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ianoid
February 21st, 2017, 02:51 AM
I have a couple of nice condition units but neither works perfectly.

The C64 starts up but has a few bad screen characters. I have three diagnostic carts. One diagnostic cart starts up with "Out of memory". Another has random RAM chips marked as problems, inconsistent- it changes every time. Another routinely gives me U24 as the problem (which is the one I trust). I actually pulled the SID and got the same error, so I put it back in. So I'm ordering some ram and I hope to try stacking the RAM on the RAM chips as a test for that U24 chip before I have to desolder a chip (which is something I haven't done and will have to learn from videos). Replacing some RAM seems promising. Sound reasonable? Would anyone suggest just soldering the RAM onto the old one? Will the old one still heat up and eventually wear out the new RAM chip? The RAM is not socketed.

The VIC sadly (late model with round power adapter) just flashes the LED on and off with a totally black screen. Based on the original box (bought it 'new in box, but clearly was otherwise) which had what I would guess was submersion and mold damage (actually looked good but was flaking apart and smelled bad), I think this one has been wet before long ago. The PCB has some black dots on it. See picture below. Are these going to be symptomatic? They don't wipe off easily. Is that damage to the PCB that will be prevent this unit from working? Or is the flashing LED symptomatic of something specific? The power supply is known good, in fact it's a modern replacement PS, and I've tried multiple PSes including a Ray Carlson design.

Thanks for any help you guys can offer!

36355

Scali
February 21st, 2017, 03:27 AM
Replacing some RAM seems promising. Sound reasonable?

Yes, I bet there's some dodgy RAM there (either that, or the power supply to that RAM perhaps, some dead capacitors?). RAM chips are often the first to go in a C64.


Would anyone suggest just soldering the RAM onto the old one?

I would not.


The RAM is not socketed.

If you are going to desolder the original RAM anyway, I would put in sockets while you're at it. This makes swapping/testing RAM easier in the future.

KC9UDX
February 21st, 2017, 04:55 AM
I would recommend to desolder the RAM in the C64, and install sockets, without even testing the RAM. You very likely have RAM errors, and I don't think it's worthwhile to hunt them down. If you're good at desoldering, you can reuse any good chips afterward anyway. (I say that because you may find someone experienced to do the work.) Definitely do not solder new RAM atop old RAM.

Speaking of power supplies, I assume you have the original potted "brick" power supply (it feels totally solid when you kick it). I highly recommend not using that. It *will* destroy your C64 sooner or later, and it's on borrowed time. I don't recall if it will destroy a Vic; I think even the later Vics generate the +5V internally, but don't quote me on that. The problem is that the voltage regulator for the +5V inside that brick will fail, and in my experience always fails with high voltage.

If the Vic-20 LED flashes regularly, there must be some diagnostic software in the ROM which I'm not familiar with. (I'm not very familiar with the Vic-20 anyway.) Clean the board, there are instructions all over the place here on how to do that safely. Then, again, suspect the RAM. But maybe wait for someone who really knows the Vic to post here first.

ClassicHasClass
February 21st, 2017, 10:52 AM
If the Vic-20 LED flashes regularly, there must be some diagnostic software in the ROM which I'm not familiar with.

Although anything's possible, I've never seen a VIC-20 or 64 where the power LED was modified to display (essentially) a POST code. That would need some substantial internal modification.

My bet is that the voltage is dropping in some sort of cyclic fashion, which makes the power LED flash. If the power supply is not suspected, then one of the internal regulators probably blew.

KC9UDX
February 21st, 2017, 01:00 PM
Although anything's possible, I've never seen a VIC-20 or 64 where the power LED was modified to display (essentially) a POST code. That would need some substantial internal modification.

My bet is that the voltage is dropping in some sort of cyclic fashion, which makes the power LED flash. If the power supply is not suspected, then one of the internal regulators probably blew.

I wouldn't expect a POST code, but a flashing power LED is right up Commodore's alley for a fault indication. Though, I don't know why the Vic would have something the PET and C64 don't have. So, you are probably right.in which case, it's not a regular, crisp blink.