Part 2: Repairing the remaining Commodore C64C longboard - can I fix it ?

In this video I attempt to repair the machine that was faulty last time.
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I had two Commodore 64 machines in unknown condition. I thought they did not work. However one was working perfectly fine, but needed cleaning. The other one was faulty.
This is a follow-up to the previous video about fixing two Commodore 64's.
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[@thebyteattic]

Great effort, Tommy! Based on the small segment of the schematics you showed, I believe those two diodes are for active line termination. Commodore may have expected reflections or transients on that line, which can bring the signal below 0V and above 5V (if the reflections interfere destructively or constructively with the original signal transition, respectively). One of those diodes shunts the line to 0V if it tries to go below it, while the other clamps it to 5V if it tries to rise above it. For them to work, you want diodes with as low forward voltage drop (Vf) as possible, because, effectively, the diodes only clamp if they conduct, which means that one clamps at 5V + Vf, and the other at 0V - Vf. The higher the Vf, the less effective the diodes are at active termination.

[@Arcticretro]

Thanks. Makes sense. You know this stuff :)

[@BarnokRetro]

Great video Tommy! I appreciate how you worked through the issues and found that diode. Loved the reaction when you figured it out too!

[@Arcticretro]

Thanks :)

[@donaldblakley6796]

Great video and I'm glad you found the issue. Nice troubleshooting. That one is a keeper. I have 2 of them restored very nice. I had a 250425 board last year.. said bad control port... drove me nuts.... ended up being the diode at cr2... wow. Very rewarding when you get it. Anyways keep up the great work. Subscriber from Ontario Canada 🇨🇦

[@Arcticretro]

Thanks so much! Greetings from Norway! I been to Canada once. In Vancouver. Just for a couple of hours.

[@congestiontv]

Nice, another one saved for many decades to come. Could you believe I received a Teensy board to build my own XUM1541. I got it working while watching your video. That means it was a simple task because I always get distracted watching your very technical videos. Saying it works is actually half the truth. It works on my 1541 but not yet on my FNX1591 (a modern 1581 rebuild). I'm saving this work to watch your next video. Doing it like so makes sure many errors will happen on my side. ;)

[@nigelreed5920]

Great fault tracing tutorial Tommy, Love ya work mate.. Stay safe

[@Arcticretro]

Thank you :)

[@deborahberi3249]

Excellent troubleshooting that made for a great Video! -Mark.

[@Arcticretro]

Thank you :)

[@theEIGHTBitBox]

Nice error analysis! Learned something new today!

[@Arcticretro]

Glad to hear it!

[@horusfalcon]

Tommy, you gotta put that on a shirt! 😆😆 (I won't spoil it for anyone watching ...)

[@Arcticretro]

Hehe, what ??????

[@bwack]

Well done, great video :) Those pesky protection diodes :D I remember seeing GadgetUK replacing those diodes in his videos several times too. This is the same computer model, case, keys, motherboard and SID, that I grew up with. I still got it :) By pure luck, I bought it back from someone at school who I sold it to.

[@Arcticretro]

That is cool. Yes, a nice machine. Happy I could fix it :-)

[@fragram1976]

Good circuit analysis! Do not bother with screen capture, watching signals and you in the mirror at the same time is more interesting. 😄

[@tommyovesen]

Hehe, thanks!

[@borayurt66]

My first guess would also be the 7406, it is known to fail. But, after replacing that and noticing the short, I would go for the diodes, more likely to go bad than a DIN connector and a ceramic cap. The original is a 1N4148, jellybean small signal diode, 1N914 is the ancestor of that, but here they are interchangeable. You should get a bag on 1N4148 diodes, they are everywhere in retro computers. I also recommend getting some BAT85 diodes too, they are the Schottky version of this type of small signal use. Less voltage drop, so better for digital circuits, not interchangeable for analog use though.

[@Arcticretro]

Thanks for the advice :)

[@networkg]

Damn diode died !

[@michaelstoliker971]

I had a C64 breadbin with that motherboard.

[@Arcticretro]

Nice :)

[@LionwoodRetroStudio]

Always such good music on this channel..!

[@Arcticretro]

Thanks!

[@bluebirdpod]

Had a similar problem on this same 466 motherboard. Had serial drive communications failure. It was NOT the 7406 but right next to it the resistor pack had a partial open and allowed some of the IEC signals to go hi. I had conversed with Ray Carlsen and he said he had never seen the IEC problem caused by the RPack next to the 7406. It had lost half its values on half the pack. Changed it and it fixed it.

[@Arcticretro]

Nice to know :)

[@jasmijndekkers]

Hi Tommy, Nice to see you work again on a Commodore C64C. I enjoy your content and videos very well. Greetings from Steven from the Netherlands

[@Arcticretro]

Glad you like them! Thanks!