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

[Jimmy422]

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!)

 

[techknight]

Bad solder joints, bad capacitors, failing semiconductors to name a few. 

youll have to get in and troubleshoot. 

 

[just.in.time]

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.

 

[Jimmy422]

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.

 

[just.in.time]

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.

 

[techknight]

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. 

 

[Jimmy422]

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.

 

[techknight]

As far as I know, they dont exist. Sadly. 

 

[Jimmy422]

Well today I replaced all but one capacitor on the analog board. Absolutely no change to the issue. The big filter capacitor was the one I didn’t change out as my soldering iron couldn’t get hot enough to melt the leads, but from what I’ve read online they don’t usually go bad anyways. 

As i was working i checked again for any cracked solder joints and there’s nothing.

There was, however, a potentiometer that looked a little warped (dare I say melted?) in the middle of the analog board. I have no idea what it controls and I don’t have a part number to get a replacement for it, nor would I know what value to tune it to if i got a new one. Could that be related to the issue? I’ve attached a picture if anyone knows what that potentiometer controls. 

Any more advice would be appreciated!

 

[techknight]

your photo is too blurry and too small for me to see, but, I believe that is the voltage level adjustment for the regulation of the power supply. Blowing up the image, I dont see an issue. You could try cleaning it and setting it back to where it was. 

Outside of that, have you properly cleaned all the adjustments in the rear? 

If none of that solves the issue, then the problem is more severe and likely related to a failing or intermittent semiconductor. I had a similar problem but was much more severe in an iMac G3 years ago, and never did solve it. I ended up yanking the CRT out and putting it in a Molar Mac G3 AIO as it had a really weak CRT. 

if you can do a video demonstrating the proiblem that would be helpful. 

 

[davidg5678]

As i was working i checked again for any cracked solder joints and there’s nothing.

Sometimes there are cracked solder joints present that are just really hard to see. Maybe you should reflow everything on the board just in case this is an issue? I once did this to one of my SE/30 boards. I added a little bit of fresh solder and flux to each and every joint, working until everything was shiny. I think it took less than an hour.  At the very least, it will help rule out this potential fault.

If you are able to take some higher resolution photos of the board, front and back, it would be very helpful.

@techknight here is a link to a video from a previous post:

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.

 

[techknight]

Wow I didnt even remember seeing the video. 

Watching that video is a dead giveaway the issue is in the power supply. Its not able to maintain proper regulation and is bouncing all over the place.

When I say power supply, it could be the Main B+ feeding the monitor circuits, or the scan derived supply coming from the flyback. Either or... 

 

[Jimmy422]

Wow I didnt even remember seeing the video. 

Watching that video is a dead giveaway the issue is in the power supply. Its not able to maintain proper regulation and is bouncing all over the place.

When I say power supply, it could be the Main B+ feeding the monitor circuits, or the scan derived supply coming from the flyback. Either or... 

I’m fairly certain the issue is with the flyback. The noise i can hear coming out of the back when it distorts sounds like it is coming from the flyback transformer, although perhaps the issue is being caused by whatever is feeding the transformer.

Unfortunately it seems like finding these transformers in 2020 is impossible. I haven’t found one anywhere online. My guess is I’m going to need a working donor board from another LC to fix it.

 

[techknight]

Could be the flyback, but I kinda doubt it. the fluctuating voltage affects EVERYTHING, as the B+ comes direct from the main power supply into the flyback before it reaches the horizontal output transistor. 

 

[Jimmy422]

Could be the flyback, but I kinda doubt it. the fluctuating voltage affects EVERYTHING, as the B+ comes direct from the main power supply into the flyback before it reaches the horizontal output transistor. 

Thanks to your post, I FIXED THE ISSUE!

Based on your post I had two theories:

A. That filter cap is actually bad and causing the input power to the whole board to be messy.

B. That melted looking pot that controls the 5v and 12v outputs is dirty and causing issues on those ends.

I was able to turn up my soldering iron heat as high as I could and was able to wiggle the old filter cap out. The bottom of it had a little bit of electrolyte leaked on it which pretty much confirmed to me it was shot.

I also hit that pot with deoxit. I wasn’t able to move the adjustment screw at all (it’s definitely melted) so I don’t know if that actually did anything to help.

I would’ve tested these fixes independently but it’s a pain to keep taking the board out. Either way, one or both of those things fixed the issue! I booted it up and the screen and brightness are perfectly stable now. 
 

Even though recapping the rest of the board was a “waste” at least I know that that the caps on that analog board are going to be rock solid for another 27 years.

Thanks a bunch for everyone’s advice! I’m gonna go enjoy my LC now

 

[techknight]

Well good! Glad you figured it out! Although, I have my concerns still, But hey at least it works now. 

 

[Jimmy422]

Well good! Glad you figured it out! Although, I have my concerns still, But hey at least it works now. 

Out of curiosity, what are your concerns? I left it up and running for a few hours yesterday and a few hours today and it's still been rock solid.

So far the only time things get questionable is when you put a CD in, it appears the screen distorts ever so slightly as the disc is read, but that might be normal? I wonder if the CD drive needs a recap as well, it smells like fish when I eject the caddy out of the drive.

 

[techknight]

Yea the CD drive is definitely the next thing to go. especially if it smells like dead rotten fish. 

My concern is the Pot, it may be melted. but it could also have been melted on purpose. without a close-up picture, I cant really know for certain. 

More importantly the actual issue being corrected. that main filter cap wouldnt cause this unless it was shorting internally, and even then that would take out the fuse. This puts my suspicions even further that the potentiometer was the problem. 

On the other hand, if the electrolytic fluid was leaking and got onto other nearby components in the power supply, THAT would cause this... Moreso than the cap itself.