Repair help: OSI 600

[ianoid]

Fantastic. Thanks so much for the key caps link. At least now I feel like I can try taking apart one of these key caps without being afraid I’ll break it. I’ve disassembled key caps before but it has been a while.

[bxdanny]

Regarding the Break (reset) key, have you tried holding it down for several seconds? Requiring this was a common mod, since people felt it was too easy to hit the key accidentally. In fact, I think that the power-on reset and the requirement that the Break key be held down for a few seconds (two?) in order to function became standard features on the 600D.

[ianoid]

You are correct, the Break key does engage, it just takes a few seconds. I’m not sure if it’s the key cap or the design, as you mentioned.

Whoops, I’m back on the bench with one of these boards. The original one I was troubleshooting, the one without the expansion area populated is in trouble. It no longer produces a picture.

I have one C1P case that I have been using for power and AV. I posted in the other thread about the power supply that I wanted to get a second power supply for one of the boards, so that I could put one in the case and still have the other one useable as a loose board.

So this one stopped responding after I tried it with the new 5V 5A power supply. Testing my second Superboard with that new 5V 5A power supply caused it to behave oddly; the keyboard was poorly responsive and text on screen was staggered oddly. For example, OK would move across the screen every time I hit RETURN. I’m thinking I should return it and get a different power supply. In the meantime, I’m testing with the known good C1P power supply.

Is there something that the new power supply could have damaged by providing low or inappropriate output? Any suggestions where to start? I’ve swapped out all the RAM, and right now just have 4K in the right banks(U31-U34,U45-U48). As well, there are two RAM chips at U77 and U59. I am using all new RAM, which I’ll swap out with the old once I can confirm the unit is working.

I did replace the largest capacitor on the board at C5 with a 47uF 25V cap, and I’m certain the polarity is correct. The old one was bulging a bit, so I thought it should go.

I’m tempted to add an LED at the LED 1 site similar to the other board I have, since it’s handy to confirm when the board has power. I believe I would just need to add a resistor next to it as shown below. I’m a total novice at reading these things, but is it a 48 ohm resistor? Is that appropriate for an LED?

One more thing, the oscillator has some corrosion on it. I feel inclined to replace it. Can someone suggest a replacement part? Or best to leave it alone? I’ll attach a picture of it below.

Dang it! Sorry for making this post so long. Do these small caps between the memory chips need to be replaced? They are damaged. I checked for replacements, they seem to be 0.1uF 10V, but the only ones available have higher voltage tolerance, which I assume shouldn’t be a problem.

Thanks again,

Ian

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[bxdanny]

>>> text on screen was staggered oddly. For example, OK would move across the screen every time I hit RETURN.

I'll just address that part. It sounds like the board got switched into 48 x 16 mode (which the 600D has, and the 600B doesn't). If you see this happening again, try entering POKE 55296,0. That would switch it back to normal.

The resistor in series with the LED should be 470 ohms, as shown on the schematic at http://marks-lab.com/osi/boards/schematics/OSI600.pdf.

[ianoid]

@bxdanny this is a rev B board, and as I soldered it in imitation of the Rev D board, the LED didn’t work. So I checked where the current was coming from on the bottom of the board and reversed the two components to make sure it is 5V->resistor->LED and now I have a board with a working green LED. Yay! This is how low level I am- getting an LED to work is exciting.

OK, as far as all my blathering on about what could be wrong with this Rev B board (I feel like I should delete that post out of embarrassment, but I won’t), it seems like the issue with the board not powering up was related to corroded contacts of the fuse holder. It must have been jarred slightly when it was sitting in the static free bag. I’ll have to work on cleaning the green corrosion off of the one on there, or I could replace it. It’s one of those things that has a lot of solder on it and I feel like the board traces are a little fragile, so if I can avoid desoldering it, It would be best.

The last thing that I am still anxious about using the 5V 5A power supply I got on Jameco with these boards (Sceptre GPSU40A-1). That is the last thing I need to deal with to have two separate working boards- a safe working power supply. I’ll try to continue to address that in the thread I started about getting a 600 PS going, which might be of benefit to future users as a separate more findable thread.

[dave]

One more thing, the oscillator has some corrosion on it. I feel inclined to replace it. Can someone suggest a replacement part? Or best to leave it alone? I’ll attach a picture of it below.

If the computer is working, no need to fiddle with the crystal. You could probably remove that corrosion with some steel wool. If you do, it should probably go without saying to be sure to clean up the metal dust

Sorry for making this post so long. Do these small caps between the memory chips need to be replaced? They are damaged. I checked for replacements, they seem to be 0.1uF 10V, but the only ones available have higher voltage tolerance, which I assume shouldn’t be a problem.

pretty much any 0.1uF disc capacitors will work fine. 10V is plenty of margin for what is supposed to be a 5V supply. Larger voltage rating won't hurt. The caps might be a bit larger and more expensive, but probably not significantly so. If you can get a good deal on bulk caps from ebay or Jameco, that should be fine for this purpose.

[ianoid]

I pulled 5 key mechs from that unit and replaced them with that perfect part. They all had response/bounce issues. I couldn’t figure out how to repair them.

If anyone wants those 5 key mechs to mess with or fix, I will send them to anyone here for free in the US. Just PM me with your address.