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Commodore 64 dreaded black screen

T

tkc8800

Experienced Member
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Jul 1, 2014
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Adelaide, South Australia
I picked up a Commodore 64 the other day that initially worked but now I'm getting the dreaded black screen when I turn the machine on. I'm just after some opinions on what the most likely problem is going by the symptoms. It initially booted fine, I could type at the keybaord, I even loaded a game from floppy disk. The video and sound all worked well. Then after warming up for a while, only a few minutes, the screen went black. I let it cool down for a while and then powering it on and off a few times initially worked, the machine booted successfully, but then afer a few minutes it went black.

Some things I've tried/tested:

I've checked voltages on the board and at the power plug and all seem fine. I'm getting 5v dc on pins 2 and 5; and 10v ac on pins 6 and 7. I think the AC should be 9v but I wouldn't have thought that was a problem.


Other things tried:

- I've removed and re-seated all removable chips, no change.
- I've tried running the machine with International Soccer cart installed, no change.
- I tried a blind disk command with working 1541 II drive, no response.
- The 1541 II disk drive spins and the green light goes on when the 64 is turned on.

Status of chips after machine has been on for a while:

93459PC (PLA): warm
6581 (SID): hot
all other chips: fairly cold

I tried removing the SID and testing the machine but no change. I've heard that the machine will still boot without the SID. At one stage when I was powering the machine on and off I got multi coloured text. From what I've read considering all the above symptoms, I think it's most likely a dead PLA, but just wanted to know what others think?

The machine is a breadbin 240v PAL model. Motherboard is ASSY No. 250407 Rev. A. I've found a website that sells the SuperPLA v3, sounds good, has anyone tried one of these?

http://www.vesalia.de/e_superpla.htm
 
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A busted SID won't stop a machine from starting up. Assuming you trust the power supply (never guaranteed with the epoxy-potted crap), a marginal PLA would also be my next suspect. Fortunately, you can fashion one out of an EPROM if you can't find the genuine article.
 
I've got a SuperPLA, but never really had a machine that needed it. Supposedly if you toggle the power switch on-off-on-off-on quickly a few times and leave it on, you might kickstart the PLA, which might be what happened when you had pulled the SID chip.

The EPROM solution is not widely recommended, and you need a fast chip to make it work. Actually there currently is a discussion on the cbm-hackers mailing list where to obtain PLA replacements, both SuperPLA and realPLA have been mentioned.
 
ClassicHasClass said:
A busted SID won't stop a machine from starting up. Assuming you trust the power supply (never guaranteed with the epoxy-potted crap), a marginal PLA would also be my next suspect. Fortunately, you can fashion one out of an EPROM if you can't find the genuine article.
Click to expand...

I had an SX-64 that would not boot and that was down to a broken SID, replaced the SID and it worked fine.
 
The 'usual' failure mode of SID chips will not cause the machine not to boot, but, it sure is possible for a SID to have shorted pins and prevent system operation. I've even had a VIC chip short, causing the PLA to quit working (and everything else).

The PLA probably is the cause of the black screen, but it could be a lot of other things too.
 
Pet Rescue said:
I had an SX-64 that would not boot and that was down to a broken SID, replaced the SID and it worked fine.
Click to expand...

As KC9UDX points out, that's not the typical failure mode. For the record, I've blown at least two SIDs on SX-64s when I forgot and hotplugged the video cable while it was on, and the machine worked fine (but no sound). There's exceptions for every rule.
 
ClassicHasClass said:
As KC9UDX points out, that's not the typical failure mode. For the record, I've blown at least two SIDs on SX-64s when I forgot and hotplugged the video cable while it was on, and the machine worked fine (but no sound). There's exceptions for every rule.
Click to expand...

If something is one in a million that it could happen, then that's my luck sometimes!

But it doesn't seem to work on the lottery though for some reason! :D
 
ClassicHasClass said:
A busted SID won't stop a machine from starting up. Assuming you trust the power supply (never guaranteed with the epoxy-potted crap), a marginal PLA would also be my next suspect. Fortunately, you can fashion one out of an EPROM if you can't find the genuine article.
Click to expand...

The power supply that it came with is the old black brick with vents on top. I took it apart to clean it and the internals are very basic. I think it even pre-dates the epoxy supplies. But it seems to work ok.
 
carlsson said:
I've got a SuperPLA, but never really had a machine that needed it. Supposedly if you toggle the power switch on-off-on-off-on quickly a few times and leave it on, you might kickstart the PLA, which might be what happened when you had pulled the SID chip.

The EPROM solution is not widely recommended, and you need a fast chip to make it work. Actually there currently is a discussion on the cbm-hackers mailing list where to obtain PLA replacements, both SuperPLA and realPLA have been mentioned.
Click to expand...

I've ordered the SuperPLA from the Vesalia site, not much money and it sounds good. As you say there's been fairly mixed results with EPROM replacements so the SuperPLA sounds like a good option. I also have some spare ram, so at least I'll be able to replace both ram and PLA.
 
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In the mean time, you can switch to using a C64C as opposed to the breadbin, I've understood it very rarely seems to suffer from broken PLA?
 
carlsson said:
In the mean time, you can switch to using a C64C as opposed to the breadbin, I've understood it very rarely seems to suffer from broken PLA?
Click to expand...

Yes, I wouldn't mind getting a few more C64's. A "C" would be nice. Seems the more you have the easier you can diagnose them.
 
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carlsson said:
The EPROM solution is not widely recommended, and you need a fast chip to make it work.
Click to expand...
Ray Carlsen uses the fast EPROM method when he does a PLA repair on a Commodore. However, since each Commodore is slightly different electrically, he has to "custom tune" the EPROM PLA with an added resistor.

FCUG celebrating 33 years,
Robert Bernardo
Fresno Commodore User Group
http://www.dickestel.com/fcug.htm
July 18-19 Commodore Vegas Expo v11 -
http://www.portcommodore.com/commvex
 
carlsson said:
In the mean time, you can switch to using a C64C as opposed to the breadbin, I've understood it very rarely seems to suffer from broken PLA?
Click to expand...
Hmm, I've had a number of C64C's (long board) with failed PLA's and a number of brown C64's with failed PLA's. I've never noticed less PLA failure in one or the other model.

FCUG celebrating 33 years,
Robert Bernardo
Fresno Commodore User Group
http://www.dickestel.com/fcug.htm
July 18-19 Commodore Vegas Expo v11 -
http://www.portcommodore.com/commvex
 
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