The UGLIEST Repair of a Commodore 64 (Part 2)

Now that the C64 is fixed and with reliable power, we focus again on diagnosing and fixing the black screen. I try to make sense out of what we're seeing and start desoldering some chips, and more, and more... It ends up being quite an ugly repair, but at least things (sort of) make sense at the end.

Chapters:
00:00 Intro
00:48 RAM?
04:52 CPU?
06:08 CIA?
06:48 New RAM
07:31 MOS?
09:15 Multiplexors?
10:25 VIC-II?
12:00 RAM again??
13:23 Broken cap!
14:33 Old RAM
16:35 What else?
19:58 Bad signals
22:36 Look ma', no RAM!
24:03 Conclusion

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#c64 #repair #commodore

[mertuckan]

I've had uglier. last week I socketed everything, recapped. I even replaced the user port socket. Checked every single line. It turned out to be a short thinner than a hair under one of the ram sockets between two data lines. It was so thin that my equipments did not pick it up. I discovered it by "inspection by eye" method. Took my 8 weeks. nice to know even proffesionals can have hard time :) Please keep sharing even something goes wrong. That is how I learn.

[redace]

We like the ugly ones! The deeper the dive, the more we thrive! Gives us a chance to see more of the circuit, inspect the schematics, get a feel for 'what's cooking in the oven'. 😁 Keep it going!

[phil.pinsky]

Not sure why you keep apologizing for the ugliness of the repair. I loved watching you go through all the steps and really fleshing out what could or did go wrong. I have a breadbin is in a similar situation and I feel like I could watch this series as a how to on how to fix it. Thank you so much!

[RetroRecipes]

Apologies for the conclusion confusion! Great video 👍🕹️

[FPVcondor]

Hi Noel, I just recently run into your channel (algorithm is working) and I really had a great time. Your videos are very well done, the topics are so captivating and you transmit the concepts with a teacher's perfection. I used to spend my life playing with Commodore 64's, CZ Spectrum's, Talent MSX and TI99's, 35 years ago, and have never touch one since then. Anyways, your channel restarted a sparkle I thought it was gone for ever. I wish I had a C64k again. Sorry about my English, I am not a native English speaker. Thank you.

[obiwanjacobi]

The ugly ones are the most interesting IMO...

[MichaelEhling]

So NOT ugly. There is beauty and wisdom in each step...especially the, "Start by replacing the MOS chip" part :)

[morantaylor]

Lol I watched a video whe Bil Herd said he is surprised MOS chips are stiill functioning after 40 years they had a 5 year life based on the materials used with NMOS technology available at the time.

[Skyfox]

The ugly repairs can be the best repairs because we can learn so much about them as so many things go through troubleshooting. The more unexpected things the better because it adds to the collective knowledge of Commodore repair.

[elfenmagix]

Ugly or not, it was a beautiful repair. 99% of all C64 repairs is "It must be this or must be that... Oh, it is the PLA!"

You did great work here.

[ctrlaltrees]

I like your new announcer, he sounds like a real chip dipper 😉

[CaptainKirk]

It wasn't ugly sir, you were learning. It's easy to jump to the common problems you're used to seeing first, but iv learned to always do a thorough visual inspection of anything new I'm working on first. Visuals are free.

[paulkoopmans]

I agree with everyone Noel. These harder ones are just more fun to watch! Your content is great.. keep up the good work!

According to the schematics and the datasheets for the 6510 and 6567 (VIC-II) that second AND gate at 21:00 is actually pretty important I think. As you discovered the output (CAEC) goes to the processor AEC pin (5). The processor needs to free up the address bus in two conditions: 1. When the VIC-II needs to access memory, 2. when DMA is used (for example by REU's). the AEC pin (16) on the VIC-II is an output pin and was designed to attach to the AEC input pin on the 6510 directly. As the name suggests the processor can drive the address bus as long as the AEC is high. When AEC is low the processor enters high impedance on the address bus.

The DMA is active-low and in normal conditions pulled up to 5V, therefore pin 5 on the 74LS08 is normally always high. The VIC-II will switch it's AEC output pin low when it wants to access memory.

If cartridges like REU's want to perform DMA, they have to adhere to very strict rules: BA from the VIC-II to the cart must be high and in the right clock phase the cart can then pull DMA low to take control over the address bus.

So this second gate is crucial in bringing these two very important lines to the 6510. If this gate fails and outputs a high then you will get fighting on the address bus, if it is stuck low then the processor will not do anything. It runs of course... but cannot do any storage or stack operations. Obviously any garbage signal in between will mix those two behaviours.

So that second gate might not have an issue after all... it looks like and your determination on the color ram select is the exact problem.

The lesson learned, I think: When you do your normal routine of checking for clocks, ready, etc on the processor, also scope the AEC on 5; it should be pulsing. And scope the CS on the color ram.

[fkthewhat]

Hello Noel, Perifractic and Adrian! Great to see everyone working as a team haha

[lexluthermiester]

@Noel
While this video was a year ago, being someone who does low-level electronics servicing like this I can say with confidence that it was not an "ugly" fix up. Some repairs are not as simple or easy as others. This was interesting every step of the way and I could not see anything you did wrong. Complex repairs are complex, it's the way life is sometimes. Carry on good sir!

[mikezimmerman]

My dad was a typewriter repairman in the 70's and 80's (well he still is!). I lost one of my joystick controllers on my C64. I had no idea how to fix it, I was 14. My dad opened it up and figured out which part it was. I was amazed. I mean I knew my dad did know electronics to a certain degree but how he figured out it was that chip, was beyond me. Hindsight, he worked with a guy who was working as a typewriter repair guy temporarily while he got his engineering degree. That guy went on to work at IBM labs or something like that, so my dad might have had a little help, but Im pretty sure the C64 never left our house, it was unbuttoned in my room for a few weeks. Im an engineer now so Im pretty sure you can trace the closest chip to the broken port and just shotgun replace it. But still, my dad nailed it. Hero achievement UNLOCKED!!!

[ericmbusa]

I loved this series! It's so cool to see things go wrong and watch the thought process and troubleshooting steps taken. These are my favorite videos!

[winstonsmith]

Thanks for finally, unlike others, stating the obvious that perhaps because of the manufacturing process they used that MOS chips are such common culprits in failed machines. And I don't mind "ugly" repairs. They are more interesting than more routine ones.

[OtreblaMaslab]

Thanks for the tip about testing logic chips with the Minipro, so glad I have another use for it now! 👍

[robjw]

Noel, it is always the last thing you look at, as we stop looking once its found! great video of a frustrating repair.