AC In My B-

[Kevin Kuehn]

Originally Posted by Jon1967us

One question about the HV deflection caps coming off the horizontal output. Schematics call for two 300uuF and I have two 470s instead. Could this potentially be creating a problem?

Stands to reason the 470pf could pass a lower frequency but I kind of doubt they picked a lower value specifically to eliminate 60 hz leakage into the picture. I would focus on trying to nail down which pin(2 or 3) the 60 hz is modulating the CRT, and whether or not it's there with no video signal present, maybe even disconnect the cap connecting the video to the cathode. The brightness control determines the cathodes DC bias, so you need to leave it connected. Have you tried swapping the CRT with the one in your other set? It's possible the CRT has some internal cathode or grid leakage.

[dtvmcdonald]

Oh yes: I've had problems with my TV-37 and only that set with sync problems due
to small amounts of hum on the signal itself, caused by poor gounding between the baseband signal source and my modulator, or caused by hum backing up onto the transmitting antenna if I am broadcasting across the room.

This is manifest by rolling hum dark hum bars in the picture and/or
loss of horizontal sync in rolling areas of the picture, typically 1/4 inch high. Its cured by getting all the grounds good in all the attached equipment.

[old_coot88]

At this point I would do this: Locate the .05 cap which goes thru a 180K resistor to pin 5 (plate) of the 6AU6 sync separator. Disconnect that cap. This completely decouples the sweep oscillators from the video chain, leaving them free running.

Then you have to manually jockey the hor and vert holds to hang onto the pic. See if the hula'ing is still present. If it is, then the ripple is not originating in the video chain but in the sweep area.

[Jon1967us]

Originally Posted by old_coot88

At this point I would do this: Locate the .05 cap which goes thru a 180K resistor to pin 5 (plate) of the 6AU6 sync separator. Disconnect that cap. This completely decouples the sweep oscillators from the video chain, leaving them free running.

Then you have to manually jockey the hor and vert holds to hang onto the pic. See if the hula'ing is still present. If it is, then the ripple is not originating in the video chain but in the sweep area.

Per your suggestion I tried this. First video is with that cap disconnected...

https://youtu.be/QG5jBrWuftQ
https://youtu.be/EvWDVbtS3_o

Comparing a few times, no cap connect vs connected, there actually doesn't seem to be much difference other than when the cap isn't connected there's less vertical stability.

[Jon1967us]

Originally Posted by Kevin Kuehn

Stands to reason the 470pf could pass a lower frequency but I kind of doubt they picked a lower value specifically to eliminate 60 hz leakage into the picture. I would focus on trying to nail down which pin(2 or 3) the 60 hz is modulating the CRT, and whether or not it's there with no video signal present, maybe even disconnect the cap connecting the video to the cathode. The brightness control determines the cathodes DC bias, so you need to leave it connected. Have you tried swapping the CRT with the one in your other set? It's possible the CRT has some internal cathode or grid leakage.

With the CRT cathode disconnected from the .25 cap.

https://youtu.be/ZuVDtigjnQw

[Kevin Kuehn]

Apparently it's not getting in through the video signal. I had a very similar situation once on a Setchell Carlson portable. After a whole lot of head scratching the cure was to put a big B+ bypass cap very near where B+ supplied the horizontal oscillator or amp section. Never did figure out why it needed extra filtering other than there was apparently some AC ripple formed between various grounding points. That chassis was aluminum with the grounding points riveted to it.

[Jon1967us]

Yeah I'm glad I did those tests. That helps narrow it down.
I'm going to double check the B+/- plate side before and after the filters and maybe try another filter.

I intend to recheck the resistance on the B+/- pins per the Sams, since Riders doesn't have those measurements to check against.

[Kevin Kuehn]

It looks real nice under there. Shinny as new.

[Jon1967us]

Originally Posted by Kevin Kuehn

It looks real nice under there. Shinny as new.

Yeah the chassis itself, the metal was pretty clean. It was all the other metal parts, such as the ground rivets or potentiometer cases that had a thick coat of white powder.

Still no luck. Tried bridging additional caps over either halves of the power supplies. Have tested 3 different 3kp4's in it an no change, so I don't think it's a heater/cathode problem with the CRT, nor a filter cap problem.

Hard to tell if it's 60 or 120Hz hum. As I understand it, 120 is four bars of alternating light and dark and 60 is two. Unfortunately, bars are usually not visible, but momentarily and since they move and are so faint, it's hard to tell.

In Kiver's "TV SERVICING TIMESAVERS" it suggests to use a scope on all the cathodes to try to ID the largest amplitude 60Hz signal. Maybe from that I can ID an offending tube, bad lead dressing, lead, or bad socket.

Not willing to throw in the towel just yet.

[old_coot88]

From the videos you've posted, it looks like the horz sweep oscillator is what's being modulated with 60 hz. The vert osc doesn't appear affected. Watching the vids at 2X speed gives a bit clearer discernment.

Just guessing, but either a flaky ground or leakage from the heater string might be the culprit.

[Dave Tilotta]

Jon1967us,

I realize that this is an old thread, but I'm wondering if you ever solved your hula problem. I have a set that shows the same effect and I'm stuck.

Dave