Meck XB-702 restoration

[Kevin Kuehn]

If you haven't already, try powering the set plugged directly into a wall outlet. Make sure the chassis is connected to the common earth side.

[bandersen]

I have power it directly from a wall outlet and flipped the plug several times. It makes no difference.

I do have 0-400VDC supply, but it only goes up to 400mA. I'll need 600mA to power both filament strings.

[Yamamaya42]

Should not be too hard to rig something up for a temporary test, that is if the filament string is still even suspect.

1 amp isolation transformer → bridge rectifier → filter caps → power resistor if needed, and bring up on variac until heaters are at optimum range.

Any large power x-fomer should work as long as it has dual primary, example, I did not have a isolation transformer when working on the RCA radio, but made one out of an old stancor p-6378 transformer.

The dual primaries were more than it's 8 tubes & the radio w/o even getting hardly warm.

[bandersen]

It will be a tricky to wire in as it uses a voltage double and tripler so I can't just power the set from DC.

[Yamamaya42]

Originally Posted by bandersen

It will be a tricky to wire in as it uses a voltage double and tripler so I can't just power the set from DC.

in the unlikely event that this is somehow heater to cathode related, it should not be TOO HARD to rig up something to temp power the entire heater string from a DC source ( adjusted by variac via isolation transformer, etc )

and rule that out 100%

[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.

[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