C128 flickering video (40 col. mode)

[Divarin]

Hi all, recently I posted about a different issue with this computer and well that issue seems to be solved after replacing the VDC.

Now this C128 has a new issue. Actually it's an old issue which I've just kind of put up with but has now grown to the point where the system is, sometimes, pretty much unusable in 40 column mode.

The issue is that the VIC output flickers between a bright/crisp picture to a dark/muddy picture. This is what you might expect if:
A) The video cable (chroma/luma) has a break in it -- That's not the issue because I use that same cable on my C64 with no problems.
B) The video jack on the 128 is oxidized or has a cold solder joint -- That's not the issue because I've reflowed the solder joints on that connector and sprayed deoxit in the jack, also wiggling the cable has no effect.

It used to only do it when the system was first started up (cold), but by the time I loaded StrikeTerm or whatever I was loading it would "settle down" and work (hence me putting off looking into it).
Now, although it's still intermittent, it's much more common and will continue to happen even after it's fully warmed up.

However, having said that, right now, as I type this, I am attempting to get it to do it again so I can test if it is observable on the composite output but of course it knows I want it to happen so it's not happening (composite or chroma/luma)

 

[robert_sissco]

Could it be an electrolytic cap that is going bad? I am not overly familiar with the 128, but it sounds like if it "warms up" and gets better, that something is taking time to charge, but as it continues to fail it is getting worse.

I've seen a few MarkFixesStuff videos where he suspects a bad cap is the cause of video issues on devices.

 

[Divarin]

Yeah that's what I was thinking too. I don't like just recapping for the sake of recapping but it might be time to invest in an ESR meter so I can test caps without having to desolder them.

 

[Hugo Holden]

The issue is that the VIC output flickers between a bright/crisp picture to a dark/muddy picture.

The best move here, rather than jumping to conclusions about faulty electrolytic capacitors, is to put the scope on the video output and look at the signal and find what is happening to it to give the effect on the vdu. Then trace the problem back with the scope to find out where it is originating.

 

[Divarin]

The best move here, rather than jumping to conclusions about faulty electrolytic capacitors, is to put the scope on the video output and look at the signal and find what is happening to it to give the effect on the vdu. Then trace the problem back with the scope to find out where it is originating.

Good call. I'm taking the day off work tomorrow so I should have some time to investigate.

 

[Toshy5000]

Hey there! I'm having the exact same issue. I was wondering if you ever found a solution and could please share it with me? I'm stumped.

 

[Toshy5000]

"The issue is that the VIC output flickers between a bright/crisp picture to a dark/muddy picture."
Just adding that my problem was a dying Kernal Rom. After replacing that, the Commodore was fully functional. No more flickering.

 

[Divarin]

Hey there! I'm having the exact same issue. I was wondering if you ever found a solution and could please share it with me? I'm stumped.

Yes and it was not at all what I expected. I only discovered it by accident when my fidgetty finger was resting on the power switch I noticed the flickering video would stop if I applied pressure to the power switch.
I opened up my c128, sprayed some deox-it into the switch, worked it off and on a few times, and the problem went away.
I would expect a faulty power switch would have different symptoms like freeze-ups or something but nope, just occasionally flickering video (at least in my case).