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Jimmy422

LC 520 Screen Flickers and Distorts - What should I check?

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Hi all! I picked up an LC 520 off ebay a few weeks ago. I cleaned and re-capped the motherboard and to my surprise it booted up just fine.

 

However, the screen will flicker and distort as it runs. A couple times it has flickered and gone to black, which made me have to reboot the machine entirely to get it back up. I believe I can hear the hard drive spin down slightly when the screen flickers, which makes me think the issue is power related.

 

I took a look at the analog board and all of the capacitors look visually okay - I don't see any spilled electrolyte or bulging caps, nor do I see any cold solder joints on the bottom of the board. The flyback transformer is solidly soldered in place and doesn't seem to have any cracks in it.

 

Does anyone have any advice as to what I should look into to fixing this? Should the analog board be re-capped as well given the machine is 27 years old? (I hope not, because there are a LOT of capacitors!)

Edited by Jimmy422

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I agree with @techknight, your analog board capacities are probably just phoning it in at this point.

 

but just in case, you can try reseating your vram, logic board, and analog board (it slides into a connector under that metal shield much like how the logic board down below does. If you can get it to start, bring the screen brightness/contrast down to the lowest you can still somewhat comfortably see. This will reduce load on the Analog board. If this resolves your issue, it’s probably in need of a recap.

 

As a way out of left field possible solution, try taking the LC 520 to a different room, on a different circuit breaker and give it a run. My LC 520 had an issue at one of my apartments where, in a specific room on a specific plug, it would get weird display issues when the air conditioner would be running. Different room, or just turning off the air conditioner resolved these issues. Machine is still going strong almost 4 years later. Though I never had mine completely shut off on me.

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So I did reseat the ram, logic board, and analog board sadly with no luck. I set the brightness as low as it would go and it didn’t change anything (and neither did maxing out the brightness, not sure if that means anything?). Also tried another circuit in the house and no dice with that either :(

 

Here’s a video of the issue in action. You can see the brightness will max out at random and the picture size is all over the place. I guess that very well could be a capacitor issue. I bought an ESR meter so I’ll see if I can find which ones need to be replaced.

Edited by Jimmy422

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Hmm, very interesting. Watched the video. Still thinking @techknight's advice about capacitors is on point due to age.

 

Another side idea though, the adjustment pots on the analog board (for dialing in all the display parameters with a hex wrench) may have some corrosion on them. I believe one does the vertical height of the picture. If you decide to look into that possibility, use caution as that analog board has some very high voltage components on it that can give a nasty shock, even with the computer unplugged. I think the service manual has more details on which one does what and their location.

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The fact that the vertical height is fluctuating with the brightness, means you have B+ issues. Either Scan-derived, or from the main power supply. 

 

It is possible the vertical height pot is dirty and needs cleaned with deoxit, but its unlikely. I think you either have solder joint issues around the flyback, vertical output or associated circuitry, or you have a bad/leaking capacitor somewhere and something is intermittently shorting out. 

 

This could also be a semiconductor fault too, but its really hard to tell without troubleshooting the circuitry. 

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17 hours ago, techknight said:

The fact that the vertical height is fluctuating with the brightness, means you have B+ issues. Either Scan-derived, or from the main power supply. 

 

It is possible the vertical height pot is dirty and needs cleaned with deoxit, but its unlikely. I think you either have solder joint issues around the flyback, vertical output or associated circuitry, or you have a bad/leaking capacitor somewhere and something is intermittently shorting out. 

 

This could also be a semiconductor fault too, but its really hard to tell without troubleshooting the circuitry. 

It’s probably a long shot but do you know if there’s any schematics out there for the LC 520 analog board? It’s a little hard to troubleshoot what’s going where without em.

 

I pulled the analog board out today and started testing capacitors with my ESR meter. Out of the 10 I’ve tested so far only one had an ESR reading that out of spec (and it was significantly out of spec, so maybe that’s part of the issue?). All the others were all in range. There’s 30+ capacitors on the board so I’ve still got a bit to go, but the bad one is definitely getting replaced.

 

I also double checked for cracked solder joints and I still don’t see any anywhere. Triple checked the flyback and surrounding areas and it’s all good. I’ll try hitting the pots with deoxit for good measure though.

Edited by Jimmy422

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