Magnavox CD4756 (T989 chassis) dark bands down left edge

[James_S]

A couple months ago I found a 1974 Magnavox 25" console with a T989 chassis in a thrift store for 10 bucks and took it home with me. Initially it had audio but no picture, I discovered the xray protection was triggering and tracked that down to a big 190M power resistor that is part of a voltage divider monitoring the EHT which had failed open then started arcing. I replaced that and that got the picture back. Then I found that after about an hour the picture would fade darker and darker, I found that the CRT heater voltage was slowly dropping as a large oil capacitor associated with a ferroresonant transformer was getting very hot, replacing that fixed that problem. Now there are a couple of significant problems remaining that are stumping me. The issues are these:

With the B+ set to the specified 120VDC the EHT is nearly 35kV, substantially higher than the 30kV max that is on the label inside the set. Also there are some darker bands at regular intervals down the left edge of the screen that appear to be some sort of ringing. If I dial down the B+ to about 112V the EHT is at a reasonable level but the horizontal width doesn't quite fill the screen.

If I adjust the horizontal centering the dark bands move with the picture. I tried tacking an extra 2.2nf capacitor across the 3 safety capacitors across the HOT and that brought the HV down and allowed the B+ to be increased so that the width filled the screen however the dark bands were still present. I tried replacing all three of the safety caps and that made no difference at all. I found multiple ground connections where the pins for the plug-in modules connect to the chassis that had cracked loose so I resoldered those and it made no difference at all. I found C36 in the horizontal drive section was bad, ESR over 25 ohms, so I replaced that and it made no difference. Finally I went through and replaced all of the electrolytic capacitors on the PCBs, something I typically avoid doing, that made no difference at all. I replaced a handful of resistors that measured well out of spec, no difference. I poked around with a scope and didn't find any smoking gun. I'm not sure if the dark bars are due to something in the video signal, or ringing in the yoke modulating the scan velocity but looking at the waveform across the HOT as well as at the CRT cathodes failed to find any obvious issue. Anyone ever see something like this before? I don't know what else to try.

[Alex KL-1]

These ferro resonant is a 60Hz trafo in the mains line, or is in the HOT circuit? I've seen once a TV with resonant regulator in the H circuit.
OBS: resonant or... saturating (about the one in the H circuit)

[James_S]

Originally Posted by Alex KL-1

These ferro resonant is a 60Hz trafo in the mains line, or is in the HOT circuit? I've seen once a TV with resonant regulator in the H circuit.
OBS: resonant or... saturating (about the one in the H circuit)

The ferrorresonant transformer is only for the CRT heater, in models that came with a remote it also powers the remote receiver. Unfortunately the image size limit is only 900px so this may not be very readable but this is the schematic for the power supply section.

[Alex KL-1]

Originally Posted by James_S

The ferrorresonant transformer is only for the CRT heater, in models that came with a remote it also powers the remote receiver. Unfortunately the image size limit is only 900px so this may not be very readable but this is the schematic for the power supply section.

Interesting, thanks for posting.

[old_tv_nut]

If you adjust centering, both sweep ringing and video will move together, so this cannot be used to separate the cause. Instead, can you turn the H hold back and forth a bit to change the horizontal phasing? If the picture moves but the rings stay still, they are related to horizontal sweep somehow - however, they still could be multiple things, ringing in the sweep or a horizontal transient getting into the video.

Needing a bigger cap across yoke/flyback sounds to me like something wrong with the yoke or flyback.

Edit: I don't know what the H drive waveform should look like. Can you verify that it's the correct duty cycle? Is there a drive adjustment coil you can try?

[James_S]

Originally Posted by old_tv_nut

If you adjust centering, both sweep ringing and video will move together, so this cannot be used to separate the cause. Instead, can you turn the H hold back and forth a bit to change the horizontal phasing? If the picture moves but the rings stay still, they are related to horizontal sweep somehow - however, they still could be multiple things, ringing in the sweep or a horizontal transient getting into the video.

Needing a bigger cap across yoke/flyback sounds to me like something wrong with the yoke or flyback.

Edit: I don't know what the H drive waveform should look like. Can you verify that it's the correct duty cycle? Is there a drive adjustment coil you can try?

I'll check this when I have time and get back to you. I'm hopeful that the yoke and flyback are ok as it could be difficult to find a replacement for either one. The picture mostly looks pretty good and everything seems stable, it has run for hours and nothing gets super hot or seems to drift over time.

[James_S]

Originally Posted by old_tv_nut

If you adjust centering, both sweep ringing and video will move together, so this cannot be used to separate the cause. Instead, can you turn the H hold back and forth a bit to change the horizontal phasing? If the picture moves but the rings stay still, they are related to horizontal sweep somehow - however, they still could be multiple things, ringing in the sweep or a horizontal transient getting into the video.

Needing a bigger cap across yoke/flyback sounds to me like something wrong with the yoke or flyback.

Edit: I don't know what the H drive waveform should look like. Can you verify that it's the correct duty cycle? Is there a drive adjustment coil you can try?

Ok this is a very clever idea. I tried fiddling with the H-hold adjustment and the picture shifts back and forth while the dark bands remain stationary so that suggests that it is some kind of ringing in the horizontal scan. That also suggests that this and the excessive HV are related. I don't see any adjustment for the horizontal drive, I will look around with the scope a bit more in that area of the circuit and see if I can find anything.

I've worked on a lot of monitors and TVs over the years and I can't recall ever encountering this symptom, it's really got me puzzled.

[old_tv_nut]

Didn't realize this is a transistor set. Can you post a pic of the horizontal/flyback/yoke schematic?

[James_S]

Originally Posted by old_tv_nut

Didn't realize this is a transistor set. Can you post a pic of the horizontal/flyback/yoke schematic?

I thought I mentioned that but I see now that I forgot. I did post it in the solid state section of the forum though. It is a bit of an odd one compared to what I'm used to as it's a fully solid state set but an early design with a lot of aspects that remind me of older vacuum tube sets and it has a delta gun tube.

Hopefully these are legible, the max resolution and file size for attachments on this forum are seriously limiting, even for 20 years ago but it is what it is. The HV protection circuitry in this set is simpler than what is shown in the schematic and lacks Q303 and associated parts that will trip the HV shutdown if the beam current is excessive. C36 on the output of the horizontal drive transformer was bad and when I found that I thought sure it would fix it but replacing made no difference. C305A, C305B and C305C are the safety capacitors which I replaced but again that made no difference so I will probably put the original Sprague orange drop parts back in. The schematics do have some scope captures but they are teeny tiny images without a lot of detail and no indication of the bandwidth of the scope used to capture them or even the horizontal sweep setting and they don't even show the graticule. I have looked at more of the signals with the scope than I posted here and everything I checked looked fairly close to what it shows.

[old_tv_nut]

If you have replaced all the caps in the H sweep circuit it must be something else. I don't know, check caps in horizontal oscillator and amp sections?

Cracked circuit/solder traces taking one of the safety caps out of the circuit? Can you check connectivity from the horizontal output collector to the lead of each safety cap?

I was going to suggest looking for a capacitor mounted on the yoke, but I don't see one on the schematic.

[James_S]

I checked the safety caps and they're all in the circuit, they're distributed around the horizontal/HV subchassis and a bad connection would result in other issues. I've replaced almost all of the electrolytic caps in the whole set except for a few that are on the chassis and those test ok in circuit. I don't normally do shotgun re-capping but I was running out of ideas. I wondered if there was a damper resistor somewhere that had opened but I can't see anything on the schematic, I don't see any components on the yoke. I'll keep looking.

[old_tv_nut]

I'm stumped now, as the only things that came to mind are not enough capacitance on the flyback/yoke (now thoroughly checked), or maybe wrong duty cycle drive pulse to the horizontal output (?).

[Alex KL-1]

I see the image issue again in the post#1...
One color TV of mine have this bars, but are more apparent if I adjust the H.size coil for less width/less overscan. For factory adjust, only is possible to see the last one bar.
I also saw it with a B&W tube Telefunken set, perhaps related to linearity control.
My Admiral '60 tube TV (recently working!) also have some of these bars.
Some systems have it "inherent"/more evident due to manner that linearity, width, flyback, yoke (and convergence) interacts. Supposedly, the bars are to be outside to visible area...

Of course, your TV have the H. size issue, so this is a misterious for now (I also don't have a clue about the real culprit here).