Help! Dead 128!

commodorejohn · May 18, 2010, 10:16 AM

So I acquired a flat 128 today and brought it home to try it out. Sadly, it doesn't work at the moment. No VIC, no VDC, no audio, no serial, nothing. After a little TLC and reseating of chips, it got to the point where it flashes the screen white when I turn the power on or off, but I've made no further progress.

I do have a handful of chips from other dead Commodores, so I can try replacing parts to see if it helps, but from the troubleshooting guide on zimmers.net it looked like the only (socketed) chip that would cause this kind of total failure would be the 8701 clock generator. I replaced that to no effect.

I'm not desperate to repair this thing; I have my own 128 that works just fine, and I was just going to sell this one. Still, I hate to write off a good computer if there's a simple way to fix it. So if anyone has any ideas on getting this thing working, I'd like to hear them. If I can't get it working, I'll probably sell it for parts, if anyone wants it.

The behaviors displayed, for reference:

On power-on or power-off, the VIC screen flashes white, then phosphor-fades down to black. There is no other VIC output.
There is no VDC output at all.
Some sort of serial-bus reset signal gets sent out on power-on (it makes any attached drives do their reset move-to-track-0 bit.) However, it doesn't do anything else with the disk drives, even with a 128-bootable disk like CP/M inserted, so I think this is probably just a reflex action.
Typing in BASIC commands blind does nothing I can discern; I've tried both disk access and sound functions with no result. Whether this means the keyboard isn't working or BASIC isn't working, I don't know.

I've tried this with both the power supply it came with and the power supply from my own 128; there is no change in behavior.

Any thoughts?

Hydrophilic · May 18, 2010, 12:40 PM

I would check the voltages inside the unit. The power supply provides 5v dc and 9v ac, but the C128 doesn't use 9v ac directly... it converts it to 12v dc for the SID and, as I recall, the Z80 clock.

Near the power connector you should find a large capacitor, C106 which should have a voltage of about 12.7 dc (unregulated). Nearby is voltage regulator 7812, marked U59. It takes produces a regulated 12.0 dc output. The computer simply won't work without these. If you don't get those voltages then U59 or perhaps the full-wave rectifier is bad. I don't think there is U# for the rectifier, but it's a small 4-pin IC nearby. It may be the only 4-pin IC on the board. If it helps, it's connected to C106 and CR11.

If you don't have a voltage meter, get one. They're handy for lots of things, cost less than $20, and available in a lot of places (besides electronics shops) like auto shops and Wal-Mart.

Now if the voltage is good, you might try replacing the VIC itself as it generates the clock signals for the CPUs and I/O chips. The 8701 clock generator you replaced feeds the VIC which divides the frequency into various amounts (1, 2, and 4 MHz).

That's all I can think of. Good luck!

commodorejohn · May 19, 2010, 08:47 AM

Thanks for the tip. I dropped the VIC into my working 128 and it displayed the same behavior, so I guess we can chalk this one up to a dead VIC. Unfortunately, that's one part I don't have a spare for, so I guess I'm going to have to sell this for a parts machine after all. If anyone's interested, I'd like maybe $10 + S/H.

redrumloa · May 19, 2010, 11:04 AM

Quote from: commodorejohn on May 19, 2010, 08:47 AM
Thanks for the tip. I dropped the VIC into my working 128 and it displayed the same behavior, so I guess we can chalk this one up to a dead VIC. Unfortunately, that's one part I don't have a spare for, so I guess I'm going to have to sell this for a parts machine after all. If anyone's interested, I'd like maybe $10 + S/H.

I'd have to dig through my pile to find one and test it... But I could do it if you wish. Are you in the USA?

commodorejohn · May 19, 2010, 11:13 AM

Yes, I'm in the US, but just to be clear: I'm not really looking to buy a replacement VIC. I already have a working 128, so I'm just going to write this one off and offer it up for parts if anyone wants it.

RobertB · May 19, 2010, 04:41 PM

Quote from: commodorejohn on May 18, 2010, 10:16 AMNo VIC, no VDC, no audio, no serial, nothing. After a little TLC and reseating of chips, it got to the point where it flashes the screen white when I turn the power on or off, but I've made no further progress.

(snip)

Any thoughts?

Hmm, a PLA problem?

Truly,
Robert Bernardo
Fresno Commodore User Group
http://videocam.net.au/fcug
July 24-25 Commodore Vegas Expo 2010 - http://www.commodore.ca/forum
and click on ComVEX

redrumloa · May 20, 2010, 06:45 AM

Quote from: commodorejohn on May 19, 2010, 11:13 AM
Yes, I'm in the US, but just to be clear: I'm not really looking to buy a replacement VIC. I already have a working 128, so I'm just going to write this one off and offer it up for parts if anyone wants it.

Oh, sorry. I read your post late and misread it

dabone · May 31, 2010, 06:35 AM

Do you still want to sell this as a parts machine?
If so, how are the plastics?? (Looking for a donor to retrobrite my 128 and make it look nice).
(Mine has the screw holes stripped out/ perm marker on the bottom)

Later,
dabone

commodorejohn · May 31, 2010, 10:45 AM

Yeah, it's still up for grabs. The plastic is a bit dingy and dusty,and there's some streaks of something on the surface (not marker, somekind of residue - it was out in a garage when I got it,) but it doesn'tlook like anything Retrobrite and a little TLC can't fix. Structurallyit's in fine condition; the only problem in that respect is that theF3/F4 keycap is missing (but that's not too difficult to replace.)Here's a picture for you to judge:

(A bit large for forum posting, but I wanted to give a good view of the surface.)

If it's what you're looking for, does $15 + S/H sound appropriate?

dabone · May 31, 2010, 02:24 PM

Sure, shipping is to 37421, cheapest possible shipping please. and is paypal ok?

Thanks.
later,
dabone

RobertB · May 31, 2010, 05:52 PM

Quote from: commodorejohn on May 31, 2010, 10:45 AM...it doesn't look like anything Retrobrite and a little TLC can't fix.

If going the Retr0Brite way is too complicated, I find that Spic 'n Span Cinch spray liquid cleaner will do a good job at cleaning and gives a slight bleaching effect.

Though it will not give full Retr0Brite results,
Robert Bernardo
Fresno Commodore User Group
http://videocam.net.au/fcug
July 24-25 Commodore Vegas Expo 2010 - http://www.portcommodore.com/commvex

commodorejohn · June 01, 2010, 01:34 PM

Quote from: dabone on May 31, 2010, 02:24 PMSure, shipping is to 37421, cheapest possible shipping please. and is paypal ok?

Left you a PM with the shipping cost. Let me know what your address is and I can get it shipped off sometime this week.

dabone · June 02, 2010, 02:22 AM

Paid, and left shipping details in a pm. Thanks.
This is going to be a round to it project, so maybe I'll get to it this summer, but most likely it will be fall.

Later,
dabone

dmackey828 · June 03, 2010, 08:32 AM

Quote from: commodorejohn on May 19, 2010, 08:47 AM
Thanks for the tip. I dropped the VIC into my working 128 and it displayed the same behavior, so I guess we can chalk this one up to a dead VIC. Unfortunately, that's one part I don't have a spare for, so I guess I'm going to have to sell this for a parts machine after all. If anyone's interested, I'd like maybe $10 + S/H.

Nice easy fix, I'm sure someone has a VIC Chip they could get to you...

commodorejohn · June 03, 2010, 08:42 AM

Well, as I said, I'm not looking to spend time or money repairing because I already have a 128; in addition, dabone purchased it for a parts machine, so problem solved!