C64 - 250425 Mobo Variations

[Jazzmarazz]

Two of my C64s do not work, both of which are Assembly number 250425 from 1984. After hours and days of analyzing them for faults and cold joints, etc, I have noticed several seemingly large variations that these boards may have. (These 2 are not mine, but I will post pics soon):

Here is a normal C64 mobo with nothing noticeable different. There is one electrolytic capacitor that has replaced a ceramic one in the center, but I don't think it changes anything.:

And Here we have several diodes that are not present in either of my C64s. Diodes visible are located on the left-most side and also to the left of the serial port. Another thing is the capasitor that you can see above the fuse. It is not there in any other board I have seen.

One of my mobos (to be pictured shortly) has a resistor in place of C204, a green cap located above the ram. I plan on replacing this with the appropriate cap (151J) which I think should be there instead, but I don't know why any of these changes have been made.

Does anyone think that I should replace the resistor labeled C204?
Does anyone know what these differences do or why they were made?

Also, while we are on the topic, I found 4 or 6 diodes that have replaced 2 Ferrite beads in my non-working C16. If anyone wants to see this, just ask and I will take a photo.
I received all of these computers well after prior owners had them and have no clue if they have been modified. I would really like to fix them but want insight from all of you because I am running out of options.

 

[rkauer]

As a rule of thumb, C= 8bit machines have insider chips that heats a lot.

Just power the boards and wait 30s and then put your finger on the custom chips & ROMs.

Chances are the stone-cold ones you'll find are the culprits.

But on C64 the main faults are the CPU, CIAs, ROMs & VIC (video chip). The SID only affect audio and the board might work with a defective one (pending the problem).

Remember to use a C= PSU with 9Vac on the alternate current section for tests! Some C64/128 PSU have 12Vac on the alternate pins that can fry the older board's SID!

 

[gazcbm]

Yep the board will run without a SID. First thing I would check is the PLA (6114) most black screens I've had is a blown PLA. I have however had a few RAM issues and 2 boards that almost all chips on them had blown (including my Sx64 when I bought that)

Sent from my GT-I9100 using Tapatalk

 

[Jazzmarazz]

Chances are the stone-cold ones you'll find are the culprits.

Remember to use a C= PSU with 9Vac on the alternate current section for tests! Some C64/128 PSU have 12Vac on the alternate pins that can fry the older board's SID!

Now, everything I have read is that the most hot chips were suspect, but cold chips would be more logical.
I have been slowly de-soldering chips from my working 1983 mobo and inserting sockets to test the chips from my bad boards. Its a real pain becuse none of the chip from 1983 were socketed, but I have so far found one good PLA and one bad PLA. The RAM that I have pulled have all been good, but I have not pulled all of them yet. The sad thing though is that putting a known good PLA into these mobos does not fix them...

I will continue testing other chis.

Yep the board will run without a SID. First thing I would check is the PLA (6114) most black screens I've had is a blown PLA. I have however had a few RAM issues and 2 boards that almost all chips on them had blown (including my Sx64 when I bought that)

First thing I thought as well. THe "bad" PLA that I have will load random text and colors in both my good mobo and one of the bad ones, which is strange since a good pla just gives a black screen in the bad mobos. :huh: