Commodore 64 REV A board (ASSY 326298) Black Screen- All attempts at troubleshooting have failed HELP!!

[SkarloeyFell]

Hello, I picked up a cheap (not so cheap) Commodore 64 to gift to my father for Christmas, I was told when I bought it that the only issue with it was they didn't have a video cable. Turns out that was not the issue. The computer only outputs a black screen, not nothing. The output has the vertical blanking interval and the screen flashes when you give it power like I've heard it should. I have a backgroung in troubleshooting and technology but that experience is with analogue old tech and modern technology; the Commodore is sat in a valley of complexity and archaic difference that I just haven't been able to wrap my head around properly. Using my best capabilities and troubleshooting from the internet over the last month I have been unable to pin down what issue is causing this. At this point I've replaced the PLA, the VIC-II, and the 6510. I've picked up a dead test cartridge and with it inserted there is no change. Just the same black screen. The replacement I got for the VIC-II is a Kawari LG and the PLA is another modern equivalent, says C64_U17 on the chip but again it's been a month since I ordered it and neither of the 2 I received gave any change to the behavior to the system. The Kawari has an HDMI output but it again is a black screen; it's outputting a signal just a black signal. Putting in the "new" 6510 created again no change and at this point I have nothing. I've exhausted the length of my knowledge and the help I can find just searching online. I can't find any bad traces or solder joints that are causing problems but nearly all the chips have been either replaced or socketed just looking at it and I'm just lost as to where to look. I have a multimeter and an oscilascope and have used both in my attempts to figure out what's wrong but couldn't find anything with the knowledge I have. The clock seemed to be running properly when I last tested it, again this has been just such a long process I fear I'm at the point of running back through everything I've tested and testing again in some vain hope of finding it this time. If anyone has experienced an issue like this please let me know how you resolved it.
Also if anyone knows anyone or anywhere within the greater Philadelphia area that would have the know-how to diagnose and fix an issue like this let me know so far I've found one store that seems promising but most focus on far newer technology.
I've attached pictures of the front and back of the board; the visibly damaged trace in the bottom right still has proper continuity (just to clarify before ppl mention it)

 

[Timo W.]

Black screen with a while line means that a lot of stuff is actually working. Suspects in that case would be PLA, CPU, ROMs, RAM (more or less in that order).

You certainly wasted money for the Kawari.

As for the Dead Test: did you wait some time? It is normal to get a black screen for >10 seconds before it shows something - unless there's bad RAM, then it flashes the screen.

You may want to get a DesTest MAX or DesTest Switch cart instead. They still work even when Dead Test fails.

 

[MicroCoreLabs]

One possibility would be to try an MCL64: MCL64

 

[xp0nentz]

Hmmm, how long did you wait on the black screen? When I hook my commodore to a modern tv, I have to wait 3 seconds on a flickering black screen before it shows picture. I'd guess this is maybe a ROM/RAM or video issue. Could be a CPU issue too. Try checking those.

 

[SkarloeyFell]

Thank you al!! I've let it run for upwards of a minute generally when testing at every phase. I have also allowed the system to sit on the black screen at least 30 minutes once in a hope that letting the system warm up might have helped, it did not. The Kawari was gotten simply because I figured the digital output would be nice and shipping/availability wise it was the most reasonable option for that before Christmas (as this was supposed to be a gift).

• I have tested with both the original composite video out on a CRT from the early 80's and the HDMI of the Kawari so I know there isn't any issue with it being hooked up to a modern display.
• With the Dead Test: I have let it sit for a while, for the majority of the tests I did with it I waited at least 30 seconds before resetting the console and trying again.

I will look into the DesTest cartridges they seem promising as the PLA, CPU, and VIC SHOULD all work as all have had new replacements installed for them and likely even the original chips were functional seeing as doing so saw no change in behavior.
The MCL64 is interesting looking but seeing as I did just buy another CPU I think I may try the DesTest first as it is the cheaper option. If that fails I suppose the next step will be the MCL64 but I hope it won't come to that.
I will reply again when I do get a DesTest, I've spent quite a bit on this computer already and I'm probably going to wait until I can put some more money in the bank to crack at it again. Thank you all again for the responses!

 

[xp0nentz]

Thank you al!! I've let it run for upwards of a minute generally when testing at every phase. I have also allowed the system to sit on the black screen at least 30 minutes once in a hope that letting the system warm up might have helped, it did not. The Kawari was gotten simply because I figured the digital output would be nice and shipping/availability wise it was the most reasonable option for that before Christmas (as this was supposed to be a gift).

• I have tested with both the original composite video out on a CRT from the early 80's and the HDMI of the Kawari so I know there isn't any issue with it being hooked up to a modern display.
• With the Dead Test: I have let it sit for a while, for the majority of the tests I did with it I waited at least 30 seconds before resetting the console and trying again.

I will look into the DesTest cartridges they seem promising as the PLA, CPU, and VIC SHOULD all work as all have had new replacements installed for them and likely even the original chips were functional seeing as doing so saw no change in behavior.
The MCL64 is interesting looking but seeing as I did just buy another CPU I think I may try the DesTest first as it is the cheaper option. If that fails I suppose the next step will be the MCL64 but I hope it won't come to that.
I will reply again when I do get a DesTest, I've spent quite a bit on this computer already and I'm probably going to wait until I can put some more money in the bank to crack at it again. Thank you all again for the responses!

Try peeking (see what I did there) into the ROM and RAM.

 

[MicroCoreLabs]

The MCL64 is interesting looking but seeing as I did just buy another CPU I think I may try the DesTest first as it is the cheaper option.

I can loan you one if you like.

I suppose the next step will be the MCL64 but I hope it won't come to that.

Lol... It's not going to bite you! (Actually, it appears that you have already been bitten a time or two...)

 

[xp0nentz]

One possibility would be to try an MCL64: MCL64

Just read what the mcl is, yeah try using an MCL.

 

[daver2]

First of all, welcome to VCFED.

Second, as you bought this unit as a 'working computer' (despite the fact that the seller couldn't demonstrate it working - which is the first 'red flag'), you may be able to return it for a refund?

However, as you have already started to fault find on it, this may not be an option now.

If you wish to continue, I can suggest a few places to probe with your oscilloscope in the morning (it is bedtime now in the UK).

Don't forget you will be under moderation for the first 10 posts, so your posts could be delayed before we see them.

Dave

 

[robert_sissco]

Odd question, but have you used your multi-meter to check the voltages at some chips? (CPU, VIC, etc.)? I have not personally seen it, but I have seen videos on YouTube where the issue was while everything was being powered, the voltage was a bit to low for the chip to properly operation. One I remember it was the 5-volt regulator that needed to be replaces (I think this was an Atari 2600 video) but when they swapped that out the machine worked.

I am just asking because you noted in your original post that you used one, I just didn't see you say anything about the voltages

 

[daver2]

If you have an oscilloscope, use this in preference to the multimeter to measure voltages. It is the better piece of test equipment because it can display 'glitches' in the voltage that the multimeter ignores!

This is always my starting point. Measure all of the DC rails on the computer identifying the DC value, and then switching to AC coupling and looking at the ripple and noise on the supply rails at varying oscilloscope timebase settings.

Dave

 

[MikeI]

I got a Chip Tester from backbit.io to help me troubleshoot my Altos system and just started brute force testing every single chip on the cpu board ... in the process I found 3 bad memory chips and once replaced got a monitor prompt! I had been trying to use a logic analyzer and scope to figure things out along with replacing chips I thought "might" be the one, but without much success. The maker has a bunch of youtube videos regarding Commodore units, Evie's revue I think it's called. Good Luck!

 

[SkarloeyFell]

If you have an oscilloscope, use this in preference to the multimeter to measure voltages. It is the better piece of test equipment because it can display 'glitches' in the voltage that the multimeter ignores!

This is always my starting point. Measure all of the DC rails on the computer identifying the DC value, and then switching to AC coupling and looking at the ripple and noise on the supply rails at varying oscilloscope timebase settings.

Dave

I have an Oscilloscope (A nice Hantek) and did use it to check power and clock first thing when It wasn't working both before and after getting a new power supply as this was the first thing I replaced. (It came with the OG and was visibly in poor condition and didn't trust it). I think I will test again before I go buying anything new. Some direct info on good test points would be appreciated as I have been working with some frankly painful to parse information from very old resources. (As with many things retro all the in-depth information is locked to websites that haven't been updated since before I was born or is split between 100 youtube videos).

First of all, welcome to VCFED.

Second, as you bought this unit as a 'working computer' (despite the fact that the seller couldn't demonstrate it working - which is the first 'red flag'), you may be able to return it for a refund?
...

I bought it as "probrobly working" in cash from a local shop for less than a for parts 100% known faulty one online, I was acceptant of the fact that it probably wasn't gonna be working and I'd need to do some work on it and even still with all I've ordered for it I am still in the green compared to a guranteed working one (that wouldn't be as featured as what I've done (new power supply & digital out) so I'm fine with the financial, I just try to be as fiscal as I can.

I got a Chip Tester from backbit.io to help me troubleshoot my Altos system and just started brute force testing every single chip on the cpu board ... in the process I found 3 bad memory chips and once replaced got a monitor prompt! I had been trying to use a logic analyzer and scope to figure things out along with replacing chips I thought "might" be the one, but without much success. The maker has a bunch of youtube videos regarding Commodore units, Evie's revue I think it's called. Good Luck!

A chip tester's not a bad shout but for $200 I feel with all that I've already replaced I surely cannot be far from finding the thing that's keeping me from progressing, if I had known from the start that I'd replace all that I had it may have been the move but as of right now I think I am better off testing via other means unfortunately

Just read what the mcl is, yeah try using an MCL.

I suppose I'll swap the order of MCL and DesTest in priority then, again I think I will wait a tad before ordering just to mull it all over but the current order of affairs I plan to move forward is as follows:

1. Retest continuity, power, and clock everywhere I can (I had already attempted this but to go through fully comprehensively I think may bear fruit)
2. Since my last post I have done some visual analysis and it appears that the caps have been replaced before but I have grown suspect of a few, so I will look into those in detail (I believe my scope has capacitiance testing aswell so I'll need to read up on that)
3. Order or see about getting an MCL loaned, I really appreciate the offer but I've always been the type to worry when borrowing things and the faster speed operation possible with the MCL intrigues me enough to consider it outright beyond just as a diagnostic tool. Although part of me still thinks I may go for a DESTest beforehand, I'm unsure how much more I want to ship of theseus this poor computer.

I have unfortunately come down with something nasty and have been slowly getting better so it might be the new year before I do anything, thank you all for the help though it has definitely given me a lot more direction which I was sorely lacking with this project. Next time I post I will have at least done the retesting, thank you all again.

 

[daver2]

No problem and get well soon.

Let us know when you are ready to continue.

Dave

 

[Intex]

Generally, check voltages at the chips. All of them. The VIC gets its power from another rail than the rest. So you will get the black screen with white line, but the rest of the machine may not get power. So check every IC. Especially when someone tinkered on the machine.

Then, check the reset signal at the CPU. Should be low after switching on for a fraction of a second, max 1 second, then go to high.

After that, check for clock signal around 1 MHz on the CPU.

If that all checks out, it is time to check the datalines to find if one of them is stuck. Check datalines on the CPU for suspicious lines. After that, check the address lines the same way.

Also do the heat test, check with your fingers if any IC gets hot.

If that all checks out, let us know.

P.S.: remove the SID before testing. They are valuable and sensitive. Also you do not need it for the machine to boot. Also you can take out the two CIAs (6526) and see what happens.

 

[SkarloeyFell]

Hello everyone! Sorry it's been actually months but I figured I should update with the images of it fixed!
I was unable to get it fixed myself, it was some ram logic controller chip according to the person I brought it to to have it fixed. (Sloopy if you know him). So I just don't think I would have ever figured it out without someone else more knowledgeable and with parts to swap out.
Everything's working now, I put in the VIC replacement and have it hooked up to an LCD for my dad and he's really happy with it. I have all of the original parts as well of course in safe storage. I was advised by him to do some stuff to improve it's longevity since it is a REV A board, so it should last a good few more years. (Fan in case and heatsink on the CPU)

Thank you all for your help! If it had been some like, reasonable issue the troubleshooting advice y'all gave would have helped but I still really appreciate all the time you gave anyway! I've attached some images of it running, it looks even better in person.