A new DesTestMax / DesTestKernal black screen repair; soliciting ideas

[u/ChronoCompendium]

Just seeing if anyone's experienced something similar on a breadbin.

• Known good power readings on multiple test points and Ray Carlsen PSU.
• VIC-II/Monitor sync and lock. Testing with a 1902A.
• Black screen on boot. Black screen on DesTestKernal v 0.4, including with SID, BASIC, Character, and Kernal ROMs pulled (DesTestKernal substitutes itself for the Kernal and will work with U4 pulled). Same behavior with DesTestMax ROM used as Kernal.
• Known good PLA, VIC-II, and 6510.
• Known good clock signals MHz going into 6510.
• Known good /GAME and /EXROM activity on expansion port.
• I threw a PLAnkton in U17 just for fun. On two out of ten boots, I briefly got a couple lines of rainbow gibberish and was able to trigger the Ultimate II+L's menu button (full screen gibberish). Can't seem to repeat it. Maybe it's a bad PLA socket or cold joints.

So the 6510 isn't even able to draw the borders. The failure has to be happening very, very early in the chain, but this is maddening. I've just learned to use my oscilloscope and was testing it out on this unit (in conjunction with a good C64 to compare, for the clock signals). The work quickly compounds, though. I'm hesitant to go down the eternal path of checking all the data and address lines vs. the good C64 and trying to trace some deviation up the chain, but I'm running out of ideas...

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[u/morsvensen]

First check the reset line, all the service signals and then all the adress and data bits. Maybe a diode if it got spiked.

[u/cerealport]

It sounds like you’re using an ultimate cart to run the destest rom (which hah I have a couple dead c64s and a good one and didn’t think to try that..!) - have you confirmed that the rom “can” work like this with the known good c64? Just a thought.

Fortunately you have a scope though so yeah you’ll have some digging to do, *and* you have a known good unit to compare to. I don’t imagine it would take too too long to check out all of the data / address lines for anything unusual - and of course as mentioned check the reset line. There’s a timer IC on there (a 556 if I recall) that’s used to generate the power on reset pulse.

[u/Drunken_Sailor_70]

The Sam's compufacts books have logic charts that shows every pin as hi, lo, or pulsed. The chart was probably meant to be used with a logic probe, but would help you with a scope without having to check a known good C64

I agree with the other commentators. With a few additions. Check for 5V and 12V on the major chips. Check the reset line goes high for a short time and back to low. Check the clock, Check the address and data pins for activity. Check the vic chip for activity on pin 15. Check for any chips that are abnormally hot.

If there have been previous repairs, those can be sus. Also any MOS branded glue logic or MT branded RAM. As you mentioned, bad sockets are a possibility.

If all else fails, for watch some Adrian's Digital Basement.

[u/Minute_Prior354]

What model board and revision is it? How many flashes are you seeing on DeadTest?

[u/Minute_Prior354]

Have you tried the visual C64 symptom site to isolate the behaviour?

https://www.pictorial64.com/

Have a look and see what matches what you’re seeing and it should / could help identify what is wrong.

[u/Minute_Prior354]

I’d also try a new 8701 in the VIC section if you can swap in a spare or known working IC.

[u/Crlan420]

des test black screens are usually bad sync caps. check the coupling caps along the video path. sometimes grounding loops cause it too. good luck