Diagnosing a Commodore 16

[Troy_F]

I'm hoping that someone can point me in the right direction for a troublesome Commodore 16 I picked up recently. The unit was a bargain, in it's original box and in great condition, however when I got it home I realized there was a problem with it. The unit seems to power up fine, however it begins to freeze after only a few minutes of use. When frozen the reset button doesn't work, but just puts a few random characters on the screen. I tested the power supply and noticed that it was outputting almost 14V, more than I thought it should for being a 9V supply. I tried the system with a known good power supply and there was no change in behavior. Lockups seem to happen within less than five minutes. I opened the machine up and the board is super clean, in really good shape. I don't see any obviously defective components (leaky capacitors, burnt components etc). I also pulled both of the ROM chips and read from them, they are both clean and match known good dumps. From what I've been reading online, the PLA may be the problem. Does anyone know how to test the PLA, or if my symptoms would make sense in this case? I've read about some people replacing the PLA on a C64 with other chips, does anyone know if this can be done with the C16?

Thanks!

[rbernardo]

I've read about some people replacing the PLA on a C64 with other chips, does anyone know if this can be done with the C16?

Replace a C16 PLA with a C64 PLA? I've never heard of that. For a proper diagnosis of the machine, you should have it examined by a service tech, like Ray Carlsen.

In Las Vegas,
Robert Bernardo
Fresno Commodore User Group
http://videocam.net.au/fcug
The Other Group of Amigoids
http://www.calweb.com/~rabel1/

[Troy_F]

I continued to troubleshoot the unit myself (I have a hard time taking the machine to a technician and paying them to have all of the fun), and traced the issue back to an overheating CPU. I've fixed a heatsink to the 7501 and there have been no further freezes or crashes. Here is some more background on the issue in case it helps someone in the future.

For testing I wrote a very simple program that just printed numbers to the screen:

10 A=1
20 PRINT A
30 A=A+1
40 GOTO 20

I noticed that after some variable amount of time I would start to see lines which were blank injected among the numbers, or I would see numbers repeated or out of order. Often the BASIC program would crash shortly thereafter, dumping into the machine language monitor at an address of DAED. I could exit the monitor, re-run the program and the system would print more numbers for a few seconds and then crash again. I initially tried spraying the voltage regulator, TED and PLA with cold spray but this didn't have a concrete effect. Eventually I sprayed the CPU and found that the system would then run for quite a while without interruption. Since attaching an aluminum heatsink the system seems rock solid, running for hours without crashing.

Hopefully this helps someone in the future.

[rbernardo]

...and traced the issue back to an overheating CPU. I've fixed a heatsink to the 7501 and there have been no further freezes or crashes.

Hey, that's great to know! I knew that the chips in a Plus/4 should be heat-sinked (including its 7501), but, as you have proved, the same chips should be heat-sinked in a C16.

Truly,