dead C128 - black screen

Kremlar · Newbie Joined: 30 Mar 2012 Posts: 17

Picked up a bunch of items from Craigslist the other day. One of the items was a C128, which powers up but I only get a black screen on both composite and RGB, also whether booting normally or trying to force into C64 mode.

I have another working C128 here, so I swapped the power supplies and confirmed the power supply is OK.

I opened it up, and it looks like it's been pulled apart before - probably for this same issue. Solder holding the RF shield in place was broken, and one of the chips was 1/2 out of the socket!

I ended up pulling every chip and at least 3 had bent pins. Thought for sure I'd put it back together and it would power right up, but no such luck.

There was some green corrision on some of the pins for the CN1 expansion port which I thought was odd. Also some corrosion near some components near CN2 cassette port. I cleaned it up as best as I could. If something is leaking, I can't tell what it is.

Found some troubleshooting docs off the internet and tried swapping the 8722 from a working C128DCR but still no boot.

I also checked and get ~5V at CR15 and ~12V at pin 3 of voltage regulator IC U59.

I don't have an oscilloscope and never used one, so don't think I can go any further testing the microprocessor operation in the troubleshooting manual I found.

I guess I could swap every socketed chip from my working C128 to test with but I'd hate to do that since it's my only working C128.

Is this thing a loss cause, or is there anything else someone can suggest that would cause black screen in all modes (40/80/c64/c128)?

Thanks!

dabone · Posted: Wed Apr 11, 2012 11:07 pm Post subject:

Start here.

http://personalpages.tds.net/~...128-ic.txt

A great guide of chips versus the failures they cause.

Are any of the ram chips hotter than the others?

Also I have 2 completely dead 128 boards myself that seem to have gotten a large surge on the +5 rail. (Every ttl chip I've removed and tested on them has been dead)

later,
dabone

e5frog · Posted: Thu Apr 12, 2012 12:10 am Post subject:

Green oxidation can indicate a leaking electrolytic capacitor, you should check the ones close to that spot and replace it/them if you don't have an ESR meter to check it/them.

Kremlar · Newbie Joined: 30 Mar 2012 Posts: 17

Quote:

A great guide of chips versus the failures they cause.

I did check that, thanks.... Tried swapping all socketed chips that, according to that guide, would cause the 128 to not boot up in any mode but no dice.

Quote:

Are any of the ram chips hotter than the others?

No don't seem to be...

Quote:

Green oxidation can indicate a leaking electrolytic capacitor, you should check the ones close to that spot and replace it/them if you don't have an ESR meter to check it/them.

You know, the oxidation near the CN2 connector seems to come from one of the leads on a ceramic capacitor (C70), but I don't believe they are supposed to leak. Am I wrong?

Not really any electrolytic capacitors in the vicinity...

e5frog · Posted: Thu Apr 12, 2012 12:47 am Post subject:

Nope, only the electrolytic caps have an electrolyte. Wink

Depending on the location when stored it could have dripped from somewhere further away. Be sure to check carefully if you don't have an ESR meter.