C64 with Black Screen

rob_santry

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Hi Group,
The other day I picked up a used Commodore 64 with PSU as untested. I got it cheap. So when I got it home and plugged it in, it did show a blue screen with no text characters. I thought I'd flip the on/off switch a few times to see if that would 'wake it up'. Instead, I now have the black screen of death.

So I pulled the top off and here's what I have:
The board is a Commodore 250407 assembly. The 90125, 90126, 90127 chips have red printing on them. The circuit board shows no obvious signs of corrosion. Inside the metal cover sits the VIC II chip which is a ceramic and gold chip with still pliable heat sink compound on the metal cover on the chip. While running, none of the chips feel too hot. I checked the voltage regulators and the 12v regulator had 20.1v input with 11.8v out. The 5 v regulator has 11.4v in and 5.0v out. My question is, is the input voltage on the 12v regulator a little high?

I have pulled the sid chip and restarted with no change. I next pulled both CIA's and restarted with no change. Actually, I pulled and re-seated every chip that I could. Any ideas. I will order a new PLA chip and replace, but I want to be sure my voltages are right before I put it in. Oh yes, I checked the fuse and it is O.K. Thanks in advance...

Rob
 

roy_bates

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did you check the other voltages?

pick two pins on something like the mpu or drams to see if the 5 volts is there and is correct.
and if your wondering,no,the regulators on the board dont supply those parts.
 

rob_santry

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Thanks Roy,
I checked the voltage between ground and the vcc pin (pin 6) on the mpu and it was 5 volts. The drams all had 5 volts. I guess I'm going to substitute a good PLA chip when I get one. I should get it in a couple of days. I'm reluctant to swap out the one from my only good C-64.

Two other things that I've read about to try are checking if the CPU clock input has a signal and check if the reset circuit works. I don't know how to do either of those tasks. Do you have any advice? Thanks inadvance...

Rob
 

stephenfalken

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If you have a good C64, you could try the suspected faulty PLA in it to either confirm or rule it out.

Black screen can be caused by many different parts so having another working machine is handy for troubleshooting.

Regards
 

Sardine

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If you have a good C64, you could try the suspected faulty PLA in it to either confirm or rule it out.

Black screen can be caused by many different parts so having another working machine is handy for troubleshooting.

Regards

im with rob on this, I too wouldnt want to disturb a working C64 incase the chip breaks the working C64 or the broken C64 breaks the chip.
 

stephenfalken

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I think Rob was saying he didn't want to try his working PLA in the non working machine which I agree is a definate no-no.

I have often found dead PLAs by trying them in known working boards with no ill consequences, but that's just me I must be a risk taker :)

Regards
 

rob_santry

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Hi guys!

Yea, who would have thought that this would be a test of character thing :) I just can't do it. So, I do have a PLA chip coming in the mail, any day now. I'm hoping that will solve the puzzle...

Regards,

Rob
 
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