C64 With Black Screen

[C64Person]

I successfully soldered jumpers to repair the faulty traces. Continuity seems good, so that's a step in the right direction.

Unfortunately, pin 13 on the U9 RAM was already missing half of it, and it got bent in the process of taking it in and out so many times. I tried to repair it, but it then broke off completely, so I am going to have to order some new RAM before I can go any further.

[rmzalbar]

Those half-pins on the end are a little more fragile as they are milled off there. A little contact cleaner or alcohol may help lubricate insertion/removal.

If you're in the US, you can get new old stock DRAM from Unicorn Electronics.
Any of the 4164 16-pin DRAM on this page will work:
https://www.unicornelectronics.com/IC/DYNAMIC.html

41256 can also be made to work, with a minor modification. Here's a cross-reference guide.
https://www.c64-wiki.com/wiki/RAM

[C64Person]

Thanks. I have ordered some replacement RAM, and I will see what happens when it arrives.

[C64Person]

Hi.

I replaced the RAM at U9, and the computer is still giving a black screen. pin 13 is connected now on all RAM, but my oscilloscope seems to be acting up so I can't check how it looks. It's showing some really weird patterns on the screen even without the probe touching anything (pictured below).

I have 4 new 4164 RAMs and 1 74LS257 multiplexer. Any advice on next steps?

Thanks.

[rmzalbar]

Scopes can pick up noise very easily when unconnected. Make sure your ground lead is well connected both at the probe and at the ground point on the C64. Make sure the gain is set the same on the probe and the scope (10X on both for example.) Does it read OK when you for example check a 5V source on the board, or at the calibration sources on the oscilloscope itself? You really need to be able to check the signals on the RAM lines.

[C64Person]

Scope seems to be reading normally again! The first image is pin 13. 2nd is 11 which I have had no reason to doubt, and the 3rd is 9 which still seems low. I'm going to keep checking all the lines for any problems.

[rmzalbar]

You can try this trick - take one of your new RAM chips and "piggyback" it on top of each RAM chip in turn, just line up the legs and slip it over the top so that it makes good contact. If one of the RAM chips isn't doing its job (but isn't causing any other problem), sometimes this will let it boot up and then you have identified which one it is.

Won't definitely work, but is easy to try.

[C64Person]

Thanks, I tried piggybacking them when I got the new RAM but still nothing. Do any of the data lines look wrong or do they seem better now?

Does 4.43VDC seem normal for power intake from the power port of the C64? I know it's supposed to be 5V. The power supply is sending about 5.1V when the computer is off, but only 4.43 when it's on. Just thought I'd double check.

[rmzalbar]

4.43v, that's too low! Either your power supply is bad, or a chip on the board has failed and is pulling too much current. The VIC-II has its own personal 5V power supply derived from the 9V side, so it won't be the VIC-II, and indeed, you can see the VIC-II is (trying to) do its job of refreshing the memory by the activity on the address lines. However, those address lines run to the logic ICs and so the signals are powered by the main +5VDC voltage to those ICs past that point, and all other main chips are powered by the same +5vdc supply too.

If any of the other chips are in sockets, pull them out, and then recheck voltage to see if any of those were responsible. You need at least 4.8v at the Vcc of all chips. A good place to check is between pin 7 (GND) and pin 14 (Vcc) of the 7406 logic IC at U8.

You might have a bad PLA, DRAM, CPU, or logic IC.

[C64Person]

That's good to know, maybe that's the problem (or at lease one of them). The PLA is new, so that's fine, I removed the 4 socketed RAMs and the 7406, and swapped the DC power supply. None of the removed chips changed anything, but the DC PSU swap put me up to 4.6V, still too low though. CPU, 4 RAMs and all logics except 7406 are soldered to the board. All chips are getting 4.6V now at VCC.

Thanks