1972 Motorola CRT Monitor - Losing Focus

[MightyFrame]

Thanks to many of you here, I was able to successfully change a bad CRT on this 1972 Motorola Chassis that had "gone to air".

But now, new problems arise, apparently with the circuitry that drives the video signal on this chassis.

With 50-year-old components, I fully expect several, if not many, capacitors (or any components) to go bad. But which ones?

The Symptoms:
On first power up, I had a picture, but it appeared "smeared" and harder to read than I had hoped.

FirstPowerUp.jpg

After about 5 minutes, the image has seriously degraded, and while the smearing seemed to go away, it gradually was replaced with what appears to be a lack of focus, where each character now apppears to be a circular blob, and while it was hard to recognize them at the beginning, now it's impossible.

LostFocus.jpg

For reference we're looking at here are somewhat random characters generated by this ancient and rare computer terminal, which when properly displayed, should resemble this:



So, my question is this: What area of the driver circuitry controls these aspects? There is no "focus" control marked as such, and even if there were, I suspect that I shouldn't touch it, as this change occured without any adjustment. All of my research so far suggests that one or more components are failing, and any adjustment won't help until the component(s) is/are identified and replaced.

Thankfully, based on this schematic for this chassis, the circuit seems relatively simple:

Motorola Monitor-TV Chassis Schmetics

I'm hoping that a few of you vintage TV experts here could help me isolate the proper area of the circuit to start testing and replacing components for symptoms such as this.

FWIW, I fully expect, and will fully heed, every and all warnings about safely and completely discharging the high voltage section of this assembly before touching anything else. Especially since it seems likely that this problem is indeed related to the high voltage section of this circuit.

Thank you all for your kind and knowledgeable help.

Best,
AJ

[MadMan]

The hv won't kill you. It just hurts like a mother.

I can't really tell, is there a focus magnet ring on the neck of the tube? That's what I would assume. Cuz I feel like if it had electrostatic or electromagnetic focus, there'd be an adjustment for it. There'd have to be. Unless the focusing bit is part of the 'raster centering magnets.'

[MightyFrame]

Originally Posted by MadMan

..., is there a focus magnet ring on the neck of the tube? That's what I would assume. Cuz I feel like if it had electrostatic or electromagnetic focus, there'd be an adjustment for it. There'd have to be. Unless the focusing bit is part of the 'raster centering magnets.'

Thanks, ManMan, that's a really excellent question, as I'm still learning how these things work.

Here's close-up images of the yoke:

Back Of Yoke Right

Back Of Yoke Left

Top Of Yoke

Could you be talking about those metal rings with the tabs on the back-most end of the yoke?

Thank you!

Best,
AJ

[MadMan]

Yes, those are the raster centering magnets for sure. Now... they might also affect focus, I'm not sure. One thing to check first, make sure the whole yoke hasn't just slipped backwards. Failing that, you could play with the magnet settings. I would mark the original positions beforehand, though. However, those are not going to just change over a short period of time, like a drifting electronic component might.

My money is on those magnets, I don't know much about these types of monitors, but I don't think any kind of electronic focus was really a thing. Now, why it would start good and degrade is beyond me.

I suppose it could be an electronic problem though... I doubt focus. Maybe the cathode is being over-driven or something? Hopefully someone else will chime in here.

[zeno]

Start by checking the 87 volt supply. Schematic is fuzzy but it
looks to come from C25 & D5. C25 most likely & is a very common
fail on almost any solid state TV. Part of the problem may be the CRT
also but CRT's usually get better as they warm up.

good luck
Zeno
LFOD !

[MightyFrame]

Originally Posted by zeno

Start by checking the 87 volt supply. Schematic is fuzzy but it
looks to come from C25 & D5. C25 most likely & is a very common
fail on almost any solid state TV. Part of the problem may be the CRT
also but CRT's usually get better as they warm up.

Zeno, thank you for this pointer. I just checked this point on the schematic, and it reads 103v DC.

I take it this is higher than it should be, because you say it should be 87v?

Could this be the CRT overdriving now?

Thank you again!
Best,
AJ

[zeno]

According to the manual should be apx 87 VDC. Also be sure the
12 V you are giving it IS 12V. No probable cause for it to be high
but I would try the cap.


Zeno