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  #61  
Old 08-28-2017, 03:55 PM
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Given the rate of wobble I'd reckon that the power supply filtering is insufficient, your AC outlet voltage is fluctuating, or some component is shorting/leaking/arcing intermittently at a regular interval.

I've seen problems with AC utility voltage fluctuations causing picture size to change 1 a minute to ~3 times a sec. There is a business park about 1 mile away with manufacturing operations there, it was worst for the first 2 years I lived there, but still happens occasionally....I knew it was the power line since I could have 2-3 working tube era sets and one new set fed the same signal source on at once, the 3 tube sets would 'wobble' identically, but the new set would not wobble (due to having actively regulated B+ rails)...The cure was to run the tube sets through a SOLA VRT (AC voltage regulating line isolation transformer).

As for contrast check the video amplitude with a scope and compare to the schematic. If it is low there are a number of things that could be affecting it.

Your AGC may be tied to the low contrast issue....For instance the tube IF grids should be biased negative enough relative to the cathodes to prevent conducting more than their max rated plate current....If the IF tubes are weak and not amplifying enough then the AGC could be be pushing the grids as positive as the biasing design wall allows...When it hits the wall it can't make any more gain, but still it trying to.

It might be interesting to connect that external bias supply back to the AGC line, see how much more contrast you get and how the IF and tuner voltages (especially the AGC controlled ones) compare to the schematic with the external AGC voltage.

May also be good to check the sync amplitude entering the vert mult/osc system. Bad vert hold can be weak sync or osc off freq.
If you can get the vertical to roll both ways (up and then down) slowly, even 'float' without rolling but not fully synced for a while, then the problem is in the sync pulse level/cleanliness and or the osc circuits handling of it.
If vertical only rolls one way then it is off freq and the vert osc/mult/output stages are to be suspected (and sync signal can be ignored till float/dual direction roll is achievable).
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  #62  
Old 09-10-2017, 08:31 PM
FrankieKat FrankieKat is offline
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I've gone through and checked through all scope traces and voltages several times and everything is generally within spec, except for a few places.

The biggest anomaly is that the grid on the sound IF / sync amp should be, according to the schematic, -1.8V however I measure between -0.3 and -0.5V. The plate voltage should be 60V but I see about 54V when the set is cold, and it falls down to about 45V. When first powered on cold, I get actually watchable contrast and fairly strong sound, however... after it warms up for about 5 minutes the contrast, sound and to a lesser extent sync gets worse (as that plate voltage goes down).

I have tried no less than six different 6AN8 tubes and all the good ones are the same. Have verified and/or replaced all nearby resistors (nearly of the originals were still within 20% spec) and replaced all but the mica and ceramic caps. And still absolutely nothing from the AGC control. The DC voltages on the IF stages are pretty much right on.

What could be the temperature sensitive components that would cause that?

FK
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  #63  
Old 09-11-2017, 09:48 AM
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Resistors are usually among the more temperature sensitive parts. Usually the best way to find a thermal sensitive part is to run the set till symptoms manifest and hit sections with freeze spray (or something similar like canned air) to see what makes it clear up. Start with general coverage of suspicious areas and once you get a good reaction to the spray, target specific components in the area of sensitivity to narrow it down.

Looking at the schematic you attached earlier I'm not seeing the -1.8 volts grid or 60V plate specs on either section of the 6AN8 sound IF/sync separator tube...Are you referencing a different listing of voltages? Also on multi section tubes like that one it is good to specify which section's plate and which sections grid when you say one is not right so we know which stage to look at.
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  #64  
Old 09-12-2017, 01:16 PM
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Resistors are usually among the more temperature sensitive parts. Usually the best way to find a thermal sensitive part is to run the set till symptoms manifest and hit sections with freeze spray (or something similar like canned air) to see what makes it clear up. Start with general coverage of suspicious areas and once you get a good reaction to the spray, target specific components in the area of sensitivity to narrow it down.

Looking at the schematic you attached earlier I'm not seeing the -1.8 volts grid or 60V plate specs on either section of the 6AN8 sound IF/sync separator tube...Are you referencing a different listing of voltages? Also on multi section tubes like that one it is good to specify which section's plate and which sections grid when you say one is not right so we know which stage to look at.
Sorry about that, I meant V7B. Attaching a close up of that part of the schematic. I picked up a fresh can of duster and will try cooling off some components to see what might be heat affected.

A few discoveries I've found:
  1. By fiddling with all of the controls I can pretty much dial in most of the DC voltage measurements called out in the schematic (thought it only produces pretty much a bright white raster). The exceptions are the grid of V7B that measures -1.8V and no matter what I do the range is about -0.1 to -0.5. V11 (6CS6 SYNC SEP) pin 7 shows -10V and closest I can ever get it to is about -2V.
    The scope trace shown for the grid is somewhat confusing to but I assume I'm looking for some kind of beat on a 60Hz pulse there? (See attached)
  2. By feeding in an incredibly strong signal (from the Blonder Tongue) I can get the plate voltage on V7B up to 60V, however at that point the signal is incredibly overloaded and has a negative image and poor sync (both horiz and vert)
  3. The scope traces on the AGC keying (V8A) look consistent with the traces on the schematic. I cannot measure the plate on it since the voltage is beyond the range of my scope and can't measure DC since those huge pulses confuse my DVM, however visually it seems to be correct. G1 (pin 8) measures pretty much the same as the plate of V7B as the schematic suggests (see attached)
  4. Adding -9V bias to the wiper of the AGC control has what would be the effect of the AGC control, and at that point the AGC control actually affects the picture. However, the control has no effect without it.
  5. Sound, contrast and sync continue to degrade as set warms (will try freezing components tonight)
At this point, is it possible that some of this could be alignment-related? The multiburst bandwidth test from the VA-62 looks pretty close to the trace from the manual (see attached) and there are distinct vertical lines on screen up through 3.0MHz, so would that imply alignment is healthy?

Thx!!

FK
Attached Images
File Type: png 0294-11n-3-v7b.png (93.2 KB, 8 views)
File Type: jpg 0294-W5_0910-1.jpg (101.3 KB, 11 views)
File Type: jpg 0294-W4_0910-1.jpg (91.9 KB, 9 views)
File Type: jpg 0294-MBBW_0910-2.jpg (113.1 KB, 13 views)

Last edited by FrankieKat; 09-12-2017 at 01:29 PM.
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  #65  
Old 09-12-2017, 02:12 PM
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On V7B: Tube biasing 101 says the less negative the grid the more plate current is drawn, the more plate current the less plate voltage (at least in a common cathode circuit like V7B)....If your grid bias is off it will effect the plate.
I'd like to see more of the schematic to the right of V7B. The grid circuit wanders off the edge of your attached scans...Seeing where it goes could give some insight into why it is not negative enough.

There is a slight chance the video detector diode is going bad (I've seen it in some TVs from time to time) and reducing video amplitude.

The AGC wiper appears to basically be the start of the AGC bus that biases the tuner and IF grids. If the control is dead till a voltage is injected it probably means the AGC circuit is not working. If all the tube voltages look right I'd be looking that fly winding between the plate of the AGC tube and the AGC control.

Unless you can get proper/much better contrast by injecting -9V at the AGC control (and adjusting the control) I think the AGC is not the main issue....Even if you can I'm still a bit more concerned about V7B.

With what I see I'm not inclined to think it is an IF alignment issue....And even if it is part of the problem, you need functional AGC and to work out the issue with V7B first before any alignment can be attempted. If you try to align circuits with defects the results often are not an improvement over the defective circuit as it was.

I'm running on ~3 hours of sleep presently so take this advice with a grain of salt...
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  #66  
Old 09-12-2017, 02:59 PM
FrankieKat FrankieKat is offline
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On V7B: Tube biasing 101 says the less negative the grid the more plate current is drawn, the more plate current the less plate voltage (at least in a common cathode circuit like V7B)....If your grid bias is off it will effect the plate.
I'd like to see more of the schematic to the right of V7B. The grid circuit wanders off the edge of your attached scans...Seeing where it goes could give some insight into why it is not negative enough.

There is a slight chance the video detector diode is going bad (I've seen it in some TVs from time to time) and reducing video amplitude.
Attached the second page of the schematic. Makes sense that the issue could be in the detector since the DC voltages on the IF stages are spot on. I assume I'm going to need to dig up some germanium diode or equivalent, but worth trying.

Quote:
Originally Posted by Electronic M View Post
The AGC wiper appears to basically be the start of the AGC bus that biases the tuner and IF grids. If the control is dead till a voltage is injected it probably means the AGC circuit is not working. If all the tube voltages look right I'd be looking that fly winding between the plate of the AGC tube and the AGC control.
Yeah, checked that -- the flyback winding is 5 ohms, right on the schematic value. Glad it's not that at least! Since I am seeing those 15,750KHz pulses on the plate of V8A, would that imply the AGC is at least partly working?

I've replaced every passive component on the AGC circuit between the grid of the 1st IF and the flyback. The control works perfectly too... no jumps and smooth resistance all the way across. I suppose I could try changing out some of the small mica and disc caps in the adjacent circuits.

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Originally Posted by Electronic M View Post
Unless you can get proper/much better contrast by injecting -9V at the AGC control (and adjusting the control) I think the AGC is not the main issue....Even if you can I'm still a bit more concerned about V7B.

With what I see I'm not inclined to think it is an IF alignment issue....And even if it is part of the problem, you need functional AGC and to work out the issue with V7B first before any alignment can be attempted. If you try to align circuits with defects the results often are not an improvement over the defective circuit as it was.
I've tried at least six different 6AN8 tubes that have all tested way-off-the-charts-good and all produce the same results, so not a tube. Adding the bias certainly affects the contrast but not really better. It's more like if you turn the AGC too far where it whites out the picture.

Quote:
Originally Posted by Electronic M View Post
I'm running on ~3 hours of sleep presently so take this advice with a grain of salt...
No problem at all! I really appreciate all of your help and thoughts!

I'll try changing out that diode and see what that does. I think I have some low forward voltage drop diodes... somewhere...

FK
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File Type: pdf 0294-11n-4.pdf (151.1 KB, 9 views)
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  #67  
Old 09-13-2017, 10:35 AM
FrankieKat FrankieKat is offline
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There is a slight chance the video detector diode is going bad (I've seen it in some TVs from time to time) and reducing video amplitude.
I pulled out the old diode and tested it and it was showing a 0.7V voltage drop one way and 0.1V drop the other way, so it was definitely suspect. I changed it out with a new 1N60 and it seems to have made a difference in terms of how the signal degrades as the set warms. Contrast and sound seem more stable as the set warms. No change to the grid or plate voltages on the V7B tube though.

What I do see is that there is a slow brightness increase over 5 mins or so when starting from cold. As in, having adjusted the bright/contrast the night before, it will start very dim - almost not visible. Could this just be the age of the CRT and it's poor life test?

I can set all of the controls so that all of the voltages on the CRT are exactly on spec, and this is how the picture appears https://youtu.be/jzV57jRqrcI. I can adjust for better picture, but it changes the cathode and g1 voltage by 50-60V away from the schematic spec. Does it make logical sense that I would want to keep the controls set to produce the schematic voltages on the CRT because that will always be it's optimum operating point? As in, this should be the best picture, and there's something else causing it to be whited out like this that still needs to be found.

FK

Last edited by FrankieKat; 09-13-2017 at 10:40 AM.
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  #68  
Old 09-13-2017, 11:25 AM
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When a CRT is weak or old it can have a long warmup time to it's stable operating point. I'd guess that if you put that CRT on a tester and watched it's emission/cutoff readings it would probably change over that same 5 minute interval.

I've got a color set with a weak CRT with poor life. All guns will start dim and the weakest one will take the longest to warmup and stabilize (which makes the colors look weird for a bit).

Generally speaking the CRT gun voltages listed in the schematic are the correct settings for a new CRT (because that is what they would have had when recording that data). The optimum CRT grid and cathode voltages are the ones that correspond to the contrast and brightness settings that look best. Those settings will drift as the CRT ages from use.
Keep in mind that you want to NOT* adjust the picture for the brightest possible image with good contrast (doing that drives the CRT hard and given it seems to be a tired CRT that will kill it faster than normal). You want to adjust for the dimmest picture (with good contrast) you deem comfortably watchable (at least if your concerned about CRT life).

*Unless you plan to ride the CRT hard till it dies, and then replace it...Some people do that when they have a replacement on hand.
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  #69  
Old 09-13-2017, 01:24 PM
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When a CRT is weak or old it can have a long warmup time to it's stable operating point. I'd guess that if you put that CRT on a tester and watched it's emission/cutoff readings it would probably change over that same 5 minute interval.

I've got a color set with a weak CRT with poor life. All guns will start dim and the weakest one will take the longest to warmup and stabilize (which makes the colors look weird for a bit).

Generally speaking the CRT gun voltages listed in the schematic are the correct settings for a new CRT (because that is what they would have had when recording that data). The optimum CRT grid and cathode voltages are the ones that correspond to the contrast and brightness settings that look best. Those settings will drift as the CRT ages from use.
Keep in mind that you want to NOT* adjust the picture for the brightest possible image with good contrast (doing that drives the CRT hard and given it seems to be a tired CRT that will kill it faster than normal). You want to adjust for the dimmest picture (with good contrast) you deem comfortably watchable (at least if your concerned about CRT life).

*Unless you plan to ride the CRT hard till it dies, and then replace it...Some people do that when they have a replacement on hand.
Interesting and got it. This CRT is clearly high hours since it is the original and was pretty dead when I got it. I zapped it with the rejuv on the CR-70 and it brought it back to life with good emissions and cut off, but yes the life test was poor. I consider it borrowed time and I don't plan to make this a daily use set. It sounds like this can explain some of the remaining warming up issues though.

FK

(attaching bonus pic of original Ratheon diode)
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  #70  
Old 09-14-2017, 10:47 AM
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Well, this set is just full of surprises. I took the cover off of the tuner to double check all of the resistors and voltage measurements, but it's really tight in there and wasn't able to safely probe everything. Resistors were all good and voltages seemed a bit high and I wasn't really watching the picture and so put it back together and turned it back on.

Good news:

So... what happens next is that for no explainable reason, the picture has massive contrast, sound is loud and clear and has flawless sync all the way across both controls. What?? And... the AGC control now actually does something. It doesn't go all the way from no picture to white picture/no sound, but it does have a noticeable affect on brightness.

Odd news:

The voltages that were previously low or in question are now massively high. The 130V B+ supply is now measuring about 175V -- despite the fact that the two other B+ voltages right above it in the power supply are still right. The V7B plate that was supposed to be 60V and was previously measuring as 45V is now measuring 95V, and the plate and grid voltages on the IF tubes which are supposed to be 115-120 are now measuring 160V. The grid 3 on V11, that's supposed to be -10V, that was previously measuring -1V is now measuring -10.5V. Oh, and now adjusting all of the CRT voltages to the schematic is right about where the best, brightest picture is.

This is how it looks, though I was able to dial it in even better with the focus, fine tuning and width controls later. The video doesn't even do it justice -- this thing looks like a brand new modern TV in person.
https://youtu.be/dKXksyLwGqs

Note the sharpness and clarity of the text:
https://youtu.be/6pDbjJgIk1Q

Bad news:

Of course it doesn't like to be run with those voltages. After about 5-10 minutes, the picture starts to take hits like this. And yes, I turned it off as soon as I recorded this:
https://youtu.be/rYIgDQ-g3ZE

The only thing that was actually changed recently was that detector diode, and this was not happening right after I did. Previously the B+ supply voltages were all dead-on correct with the schematic and the IF's being fed from the 130V line were all correct while the sync circuits fed from the same 130V were all too low. I've never seen a situation where only one single part of the power supply divider goes from being right to being 30% high, unless a significant load was removed. If that were the case, and a tube that was drawing a lot of current got disconnected why in the heck would the set all of the sudden start playing WAY better?

Has anyone ever seen something like this before?

Very befuddled,
FK

Last edited by FrankieKat; 09-14-2017 at 10:52 AM.
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  #71  
Old 09-14-2017, 11:02 AM
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Have you cleaned the contacts in the tuner? I've seen dirty tuner contacts do stuff like that.

If you have not cleaned the tuner contacts make sure to get both the ones involved in changing channel and the tube socket pins.

I recommend Deoxit, or the cheaper comparably effective De-ox-id as the cleaning solution. If you can rub the contacts with a pencil eraser (the soft white ones are especially good) while they have the cleaner on them it will be better still. Those erasers are almost good enough contact cleaners by them selves to not need the spray.
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  #72  
Old 09-14-2017, 11:21 AM
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Have you cleaned the contacts in the tuner? I've seen dirty tuner contacts do stuff like that.

If you have not cleaned the tuner contacts make sure to get both the ones involved in changing channel and the tube socket pins.

I recommend Deoxit, or the cheaper comparably effective De-ox-id as the cleaning solution. If you can rub the contacts with a pencil eraser (the soft white ones are especially good) while they have the cleaner on them it will be better still. Those erasers are almost good enough contact cleaners by them selves to not need the spray.
I had cleaned the channel changing contacts earlier with WD40-branded spray electrical contact cleaner (I ran out of deoxit), and have worked and rotated around the channel clunker quite a bit since and never saw anything like this happen. I've wiggled the OSC and RF tubes in the tuner and didn't see any reaction, but who knows. I can try cleaning the tube pin contacts next, but now I'm afraid if I touch it that it might go back to the lower voltages and sub-par performance!

Clearly getting the sync and AGC circuit voltages up directly affected all of the remaining problems so at least there's some confirmation there. I'll trace out everything from the 130V on and see what's up...

FK
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  #73  
Old 09-14-2017, 12:37 PM
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My guess is dirty contact or loose connection in the tuner. The issue with AGC and such is probably being caused by that connection and the issues in the 3rd video are probably it trying to relapse.
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  #74  
Old 09-15-2017, 12:50 AM
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I've had noisy carbon resistors cause intermittent grief. They check o/k with a meter but break down when passing current. Since it was the tuner that you disturbed before it came good it might be worth replacing the carbon resistors in it. There shouldn't be many in it but access will be tight. Be careful not to disturb coils and feed through caps etc. The turret will need to be removed to gain access. I'm not a fan of WD40 for electronic work. Use Deoxit as it leaves minimum residue. If you have a spare tuner from another similar vintage set even better. Doesn't have to be physically the same just electrically close enough.
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Old 09-15-2017, 10:01 AM
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I've had noisy carbon resistors cause intermittent grief. They check o/k with a meter but break down when passing current. Since it was the tuner that you disturbed before it came good it might be worth replacing the carbon resistors in it. There shouldn't be many in it but access will be tight. Be careful not to disturb coils and feed through caps etc. The turret will need to be removed to gain access. I'm not a fan of WD40 for electronic work. Use Deoxit as it leaves minimum residue. If you have a spare tuner from another similar vintage set even better. Doesn't have to be physically the same just electrically close enough.
Wasn't regular WD40 lubricant, it was their electrical contact cleaner that's just typical electrical contact cleaner with a fancy nozzle. And yeah, I'm very careful around those wire coils that had been hand tuned by carefully forming and pinching in the factory.

I spent some time last night cleaning the tuner contacts and tube pins and so far what I'm seeing is that when all wafer switches are making good contact I'm getting great performance but with the over spec voltages. I tested tube voltages and everything that is fed of the 240V B+ is right about on, and everything fed by the 130V B+ (the one measuring 170V now) is proportionately high. It doesn't seem like it's a bad contact on the tube socket since all voltages are present and no glitching at all if I wiggle the tubes as would normally be associated with dirty connections.

Meanwhile though, the thing is still producing a beautiful picture and sound (though I know at those voltages I'd be cooking tubes alive). This is with CRT pin and high voltages dead-on schematic measurement, so it's not even being pushed out of spec. Not bad for a tube that was found with a brightener (which is of course no longer on) and had virtually no emissions and zero cutoff. That CR70 rejuv zap is incredible!

https://youtu.be/MYtxY-Eanrk

I'll keep cleaning and maybe try replacing some resistors best I can without having to pull the tuner totally apart.

FK
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