Repairing a Supervision

Hello,

I bought a Supervision for a cheap price because it was not working. When I try to power it the LED shows it's started, there is some static from the speaker and the screen stays white.
One of the back screw is missing, there is a weird fix with the diode close to the power switch, and I believe it was (not completely) recapped.
Can you tell me what seems missing/out of place? I tried to compare it with the "Quickshot" version (from this blog) and annotated the first picture.

Thank you

 

The "quartz" is a capacitor, I think those kinds are usually high voltage. Unpopulated spots on a board are not uncommon, but I don't know this board to say if it's supposed to be there or not. That fix does look.... sad, but it also could easily be from the factory. I'm most concerned with whatever did enough damage to crack the epoxy around that chip on the right (edit: in the first picture). It also looks like that chip above it might have some physical damage as well. Can you get a better resolution picture of that area?

I did a little digging. Couldn't find a schematic, but from the specs and parts lists, I can do my best. The epoxied chip on the carrier on the right is a 6264 SRAM. The chip above it is, I would guess, the audio amplifier.

I would say there's an incredibly high chance the SRAM needs replaced. The system should still work without the audio amp, but if you get it working and there's no sound, it's pretty clear where that issue would lie. I do not believe it to have been recapped, nor do I see any failure prone cap styles or evidence of leaking. They're cheap to replace and there's not a lot of them, so don't let me stop you if you think they might be problematic.

edit: oh, flash from the camera on the tape looks like a missing wedge of epoxy. Still need a clearer picture of that area.

 

Sorry for the pictures (they are from my phone), I tried to make attached ones better.
I'm not sure where you see a crack on the epoxy, was it the tape with the camera's flash?

I found new pictures to compare:
- a repair guide from a blog (same as yesterday)
- a german forum, I believe to fix one?
- a wiki page
- a video on youtube

So:
- the blue part is definitely a quartz. On the blog page it's a "Hooray 4.000", the CPU's speed. On other pictures it's the exact same blue component
- on the top-right, pins 9 & 10 from the 74HC14A are soldered together, but it seems to be normal
- I don't think it was not recapped anymore, some pictures have the same capacitors, and the one I thought was missing is on the other side
- I believe the main issue comes from the diodes (?) because their rings are facing eachother.

 

No. Quartz is a crystal oscillator. Electronics 101.

Uh, what? You mean a bridge rectifier?

 

I put quartz in quotes because I didn't think it was a crystal, and that I thought it was a capacitor instead. I don't believe you didn't know what I meant, so you're purposely misinterpreting what I said to be rude to me for some reason. If you want to pull the electronics 101 card, then a bridge rectifier has 4 diodes.

OP: Yeah, the flash on the tape made it look like a chunk of the epoxy was missing, my bad. And it must be those two pins being soldered together that also makes it look a little funky on the other one. The video clearly shows the familiar package oscillator in that spot that I thought was a cap, but you can see why I thought it was a cap ...here's a blue monolithic: http://www.tedss.com/stock/images/1994/1994001850.jpg

Anyway, I see a few cold joints, but they still appear connected. Honestly, since the whole system is driven by an asic with so many glob top chips, I'd just follow the input voltage to make sure it's getting where it needs to go in a manner that makes sense, check for some shorts, follow what the others have tried, then call it a day. You could be chasing ghosts to a chip that can't be replaced. Not a good gamble of time.

 

Victory!

I started by removing the weird diodes soldered together.
Then I tried to power it on, but not even the red LED was on, while I'm pretty sure it was working before. I checked the power switch and the on-state was not working, so I disassembled it and removed the oxidation.
Now I had sound, but the screen was still white... until I turned the contrast trimpot! I believe it was this "problem" from the beginning since it's possible to turn it from completely white to completely black.
I applied some contact cleaner on the sound trimpot because turning it was causing parasitic noise on the speaker.

I finished by soldering the diode how it was supposed to be, now I'm pretty happy with the result