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3 Commodore 128s powering on with blank screen


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Have you tried both 40 and 80 column modes (and with the 40/80 switch in the correct position)? If they boot in 80 column mode but not 40, the VIC chip is probably bad, if the opposite is true it might be the VDC though I've never heard of one of those going bad. If you get nothing in either mode, the PLA chips are most suspect.

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You've said you get a blank screen. Does this mean a completely black screen (no background or border)? Is there a momentary change to the screen when you power up?

 

Does the drive respond if you enter a blind command (LOAD"$",8 <RETURN> for example)?

 

While you have the machine open, remove and reseat all socketed chips (be careful you don't bend any pins!) and try the machine again - maybe you'll get lucky. Most of the chips run slightly warm, so you can try powering up the machine and see if any seem abnormally hot.

 

Replacement PLA chips are available (see http://www.melon64.com/forum/viewtopic.php?f=12&t=2370). These are for the C64 so check with the supplier to verify C128 compatibility. You may also be able to find replacement Z80A's with a bit of searching. The other major chips (SID, CIAs, VIC-II, VDC, ROMs, and 8502 CPU) can generally only be found as salvage from other machines. As there are many possible faults causing a blank screen, I'd recommend trying to find the faulty chip(s) before spending money on replacements. The only good way to do that without expensive testing equipment is to, one-by-one, swap individual chips from a faulty machine into a known good one until you identify the culprit. Remember it's possible that more than one chip can be bad, and it's even possible that all chips are OK but there's a problem on the board (broken trace, bad capacitor, blown voltage regulator, etc) that's preventing the machine from booting - rare but possible. Sorry there's no easy, 100% sure way to diagnose the problem.

Edited by krslam
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This is an odd one. The only time I have hd trouble along these lines, it was only in 40 columns. I bought two C-128s, and the seller sent me a/v cables that were not for the Commodore! They had eight pin DIN on one end, but were not wired for the 64 or 128.

If it isn't as simple as the 40/80 switch being in the wrong position or a cable problem, it could any of several chips in there.

I would say it's time to get the schematics from Ray Carlsen's site, and check everything.

 

http://personalpages.tds.net/~rcarlsen/cbm.html

 

 

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Has to be a chip. I even connected to a tv set using rf and an atari 2600 switch box. Same deal. Thanks for the link.

I won't disagree with you. You might want to post this on "C-128.com" to ask Bill Herd about this problem:

 

http://c128.com/forums/commodore-and-retro

 

Chances are that he will know more about the C-128 than most people.

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Wow. He is good. Here's what I got from him:

 

Sorry to hear about the dead 128’s. Unfortunately I am not very good having not really turned on a 128 in a couple of dozen years, HOWEVER, some of the guys in the forum fix these on a regular basis. Please post there and if no-one gets you what you need let me know.

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Another thing you can try is starting it up in 64 mode. Hold the Commodore key on power on. Sams Computerfacts has troubleshooting info for no video, but it requires a multimeter and o-scope.

 

You may want to look here and download some repair manuals:

 

http://www.bombjack.org/commodore/books-commodore128.htm

 

Unfortunately the 128 is more complicated than the 64 for repair work since it's 3 computers in one. The Z80 CPU in the 128 is actually part of the power up sequence so if it's bad, you'll get nothing out of it. But it's odd you would have 3 128's with the same issue.

 

I know on a 64 bad RAM will cause a black screen. You can usually test for that using a cartridge like radar rat race that bypasses the RAM. Any Ultimax mode cartridge will do.

 

If you use a 64/128 a lot, you can always invest in a dead test cartridge and also a diagnostic harness/cartridge. This helps with troubleshooting issues (including no video (w/ dead test cartridge) ) greatly.

 

From Sams Computerfact for the 128:

 

"No video when using the RF modulator. Use a scope to check for a video waveform at pin 17 of IC U21 (this is the VIC). If the waveform is missing at pin 17 of IC U21, check IC U21 by substitution."

 

Looks like you've posted about this on lemon64.com too. Seems we should try our efforts on one site instead of 3.

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