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AC In My B-
I have a model TV-37 I'm working on. This is my second one. My first one didn't have this same issue with what appears to be AC ripple disrupting the Horizontal and slowly makes the image do the "snake" The symptom is a Hula dancing effect with the Horiz. The pulling only occurs when a video signal is present. When there's only raster, like just at start up, the image is stable.
https://youtu.be/EPdc9n_1bpI I have found a bit of AC current leaving the 35W4, which is the - rectifier. On this set the B- feeds the Sync, Horiz Osc and cathodes of vert and horiz outputs. I have found approx 2.3VAC on the plate of the 35W4 and the following AC values found on the cathodes of the following tubes. 35W4: 2.2 VAC Hor Osc: .2V Hor Out: .2V Sync: .2V HV Osc: 1.5V Vert Osc: .28V Vert Out: 1.3V AC is not found on measured cathodes fed from the B+, which is the 25Z6 dual rect, even though I measured 9vAC on the cathode of that tube! This set adheres more to the Rider version of the schematic than the Sams. Here is a link to the PDF, from Phil's site: https://www.antiqueradio.org/art/Pil...7RidersTV2.pdf Work I have done up to this point: All original filter caps have been replaced with tested new caps twice, just in case there was an undetected defect that my capacitance and ESR tester didn't catch. Added an extra third 120uF parallel to the B- filter caps which lessened the pulling. Adding an extra parallel to the B+ does nothing. All non polarized caps replaced, including all small value ceramics and coupling types. All deflection caps replaced with WIMA HV film caps, except for the cap in the HV cage which was replaced with a 10KV ceramic. Note: this problem existed before and after the in-cage cap replacement. All tubes have been swapped out with tested ones and this doesn't improve the symptom. I know this is a lot of info, but I've kind of hit a wall here and am looking for ideas if anyone can enlighten me. :thmbsp: |
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I may be using the wrong nomenclature, what I mean is that the portion of the rectifier circuit in question is negative relative to ground.
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is it ripple from the 35W4 being loaded down too much? or perhaps a socket problem where AC from the nearby heater pins is bleeding in somehow, after all. this is in series, direct off the AC line, with no x-former, who knows what odd things it may do with socket problems.
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https://i.ibb.co/qkBPMvr/Screen-Shot...7-34-04-PM.png
It's definitely the 35W4 (B-) that is the source of AC ripple. It's a single diode, whereas the 25L6 (B+) is a dual diode and does a better job in smoothing out the DC. I'm measuring 2.4VAC at the 35W4's plate and it gets dropped to 1.6v at the junction of the 10Ω and 270Ω - only .2VAC is leaving the smoothed DC coming from the 25L6. I'm wondering if I should try putting a 47uF or something between the plate of the 35W4 and the 10Ω? |
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if it IS being loaded beyond it's spec, you gotta track down what is loading it down, if not, the AC is bleeding in from something else, you should not have to mod anything to get it to work, within reason :o |
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It kind of makes sense that there will be SOME amount of ripple on the plate side of the 10 ohm resistor, how much is the question, and how much, if any should be seen on the far side at the filters.
Before you go tracking a possible red herring, you did say you had restored another one of these sets? Perhaps it would be productive to go and see what the negative power source on that looks, and how much ripple it has comparatively, who knows, it may be the same, or you may be on to something and it is too much in the set. |
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Thanks for the helpful replies. As far as the video signal goes, I haven't detected any AC on the cathodes of the IF or Video tubes, nor have I detected any on the cathodes of the RF, Osc or Mixer. I haven't tested all the grids. Maybe it's possible some AC is leaking into the video grids and coming out of the anodes of the IF or video tubes.
One thing I'm thinking about is the TV37 had at least 3 production changes. This set however is labeled "TV37" not "TV37U". There was a little work done in the past on this set and one of the multi section cans was replaced. One production change, represented in Sams shows a 3 section ballast resistor, instead of discreet ones. It's possible the ballast was removed and those resistors weren't completely replaced (only the 470Ω is present in mine, like the Rider's schematic shows) It could be that the B+/- aren't loaded down enough without the potentially "missing" (if we believe the above scenario possible) two 10Ω. Later Sams Schematic showing low power supply and ballast resistors: https://i.ibb.co/nfx1qFC/Screen-Shot...2-41-43-PM.png I'm not sure what my other options are, but I may need to bust open the working TV37 I have once again for a deeper look (I had done this before, scope and all and didn't find any significant difference in the spot checks I performed, but maybe I missed something) |
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The video amp cathode is grounded so you wouldn't see it there. If you haven't already, try subbing new 6AU6's for both the video amp and DC restorer/sync tubes.
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Swap out the 35W4 while you're at it. It too could have heater to cathode leakage.
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When we're talking heater leakage, I thought we're concerned about leaking into the Cathode. Could it be leaking into the grids? I've already subbed out all the tubes, and this makes no change. Not sure what else there is to do other than modding it by adding the ballast resistors from the Sams. Just about every relevant component has been changed. The remaining original resistors are within tolerance. |
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How are you measuring the AC on the cathodes ?
jr |
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jr |
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I also measured 2.4AC/-22DC off pin 3. Both measured to ground. These values jump around a little compared to all the other points I've tested, I guess due to video signal. That's the highest measurement of AC on a cathode in the entire circuit that I've tested so far. Raw line current goes straight into the CRT of course (which is why I always bring up on a variac :thmbsp:) Wondering about putting one of those TVS diodes across the CRT heaters to limit current and maybe it will take care of this problem as well. I feel like I need to take resistance measurements again in these areas between the sync, B- and CRT. I've looked at all the underside tube pin terminals for any shorts or crud that could be creating a short with a magnifier and wiped the undersides with contact cleaner. NOTE: many of the metal surfaces were coated with a white powder when I got the set, which I have cleaned off what was visible, but there could be some that got into an area and is conducting - who knows :saywhat: Haven't tried hanging a cap from the CRT pins 2,3 to ground yet (I assume the - terminal will be on the CRT side, right?) |
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I have a Pilot TV-37. Its got stray 60 Hz everywhere. I've verified that its there even if there is no B+ or B- because the rectifiers AC feeds are disconnected (but heaters are connected). Thus its heater line pickup. In most cases I verified that its not heater-cathode problems by moving tubes of the same type around.
I've also verified that a lot of the problem is modulation of the RF in the tuner. I managed to get OK pictures by careful adjustments. Its vitally important to get the RF level in a good range: if the gain is too high or low it gets worse. Thus, I need an external attenuator. You can't really win. The set was designed for pictures that had a constant average gray, and a constant contrast, no black or white screens. Give it such a program, and its much happier! |
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SO, w/o an oscilloscope, its hard to know what you are really looking at, other than what this "Hz function" on your meter is saying, you have no real tell what wave form it is... |
Yep, I keep assuming we all love our scopes. It's anyone's guess what AC you're picking up on without visually seeing the signal. Quite likely you're chasing a red herring.
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It definitely seems like I am chasing something misleading around. I have taken quite a few readings around the set with the scope and other than the video signals I'm seeing the same thing as the multimeter-AC current at unexpected points. Going to run this and the better running set at the same time and go through them with meter and scope and try again to spot any suspicious differences.
:thmbsp: |
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?
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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. |
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. |
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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. |
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https://youtu.be/ZuVDtigjnQw |
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.
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https://i.ibb.co/HY5vLG7/63978-BF0-F...D7105-E9-B.jpgYeah 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. |
It looks real nice under there. Shinny as new. :thmbsp:
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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. |
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. |
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