along audio so you can still go backwards but at this point there could be a problem with the alignment coils and still have sound but no audio. Try different channels 3 or 2 or maybe 6 I had one of these sets that had sound but no video blank screen and it was a think ts 4h with the adjusting coils in front so I concentrated on channel 3 and eventually got video. Remember you probably don’t know the history of the set who know who was in it or what may be bad in the IF section.

[Chris K]

OK thanks. Expecting a decent oscilloscope to arrive tomorrow and I also purchased a good 100X probe. I'm tired of trying to make this 45 year old analog Textronix work for me.

[bandersen]

Have you gone through the Sams resistance chart for the CRT and 6AU6 video amp?

[Chris K]

CRT and Video amp resistances are fine but pins 5 and 6 on the 3rd IF amp should be 1.2K and they are at <300 ohms. Both feed into the detector. The rest of the IF strip is fine. I have a parts chassis. I'm wondering if I should just swap out the detector.

[bandersen]

No, if you have sound but absolutely no vide, I seriously doubt it's the detector. A scope should give you the answer in a few minutes. Start at the detector and go through each stage to the CRT. At some point the signal will disappear and that will be the problem point. I suspect either something is open or shorted.