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 Post subject: Snakey/Wavy picture?
PostPosted: Nov Fri 15, 2019 7:59 pm 
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Before I start probing around with an oscilloscope, I'm wondering if anyone knows what can cause a picture to snake down the screen. The photo below shows a still version of this (ignore the static, the same thing happens with a clean picture). They can snake down the screen too. This is pretty stable as I adjust the horizontal sync, but once it is corrected it generally stays that way until I change channels and change back.

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snakey.jpg
snakey.jpg [ 185.5 KiB | Viewed 516 times ]

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 Post subject: Re: Snakey/Wavy picture?
PostPosted: Nov Fri 15, 2019 8:24 pm 
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If the image remains in sync and the " snaking " rolls vertically at a roughly 11 second interval then you have 60Hz AC in places it should not be.... could be a bad filter cap, a heater cathode short in a tube, a bad ground on a PCB, a lead dress issue, or a bunch of other things....It is one of those problems that you may have to chase with an oscilloscope if it ain't one of the obvious suspects I listed.


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 Post subject: Re: Snakey/Wavy picture?
PostPosted: Nov Fri 15, 2019 9:10 pm 
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You mean once every 11 seconds? I'm not sure I've left it that long - when it is snaking down the screen (shimmying?) it is going faster than that, like once per second. It is just an annoyance at the moment (because once it is corrected, it stays that way), but it will bug me until I find out what it is. I'll start poking around in it when I get some clear time.

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 Post subject: Re: Snakey/Wavy picture?
PostPosted: Nov Sat 16, 2019 2:54 am 
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I would replace the power supply filters....never mind the o-scope. Nice thing about televisions is they are an o-scope in many situations. I once found this type problem caused by a bucking coil wired backwards. I know that's a Predicta so that's for older sets. Just passing info. Craig


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 Post subject: Re: Snakey/Wavy picture?
PostPosted: Nov Sat 16, 2019 5:01 am 
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I've already replaced all the electrolytic and 'paper' caps and input diodes. Admittedly, plenty of opportunity for messing it up, and I haven't tested the choke. Plus the board had been worked on in the past - a couple of caps had been replaced and the audio wiring looked to be very different. I didn't trace that out as this TV did not have audio problems to start with. Anyway, time to trace through some of the wiring, check for dry joints and check that choke. Though I might just put an oscilloscope on it to see what I can see anyway.

A couple of interesting observations. Some of the resistors I replaced had different values in SAMS than they had on the board - and I mean the marked value, not the tested value. They looked like original resistors and they weren't resistors that changed with different revisions. Plus some of the capacitors are just plain wrong if you follow the wiring. For example, SAMS has C59 and C23 swapped. Where they are in the schematic (and by tracing the wiring) does not match the labeling on the photographs, and these don't look like they have been tampered with in the past. It looks like SAMS copied the factory manual here. That has these two swapped too - C2 and C8 on their schematic - v. their chassis drawings. It leaves me wary of the accuracy of the documentation.

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 Post subject: Re: Snakey/Wavy picture?
PostPosted: Nov Sat 16, 2019 2:40 pm 
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Give us the chassis number; one of us may have the original Philco schematic. As mentioned, a heater-cathode short in one of the tubes can cause the problem, especially in the RF, IF, and video stages.

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 Post subject: Re: Snakey/Wavy picture?
PostPosted: Nov Sat 16, 2019 5:25 pm 
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probably one of those couplate things.
there is a member here making repros .


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 Post subject: Re: Snakey/Wavy picture?
PostPosted: Nov Mon 18, 2019 4:48 am 
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judge wrote:
I've already replaced all the electrolytic and 'paper' caps and input diodes. Admittedly, plenty of opportunity for messing it up, and I haven't tested the choke. Plus the board had been worked on in the past - a couple of caps had been replaced and the audio wiring looked to be very different. I didn't trace that out as this TV did not have audio problems to start with. Anyway, time to trace through some of the wiring, check for dry joints and check that choke. Though I might just put an oscilloscope on it to see what I can see anyway.

A couple of interesting observations. Some of the resistors I replaced had different values in SAMS than they had on the board - and I mean the marked value, not the tested value. They looked like original resistors and they weren't resistors that changed with different revisions. Plus some of the capacitors are just plain wrong if you follow the wiring. For example, SAMS has C59 and C23 swapped. Where they are in the schematic (and by tracing the wiring) does not match the labeling on the photographs, and these don't look like they have been tampered with in the past. It looks like SAMS copied the factory manual here. That has these two swapped too - C2 and C8 on their schematic - v. their chassis drawings. It leaves me wary of the accuracy of the documentation.

Sam's was known for typos in their literature I've discovered 2-3 myself in a couple different makes and chassis the hard way, and I know of several others other folks have found.... I've said it before but it merrits repeating: when the original wiring looks untouched always trust it more than the documentation, and only trust documentation more than what is in front of you when the set has hack repairs or when the original part is burnt or defaced beyond positive ID.

Documentation is good with periodic typos as an exception, but there are plenty of cases of production changes that never made it to some of all of the documentation publishers.


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 Post subject: Re: Snakey/Wavy picture?
PostPosted: Nov Mon 18, 2019 6:09 pm 
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Forgot to mention: I also replaced all the couplates.

Anyway, I spent a few hours poking around with an oscilloscope and didn't really find anything. The voltages are clean - no visible ripple. As far as heater shorts go, the only tubes I haven't replaced are in the RF section. I've been using a test pattern of vertical bars which gives a nicely identifiable waveform. There is what appears to be some ringing on the video signal (W1 on SAMS). It is visible on the horizontal sync pulses and moves around if I just sit and watch it. By the time this has been inverted and amplified it leaves the sync pulse quite poorly defined. Maybe I should have worked on the RF and IF sections when I had it apart.

I adjusted the horizontal sweep circuit following the procedure according to SAMS. Interestingly, the picture was rock solid while L22 was shorted out. Or to put it another way, it was at least as stable as when it wasn't shorted out. I checked the resistance of the coil. It is fine. BTW, yet another problem with the factory documentation here. It says to short out C15U to do this (which would be C26 in SAMS), which is clearly wrong.

I'm going to leave this be for a while. I don't have the appetite ATM to remove the chassis to work on the IF and RF sections. I guess I should have paid more attention to this while it was on the bench.

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 Post subject: Re: Snakey/Wavy picture?
PostPosted: Nov Mon 18, 2019 7:22 pm 
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One thing to be mindful of is bad grounds from PCB to chassis. If one of the PCB grounds (and thus part of the PCB) is floating (is not making good chassis contact) it can cause strange symptoms like that....


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 Post subject: Re: Snakey/Wavy picture?
PostPosted: Nov Mon 18, 2019 7:44 pm 
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Electronic Memory wrote:
One thing to be mindful of is bad grounds from PCB to chassis. If one of the PCB grounds (and thus part of the PCB) is floating (is not making good chassis contact) it can cause strange symptoms like that....


I checked all the grounds on the main PCB - I checked continuity between the pins coming out of the PCB and the chassis. They test fine. A better test would be from components that should be connected to ground and the chassis - it would avoid the issue of broken traces and dry joints giving me a false sens of security! Mounting the PCB back on the chassis introduces a fair amount of stress on the mount points and other parts of the PCB.

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 Post subject: Re: Snakey/Wavy picture?
PostPosted: Nov Mon 18, 2019 8:50 pm 
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AC in the vertical sweep. From? Tube heater/cathode leakage, poor quality or wrong type of caps chosen for shotgunning in the vertical circuits. Bad chassis grounds, riveted connecting lugs are biggest offenders but also ground rings around tube bases.

Not a tube set? YOYO.

Chas

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 Post subject: Re: Snakey/Wavy picture?
PostPosted: Nov Tue 19, 2019 2:51 am 
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i vaguely remember a leaky afc dual diode causing a similar issue in an old admiral i got in the 70's


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 Post subject: Re: Snakey/Wavy picture?
PostPosted: Nov Tue 19, 2019 6:31 am 
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This set uses a 9BR7 tube for the dual diode horizontal AFC. Worth swapping if you have a spare


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 Post subject: Re: Snakey/Wavy picture?
PostPosted: Nov Tue 19, 2019 3:19 pm 
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@Chas: What might be the wrong type of caps? Also, AC in the vertical sweep? I had been assuming this was in the horizontal sync circuits?

Pretty sure I swapped out the 9BR7, but will double-check.

Well, the consensus is a grounding issue I guess! Could that be anywhere? e.g. the RF stage? Just trying to understand the scope of what I need to look at here.

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 Post subject: Re: Snakey/Wavy picture?
PostPosted: Nov Tue 19, 2019 7:34 pm 
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AC from the from either bad filters (but scoping the B+ lines would have shown this) or a heater cathode leakage seems the most likely cause.
But something else to check. Does this set have an AGC (Automatic Gain Control) adjustment? If so you could try adjusting that. If the signal passing through the IF strip is too large the sync signal can be squashed or partially clipped off.

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 Post subject: Re: Snakey/Wavy picture?
PostPosted: Nov Tue 19, 2019 9:46 pm 
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Usually when doing a horz osc set up part of it is defeating the hoz AFC.
If during that you you get a straight pix rolling side to side the problem
is in the AFC or just before it. H-K short also possible, dont trust a tube checker.
Last thing is video detector diodes can cause some strange sync & vid problems.

73 Zeno 8)
LFOD !


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