Monitor 1702 acting like it's degaussing alone

[giobbi]

Hello, my friends,
my 1702 has a strange behavior: every few minutes / seconds the image starts waving alone, like when you press the degauss button in some more recent CRT monitors.
What could it be? I'm not sure if it always did it or not, I got it recently, after many years sitting in my father-in-law attic.

 

[Hugo Holden]

Hello, my friends,
my 1702 has a strange behavior: every few minutes / seconds the image starts waving alone, like when you press the degauss button in some more recent CRT monitors.
What could it be? I'm not sure if it always did it or not, I got it recently, after many years sitting in my father-in-law attic.

A video of the problem might help. Currently my crystal ball has gone cloudy.

Or maybe there was more information in your post, but as James Bond said to Q "did you really have to use invisible ink ?"

 

[giobbi]

A video of the problem might help. Currently my crystal ball has gone cloudy.

Or maybe there was more information in your post, but as James Bond said to Q "did you really have to use invisible ink ?"

Hugo, sorry for the lack of information. I thought my explanation was clear, but it seems it was for me only.

I'm trying to capture the problem on a video (sometimes it take a while before it does that, I'm recording the screen right now) and I will post it here a.s.a.p.;
however, it's nothing more than I wrote.

Do you remember those old CRT VGA monitors with the "degauss" button? When you press the button, the image on the screen swings for a while, and then it suddenly stabilize.
My 1702 is doing the same thing, randomly.

Since I suppose there isn't an auto-degauss function in the monitor, I suppose something is going wrong

 

[Hugo Holden]

When a "waving" disturbance appears in the image, it is important to figure out if it is in the entire scanning raster , which it would be if there was a real degauss happening and in addition a vertical component, or if there was transient ripple on the B+ voltage supply feeding the H output stage and/or vertical output stages from the psu, this modulates the scan amplitude.

On the other hand, a signal disturbance can produce what looks like the same effect, the raster itself can be stable but the picture can move inside the raster time and produce ripple like effects, say if there was some instability in the H lock AFC system that locks onto the sync or hum modulation of the sync.. But turning the brightness right up it is often possible to tell if it is the raster or the picture.

Unstable power supply output can produce both simultaneous picture timing and raster scan defects.

So without actually seeing it, it is pretty hard to know where in the circuitry to look for the cause.

The number of waves that appear down the picture edge can also give a clue as to the origin, for example if they corresponded to line rate full or half wave ripple voltages, or say a higher frequency rate due to an H AFC disturbance.

It is the appearance of the screen that tells the thousand words, "waving" is only one.

After the video is posted can you post the schematic of the 1702, and we will see what sort of psu it uses.

 

[giobbi]

Hugo, many thanks for your detailed explanation.
I tried the whole afternoon to get the problem recorded: it happened only once, obviously when I wasn't recording.

As soon as I will get a video with the issue, I will post it here. Sometimes it happens many times, every few minutes, sometimes not.

You can find the schematics here: https://zimmers.net/anonftp/pub/cbm/schematics/monitors/index.html
more specifically: https://zimmers.net/anonftp/pub/cbm...ual_Preliminary_314004-001_(1985_Apr).pdf.pdf
and: https://zimmers.net/anonftp/pub/cbm/schematics/monitors/1701_Schematic.zip

I didn't find any consistent information about the difference between 1701 and 1702 (I have both and they apparently are the same monitor; the service manual seems to confirm that).
Mine are both PAL, 220v. Only the 1702 has the issue.

 

[Hugo Holden]

It has an analog power supply, it could possibly be ripple breaking through at times due to degraded main filter capacitors there, possibly a bad connection to the collector of the external pass transistor, might not be if everything is soldered and not on connectors.

It has a typical horizontal AFC loop with the standard anti-hunt network in the AFC loop, if these go awry it can cause some interesting similar effects.

Hopefully we will get a video of it and work out the likely place to look.

One thing to check, if a disturbance is injected into the AFC loop (for example a noisy wiper connection on the H hold pot) it can cause a brief oscillation due to the loop. make sure to exercise the pot in case it is something this simple.

 

[giobbi]

Hugo, thanks again. I didn't make a video with the issue yet, I hope I will find sometime this week. The monitor isn't cooperating with that, it's working for hours without any issue, now.
I'm sure it will become to show the problem soon or later; I'm just waiting for that moment...

 

[Hugo Holden]

Hugo, thanks again. I didn't make a video with the issue yet, I hope I will find sometime this week. The monitor isn't cooperating with that, it's working for hours without any issue, now.
I'm sure it will become to show the problem soon or later; I'm just waiting for that moment...

That does make it more suspicious of a power supply issue, perhaps the main filter caps are improving with time & use.

 

[giobbi]

Finally I got a clip about the problem. Sorry for the dark image, it's not easy to record it; I'm using the monitor attached to a VCR, just to make things not too boring LOL
Hope it can give you some clue about the problem.

It seems that the problem happen only when the monitor is "cold"; again, it seems to stabilize after some time. I'm not sure of that, actually. I'm testing it, trying to reproduce the problem, watching old movies every night for that ^_^

 

[Hugo Holden]

It may well be an oscillation affecting the pincushion correction. Both the H & V scans appear affected together, but more on the corners of the raster. Likely it is a power supply issue. It will need the scope put on the power supply output, first at least, to see if the power supply is stable, or not, when this disturbance occurs.

 

[giobbi]

Thank you Hugo; I will open it and put the scope. It will take a while, but I should be able to record the scope signal too.

BTW:

That does make it more suspicious of a power supply issue, perhaps the main filter caps are improving with time & use.

I didn't know it was possible; I "knew" that when the caps are faulty, they must to be replaced. But, considering the current behavior, it makes sense. This is really interesting, because those old caps are way better than the new chinese caps that will last a couple of years only... so if I can simply leave them in place instead of doing a recap, it makes me feel better.

 

[Hugo Holden]

Thank you Hugo; I will open it and put the scope. It will take a while, but I should be able to record the scope signal too.

BTW:


I didn't know it was possible; I "knew" that when the caps are faulty, they must to be replaced. But, considering the current behavior, it makes sense. This is really interesting, because those old caps are way better than the new chinese caps that will last a couple of years only... so if I can simply leave them in place instead of doing a recap, it makes me feel better.

With circuit voltages applied, the old caps, that have been sitting unused for years, tend to re-form (make new aluminium oxide on the dielectric layer) and their capacity increases, leakage drops and their ESR drops toward their previous manufactured values. This is not the case where the equipment has been used a lot over many years, in that case they dry out (or leak) and it is a one way process and extra use makes them worse rather than recovers them.

Some people have built capacitor re-formers to rehabilitate old stored electrolytic caps.