Back to the VT-71H

[timmy]

Originally Posted by Chris K

I'm building Bob's design. Waiting for the resistors. I think all I need to do is move the AC line from pin 6 to pin 5. I have 10 of the "Littlefuse" in the appropriate size for this TV. Bob's design incorporates a CL-90. Will that be enough to protect the TV from the surge?

CL-90 yup it will.

[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...

[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.

[Chris K]

Originally Posted by 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.

Morning Bob...do you mean move forward in the circuit between the detector and the CRT or check the entire RF/IF string? Timmy has commented it's possible to have sound but no video due to problems prior to the detector but others have commented if there's sound, your video information is getting lost somewhere between the detector and the CRT cathode. There's not much between the detector and the CRT...a couple of coils and capacitors and all have tested good. I have a resistance on the video amp pin 5 that's spot on by the Sams. I've checked physical wiring continuity from the 0.05uf cap to the halfway plugged in CRT pin 2 and that's all there as is continuity through the 2 small coils after the detector. I've tried a different 7JP4 and have the same raster/no video result. Totally stumped but I will look for the actual signal with a scope next.