Meck XB-702 restoration

[bandersen]

I understand and appreciate the suggestion. I really want to figure this out. Check out the schematic. For example, the HV box gets both the filament power and common reference for the DC power from the hot side of the AC line. They intentionally tie the filament and cathode together on that 12SN7. I've swapped out every single tube and the CRT so I'm think and H-K short is unlikely. Also they problem has also been there before I did any work. I powered it up on all the original parts and was able to get an image and it had ripple.

[bandersen]

The small about of ripple on B+ is finding its way to the output of the video amp.
Since the video is fed into the cathode, that's going to effect the deflection.


I threw another 220uF right next to the video amp which got it down to less than 1 volt. Didn't help the image all that much though.

I'm thinking of adding an RC B+ filter to fee the video amp to help even more with the ripple.

[Yamamaya42]

kinda reminds me of the weird R-Y amp problem i had with my GE CTC-15 clone.

each time I THOUGHT I had it fixed, it kept coming back.

https://i.imgur.com/BSJ5KpG.jpg
normal

https://i.imgur.com/rLZv8JY.jpg
messed up
I finally got tired of trying, and got some 30 Gauge Solid Bare Copper Wire and point to point wire wrapped / reinforced every trace in the areas involved, and it has not done it now for 8 months, I guess re-soldering the PCB 4 times alone was not enough!

GE did NOT live up to their name, " good enough " :O

[bandersen]

I'm curious to try feeding a signal directly into the video amp. GEs are some of the worst sets I've worked one. Sorry you had to suffer through that.

[Kevin Kuehn]

Originally Posted by bandersen

I'm curious to try feeding a signal directly into the video amp.

Did you get a chance to try feeding video directly?

[bandersen]

The issue seems to be isolated to the detector / video amp / sync separator.
I recall there's an old trick to reduce hum in a 6AL5 by running the filament lean. I'll give that a try.

I also noticed that the instant I turn off power, I get a perfect test pattern grid before it fades away.

[Kevin Kuehn]

Originally Posted by bandersen

The issue seems to be isolated to the detector / video amp / sync separator.
I recall there's an old trick to reduce hum in a 6AL5 by running the filament lean. I'll give that a try.

I also noticed that the instant I turn off power, I get a perfect test pattern grid before it fades away.

Along those lines you could try adjusting the lower video frequency response. But it may be a band-aid.

Checked for 60 hz on the DC restorer voltage feeding the grid?

[bandersen]

DC restored grid voltage is clean. AGC voltage is also very clean and responds properly as the input signal strength varies.

Now here's the weird part. I hooked up my scope to the junction of R46 and C48 (upper-right).
As the set powers up, I get a big 60Hz sine wave. Around 40vp-p. I'm trying to track down the source of that sinewave.




Once the tubes warm up, I get a waveform that slowly distorts and changes in amplitude over several seconds. Here are a few samples.

I'm thinking it's the result of two waveforms mixing together.

[bandersen]

OK, I'm going nuts. I'm focusing on V9 the 6AU6 sync separator and restore tube.

I disconnected everything from the plate except the 220K resistor going to B+

Video in on the grid is pretty clean. Cathode is clean. b+ IS CLEAN.

A few seconds after power-up, there is a 50 Vpp sine wave that slowly gives way to a mix of video and sinewave.

I've swapped out the tube several times. No change.

I examined the socket for any solder splashes or carbon tracks.

I'm going to dig up a NOS socket, build the circuit using all new parts, disconnect the old socket and wire it in. I'll just leave it unmounted until this is resolved.

[old_tv_nut]

Junction of R46 and C48 has 180k going to sync waveform and 560 ohms going to vert oscillator, so I would expect to see a lot of vertical oscillator waveform plus some sync - so maybe what you see is normal? During the several seconds the waveform is changing so drastically, do you have a picture, or has it not warmed up enough yet? The changing waveform may be meaningless if it's just some circuits running and others still starting up.

[bandersen]

I disconnected the oscillators. The only thing connected to the plate is 220K to B+.

The waveform changes start once the set warms up. It never stops

[Yamamaya42]

Originally Posted by bandersen

I disconnected the oscillators. The only thing connected to the plate is 220K to B+.

The waveform changes start once the set warms up. It never stops

What does the noise / signal look like on the bias side of R45? And the cathode for that matter? Is it there, or much different ?

[bandersen]

The other side of R45 is B+. No ripple or noise. No ripple or noise on the cathode either.

[Yamamaya42]

Quite strange

[bandersen]

Scratch all that. The 60Hz can be traced back to the detector. I also discovered that the video amp peaking coil and sound pickoff coil do not match the schematic. Also, one of the wires to the sound pickoff coil is longer than necessary. That makes me think it may be an old repair or modification. Now I'm really working in the dark.