RFT b/w tv with no video information

[tube tv fan]

Hi everyone! I am new here and sorry for my bad english (I am Greek).
I am having trouble with an rft donja 202 black and white tv. I have replaced all the paper and electrolytic capacitors (I tested them with 300 volts before puting them in), I checked all the ceramic capacitors and ALL the resistors. The tv has crystall clear audio, has static on the screen but when I tune in only the audio tunes. The picture turns from snow into a raster during tuning and starts showing fade jumping dots and bars according to the movie action, but absolutely no video information is visible. Since it is tuning, the tuner is not the case but I tried with an exactly same tuner from a donor set and of course this made no difference. I also tried with different converter boxes. In a last attempt, I opened all the video IF cans and checked everything in them. All good. Finally, all the tubes were tested and most of them have been replaced. The only weird things I found is that the 100K potentiometer called "W125" does absolutely nothing to the picture. I measured it and it is good. I did not measured the center pin though. The second weird thing is that the contrast potentiometer pulls the sound down when it is turned down. I changed it and it made no difference. The weirdest thing is that it pulls the sound down according to the antenna signal. With no signal, it does not affect the sound. Now I am completely lost. Has anyone ever experienced a problem like this?
I am going to attach the schematic.

[Electronic M]

W125 looks like the contrast control.

Sets with intercarrier sound (sound IF gets fed by the video IF) sometimes have the contrast effect sound gain if the contrast changed the video IF gain as a part of their design...Sets with AGC don't have this issue.

Your description of the picture is hard to follow. Could you post a picture to clarify?

If my guess at your meaning is correct it sounds like you get snow with no signal, and a weak picture that is unsynchronized in both sweep axes with signal...that would point to 2 problems. Bad sync separator (or issues with the sweep circuits). And poor gain in the tuner, IF and or video stages(could be a bad video detector diode too).

[tube tv fan]

Thanks for the reply. The contrast potentiometer is W821 which I replaced with no difference. W125 does nothing when turned. The I.F. can with the demodulation diode was also replaced with no difference. The tuner was also replaced with no difference. I will try and explain the problem in more detail. With no signal, I have snow and static from the speaker. When I tune in, I get crystall clear sound, the picture jumps during tuning as it should, but instead of snow turning into video, it turns into a somewhat clear raster with random dots and bars dancing according to the movie action. Tomorrow I will try filming the problem.

[Phil]

It isn't very clear to me, but possibly the triode section of Ra104 is some sort of video derived agc. It appears to be feeding back down the chain to the IF. If this is true then W125 would be an agc control. It is a bit difficult to tell connections from wires crossing each other, and not connected, on the schematic. It sounds to me as if you may have a problem in the IF causing low video output and low sync level. I had a very similar problem with a set one time that turned out to be caused by someone adjusting the IF transformers WAY out of alignment. Good luck with it and please let us know what you find.

[tube tv fan]

Oh man, that was my biggest concern...If video alignment is the case, I'm done. I don't have the equipment for such procedure. The weirdest part is that I have checked ALL the parts in the video I.F.

[tube tv fan]

I forgot to mention that the original flyback had shorted primary. I replaced it with the one from the donor set. It is the same tv with two inches smaller screen. To my knowledge, the horizontal circuit has nothing to do with the video. With the new flyback, the horizontal and high voltage are normal and the temperature of the primary is always 58 degrees Celsius.

[old_tv_nut]

According to waveform B12, there should be a positive 275 volt pulse from the flyback on the plate of the triode section of tube 104, so this definitely seems to be an AGC detector.
Is it possible this pulse is missing since you replaced the flyback?

[tube tv fan]

Here is B12. I was unable to measure volts pp as I am new to oscilloscopes.

[old_tv_nut]

Originally Posted by tube tv fan

Here is B12. I was unable to measure volts pp as I am new to oscilloscopes.

Well, you definitely have a pulse at that point, so you might assume the amplitude is OK.

Here's how to know:
Look at the vertical setting of your scope. It should tell you volts per (major) division. Count the number of major divisions of your pulse height to read the peak-to-peak voltage. If you have a 10x probe (usually the case), multiply the calculated voltage by 10.

Edit: make sure your vertical switch is set to the calibrated position, and not variable gain.

If your scope is set to 10 V/Div with a 10x probe, that makes 100 v/div, so the wavefrom you posted would be about 300 v p-p, which is just fine.

[tube tv fan]

Thanks for helping, I really appreciate it. Currently I don't have the oscilloscope so I am unable to calculate the voltage.
I had an idea last night. What if I feed composite video into the video amp and audio into audio preamp and use the tv as a monitor? Will it work?