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  #61  
Old 07-02-2017, 06:13 PM
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VintagePC VintagePC is offline
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Certainly you can wash out electronics with distilled water without harm... and it works better at air than removing dust and other debris. If you don't let it dry, or if you use water with minerals, that's when you get issues. Would I hose out my scope? No. Would I rinse a particularly nasty PCB given the right facilities and a supply of DI water/IPA? Sure.


Some interesting developments. I've re-coated the flyback with a total of three coats of dope and bake-cured it. Reinstalled it with new wires.

Powered up the set to verify all was well; after a few minutes, the image goes all jittery, dims, I hear a *plink* and I lose HV entirely. No 15KHz whine from the HV area either. Strange.

Pull the output tube, re-verified my wiring, and start measuring. Both 200V and 300V B+ are fine. Horizontal drive waveform on the HOT is fine, so no issues with the oscillator.

Odd.

Plug the tube back in, start set again. Hear the HV come up, get a picture. After about 30 seconds, picture jitters, blooms and fades, and the HV shuts down again with another *plink* noise.


One thing I take away from this is my HV woes may not have been flyback related, but rather wonkiness on the part of the horizontal drive... but I sure feel a lot better knowing the fly is likely good to go for much longer now that it's rid of the wax and coated with a better material.

Now I'll need to study the output stage in detail and figure out what's going funny. Likely going to be tricky as my scope and probes are only rated for 600V input so I can't go probing willy-nilly.

Anyone have any knee-jerk reactions as to the cause? You might save me a few hours.

Last edited by VintagePC; 07-02-2017 at 06:20 PM.
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  #62  
Old 07-03-2017, 09:16 PM
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Progress. Having found no funny components in the area, I realized I don't need direct contact to observe the HV waveforms. Hooked a small loop of wire around the HOT plate lead and to my probe... ta-da, there is the flyback pulse for easy viewing, and at a safe input level for the scope.

It occurred to me the symptoms above are very similar to the issue I had with a dud HOT I bought; it would work on startup but fail shortly after. Did some tube swapping and now have some slightly burnt fingers (Laboratory lesson #1: Hot glassware and cold glassware look identical until you pick them up. Then the hot glass is broken on the floor) and observations to show for it.

Here's what I saw:
1. Known "dud (?)" 6AV5: Oscillator comes up, along with HV pulse. There's some chitter on startup that I get in all tube combos. Seems to be caused by the HOT waveform getting out of sync with the Horizontal drive...?
Once the picture comes up, the height of the HOT pulse wavers a bit, and then it all dies (H. Oscillator shuts down too). Killed power.

2. Original 6AV5: HV, oscillator come up. Also chitters a few times as they sync up. HV pulse wavers quite a bit in sync with all the picture funny business I've been trying to troubleshoot. Sure enough, *plink* and the HV pulse disappears. H. Osc. stays running this time.

3. New 6AV5: Comes up as expected, pulses stablilize. Doesn't go out, but still funny business with the picture flickering and pulse jumping around. Shut it off and I notice the spot stays on the screen a LOT longer now. Maybe picture is a bit brighter and more in focus too, so I almost certainly had a weak HOT.

4. Swapped the damper. Lo and behold, my wavery HV issues seem to be gone. Nary a flicker after a minute or two whereas before it would have been acting funny already. The arc from discharging the HV (5meg resistor so it's a bleed-off, not a bang) also seems much stronger now.

Looks like we can chalk it up to a flaky damper diode as well. I'll swap my original 6AV5 back in to see if it's still funny, perhaps it was the damper all along.

Weird I didn't see this previously when I swapped some tubes, but at the same time, not surprising if it was convoluted with some flyback issues and a dirty rectifier socket.

I'm questioning if that 6AV5 I thought was a dud really is... the HV pulses on it were quite strong compared to the other two tubes. Calling it a night for now, but will do some more swapping tomorrow to find out once and for all.

One final observation is I'm now hearing a soft ticking coming from somewhere. Best I can pinpoint by ear is either the HV anode lead or yoke, the former being far more likely. I'll give it a good cleaning tomorrow, could just be some crud causing the odd arc

Last edited by VintagePC; 07-03-2017 at 09:21 PM.
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  #63  
Old 07-04-2017, 03:39 PM
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Observed something interesting. The chittering I hear on startup is the horizontal oscillator flipping out for a few moments. This is with the horizontal output tube removed, so the fly is not involved. (Still does it with the HOT installed, you just hear it more)
Anyone seen this before? Doesn't seem to matter if it's a hot start or a cold start, it always does this.

I scoped the waveform. The second trace is just on the B+ for reference so I can see when it comes up.

https://youtu.be/1-Gz7kmtnkY

Last edited by VintagePC; 07-04-2017 at 03:43 PM.
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  #64  
Old 07-04-2017, 09:01 PM
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By pausing the video I see what seems like a series of three near normal cycles and then a short one. Also it seems like the signal is riding on a slower sawtooth, maybe there is a circuit with that longer time constant. It would be interesting to see what the sync pulses are doing during this time.
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  #65  
Old 07-04-2017, 10:09 PM
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Interesting thought. I'm in the process of following the pulses back up the chain, so I'll keep an eye out. Maybe there's a leaky mica cap somewhere. I was looking at the sync pulses earlier, but they were a little non-sensical; I suspect I need to trigger off the vertical instead of the horizontal for them to make sense otherwise I see what I think are artifacts of the blanking period in between. Might be tricky with just a 2 channel scope but if I can get the EXT trigger to work it should be doable.

Should comment that this video was with the oscillator free-running (no video input) but if there is an input it still behaves the same. I'd be less surprised if it did this only with input as it could be explained by the sync circuits pulling the oscillator into phase.
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  #66  
Old 07-05-2017, 02:09 PM
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Oh, OK, no video input. I thought that maybe the sync pulses were screwy during start up and effecting the oscillator.
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  #67  
Old 07-08-2017, 03:36 PM
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No appreciable progress on the funny behaviour, I think it is simply how the 6SN7 warms up. It's definitely not caused by the sync pulses and is siightly different if I swap around the 6sn7s in the set.

I also disassembled the yoke to investigate the ticking I was hearing... quite possible it was arcing to the dag as the varnish on the windings is cracked and crumbly. Gave it a coat of corona dope and letting it dry now.

Also changed the 560 ohm resistors in the vertical section, they were way off at over 700 ohms each.

There's a 1.2k in the horizontal section that someone seems to have swapped for what's supposed to be a 1K. Dunno if that affects the width much, so I'm debating whether to change it back or not.
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  #68  
Old 07-09-2017, 01:22 PM
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Especially in those circuits You want to stay as close to specs as possible. If your oscillator is way off it will overload the circuit from the wrong pulse, and, picture can shrink.

Is there a large tapped resistor in, or, near the high voltage? If so If that resistor opens, it will raise havoc. That resistor is a bypass for the damper tube.
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  #69  
Old 07-09-2017, 04:34 PM
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No, don't see any resistors like that. Replaced the 1.2k in the yoke with the correct 1k, as well as another 15k that someone had swapped in a 12k for.

Still no luck though; width coil is doing nothing and the picture is still too wide.

Something's still funny with the horizontal as well; it's got a tendency to wig out and drift as things warm up... even with a new damper and output tube. Strange...

Edit:
AHA! progress! I was trying to get horizontal in sync and accidentally whapped the chassis with my screwdriver. Whaddya know, it makes the horizontal drive go funny, and come back again if I hit it again. Guess there was something to the old stereotype of whacking the side of the TV to get it to sync! *grin*

At least now I know I am on the hunt for a funny/bad connection.

Last edited by VintagePC; 07-09-2017 at 04:54 PM.
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  #70  
Old 07-09-2017, 07:36 PM
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I thought you said picture was t00 short??
00000000000000000000000000 Are you getting blooming when you turn up the brightness?
Is picture also out of focus.??
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  #71  
Old 07-09-2017, 09:35 PM
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No, focus is fine, height is fine. Maybe a little bit of bloom but it is typical for what I recall.

Main issue is funny business in the horizontal section; it's going out of whack and drifting a lot. Possibly related symptom is that the width control does not do anything. Picture is too wide.

I did also see there's a strange vertical behaviour on startup; huge divots in the top right and bottom left of the raster that gradaully stretch out to normal as the system heats up.

Flyback and power tubes also seem to get pretty warm, don't think that's normal but probably also related to my horizontal issues. HOT isn't red-plating or anything. I'm out of ideas, I'll get a video tomorrow and upload it, perhaps actually seeing the symptoms means someone can point me where to start looking.

I did check the system (300V B+ supplying sweep circuits) B+ current, it's is running around 125mA or so, not excessive. Probably reasonable given the max plate current of the 6AV5 is 110mA and there's a few other things on that B+ line.
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  #72  
Old 07-10-2017, 09:18 AM
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Were those different value resistors obviously changed after the set was made or are you assuming they were changed after the sets was made based on them not matching the schematic?.....If the latter you may be making a mistake.

I've beat this drum before, and I probably will again: Electronics of that era usually under went several production changes between new chassis designs, Schematics of the day often only show one revision and do not tell you if what is in your set was a change for the good or an old flawed design....Add to that schematic sources are known to contain typos (Sam's is especially infamous). Use the schematic as a crude guide/overview and take it with a grain of salt, but never as the Bible for your set.
Unless the solder joints, brand/vintage of part, installation methods clearly show it was replaced after leaving the factory it is usually best to assume a production change and leave it as is. At this point questioning production changes is a valid part of troubleshooting, but note the effects of changes, and note how it was so that if you need to go back to factory configuration, you can.

If the sides of the raster curve in and that curve rolls vertically at a rate of ~11 seconds with respect to vertically synced signal on screen then you have 60Hz hum leaking into your deflection stages....Which could be caused by B+ filtering/supply issues, grounding problems, Heater to Anything shorts in tubes, among other things.

If you have a variac and the set will maintain a raster when turned down to achieve slight H under scan then you can use that to check the sides of the raster for pull in after warmup.
If you have a large amount of hum leakage in the deflection stages (power supply hum will be worse) it can cause odd symptoms like disturbing oscillators during warmup like you describe.
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  #73  
Old 07-10-2017, 05:23 PM
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Sorry, I wasn't clear on those divots, they are irregularities in the vertical deflection, not the horizontal. If I can catch it on video or camera I'll post a picture tonight. Does not look like 60hz hum, the horizontal drive waveform is clean once B+ is up but before the tube warms up as far as I can tell.

Re: parts changes - I'm going off reasonable suspicion it's not right. The 560s I swapped were not "wrong parts" those were marked correctly but just way off value, as is common for old carbon-comp ones in a hot environment.

The 1.2K I swapped out in the yoke was also definitely not factory, had the same crusty electrical tape bodge-job as the work done elsewhere in the set.

The 15K was of a different type of resistor than the majority in the set. Most of the carbon comps were smooth glossy brown bodies, this one (and a few other ones that had drifted) had a very different body texture to them; pretty beat-up and pitted surfaces too like they had been floating in somebody's junk drawer for a while).

I hear you on not taking the schematic as bible, but for this set it is certainly something I need to give a little more credit than normal when working in areas that are not behaving properly. The last "tech" that worked on it couldn't be bothered to replace the bad 6AX5 rectifier and instead bodged the separate B+ systems together, probably making a bunch of these changes to correct for the voltage differences (including the doubled-up fuse hackery for the higher current draw)
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  #74  
Old 07-10-2017, 06:14 PM
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Here's the vertical "divot" issue. You can see it top right and just a hair on the bottom left. Yes, the yoke is slightly tilted but I have not been able to get a stable pattern up to fix it yet.



Also here's the video of the sync issue. Sorry for the potatovision, apparently stabilization does not like unstable CRT traces.

The tapping you hear is me hitting the side of the chassis with a screwdriver handle. Sounds like I'm hitting it pretty hard, but it's probably about the same amount of force I'd use to crack an egg, for lack of a better comparison.

https://www.youtube.com/watch?v=z0v4...ature=youtu.be

The odd thing here is if I flip the chassis on its side, it's better. It just won't work properly if I have it sitting right-side up.
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File Type: jpg 20170710_183154_sm.jpg (29.3 KB, 20 views)
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  #75  
Old 07-10-2017, 07:35 PM
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If I am looking at the right thing, those divots look more like some sort of beam shadow in those corners. I wonder if the ion trap or the like is aligned wrong.
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