Stubborn C64 Repair 

The PLA always likes to get involved in any C64 repair video.

You're on a row, Jan, great and instructive video! :-) That wonky signal is an example of three normal effects we may often confuse with a problem when interpreting the output of an oscilloscope: (1) the rising edge of a signal depends on the parasitic capacitance inherit to the network transmitting that signal; that is, it is design dependent. In the 80s, people had little care for this effect, because slow rise only slightly delays the point when the system recognizes a transition from logic zero to logic one. And since those computers operated at very low clock speeds anyway, this delay was often negligible. This is what you saw, I think. (2) Sometimes a bus line (particularly address lines) switches from zero to one, and back, without any periodicity (that is, without a discernible, repeating pattern), which confuses the trigger mechanism of the oscilloscope. The result is signals that seem to climb a step ladder up or down, displaying ambiguous logic values. This is often an artifact of the oscilloscope and not a problem in the circuit. Many a repair person has gone down a wild goose chase when, in fact, there is no problem at all. (3) Chips have internal switching because of the dynamic activity of their internal logic gates. This activity often leads to relatively small changes in the signal level visible from the outside, which is also synchronous with the system clock. Those can be confused with bus conflicts, and also lead people down a wild goose chase. In general, the oscilloscope is only as good as the operator's ability to interpret what it displays, and shouldn't be believed blindly! What you did (that is, to compare the signal to that of a working machine) is the way to go!

Jan Beta, 8-bit guy, Adrian Black and Noel Retrolab, all quality assurance seal in retro home computers repair and restoration, thumbs up!

Best thing to watch on a Sunday afternoon 👌

"The things that ruin your day are often the most obvious ones" ... so true. When nothing makes sense or when the behavior is just off the charts, then we are likely to overlook the most obvious of things. The censored middle finger was priceless :)

Those two address lines are crazy. I wonder what loads them like that.

I worked as an arcade game tech for 26 years and I had to laugh when you swore.... I remember the frustration of thinking for sure you found an issue only to have it make you feel like taking an axe to it when it proved you wrong... a familiar feeling lol

Well done for hanging in there and working it out... a true "diy" and "right to repair" inspiration. I'm sure that others (myself included!) would have given up much sooner, thrown it away and told the owner to just get another one.... oh... wait... mmmm. well done again.

Shorter legs? Crazy, never heard of that. Great video, as always. As stubborn as Noels patient in the video you linked

Wow, that was a long slog to a fix. It's really great to see that you have the same sorts of issues that I do, as sometimes we get the feeling that "Jan Beta or Gadget UK or Noel would have cracked this by now" but in reality you guys sometimes hit stupid snags like this and we are not alone! Great video, I often think that you learn more from making mistakes then you do when things just go right so well worth watching a lot of useful tips.

I know a lot of people suggest right off the bat to yank out the roms and the CIAs. Granted you only had one socketed. But a number of times when I realized you were having misleading results, I was suggesting it to the screen over and over again. lol

The BSOL! I love how you used a contemporary based term to describe a retro repair success!

I had to laugh when you gave it the middle finger!

You are my Hero. Who was "stubborner" or was it "stubbornier"? It was Mr. Beta, the Savior of C64´s. 👌 It is very Kind, that You mentioned Mr. Peters. 💚💚💚 c.u. Zui 😎

Rabbit Holes followed by giving the board the one finger salute spells frustration we have all experienced at one time or another. Last time I went down that road on a Commodore 8 bit was when two pins on one of the control ports on a 128DCR were shorted because one pin was bent. Yea had I done a full inspection of the ports I would of saved myself a lot of frustration and time. I have an old SAMS Computerfacts repair guides which have expected signals on the schematics which are helpful. When you have what you believe to be strange signals you did eventually do a comparison with a working unit which is a good practice which is not really different then swapping out IC's to test between working and non working units. Nice to see you putting the new scope to work. Now you have a behavior that you can document for future reference. if you have space around your bench you might want to put a Desktop or at least a notebook just for pulling up schematics and for documenting everything. Using Microsoft One Note is very helpful for creating your notes which can be synced to your other devices including your cell phone which is always with you so you can call up and review your notes or make new notes on the fly. I use One Note for everything at work and those notes save my behind more than you know. The other benefit for having a computer at your bench is if you ever get a microscope with a good camera you can do inspections and display points of interest on your screen and take pictures and what not but a good microscope takes up quite a bit or real-estate which is why some use those digital USB Scopes which kind of sort of work and I do have them which is better than nothing but only useful if you need a portable inspection tool. We all go down rabbit holes and once your through them they always help improve your troubleshooting skills. Never apologize for what your doing its all a learning experience both for you and those watching your videos. We all learn from the from the success and failures of others and none of us are perfect believe me I have make my fair share of mistakes which often occur when I get distracted, yea distractions for me is an on going 24-7 event so I have to do a lot of back tracking on what I was doing last before being able to move forward again.

Oh, another little tip I picked up from watching arcade board repair videos - those Fujitsu logic IC's are known for going bad ! When you see them, suspect them. I've seen it happen myself repairing a few BBC Micro's at the Centre for Computing History in Cambridge, UK where I volunteer

Thanks for sharing this and including all the mistakes Jan! It's great to know it happens to everyone and for those of us learning really it's more useful to see things go wrong than to magically go right which is hardly ever a realistic experience!

Your bad days are of interest to multitudes of us viewers. Thank you for bringing us along.

Nice to see I'm not the only one getting frustrated with a repair. Great work! Flash codes can be a rabbit hole.

A12..A15 are not connected to the VIC-II. To prevent them from floating during VIC-II half-cycles, resistor pack RP4 acts as pull-up resistor. This is what causes the wonky signals: The bus acts as a capacitor and the resistor pack needs more time to charge it, than the chips do with their MOSFETS.

Loved it Jan! Sorry it was a tough one, but it was so great to see you try everything!

I love the 425. Best picture and sound of all of the revisions imho.

Love your repairs, wrong turns and all!

Occam's Razor: "I suspect the PLA..." 48 hours later after exhaustive diagnostics and a few f-bombs..."It turns out the PLA was the problem!"

Enjoyed this video! Repairing old computers can be extremely frustrating at times, followed (hopefully) by the delight that you once again brought something back from the dead! Keep up the good work, Jan!

A great example of the "Hunt and Peck" method of troubleshooting. Once you've replaced every component and it still doesn't work then you have become your own saboteur.

Well done Jan. Another one resurrected from the dead. Stressful repair for you, but an enjoyable watch for the rest of us. Thanks for uploading 👍

That was a really exciting episode :-)

Every time there’s a shot in this guy videos that points out his window all I can think is ‘A tall nondescript building with minimal windows. Why does it look like he lives in a prison?’

Thanks for persevering with this repair video. I am part way through a similar marathon. You covered methods and techniques, showed results, found revisions in the schematic. I could go on and on. Well done. A marvellous tutorial omnibus of investigation, deduction and repair. It's my goto reference work on C64's lol. Brilliant.

Always a journey with Jan! Thank you for all the great informative content 👊 Much love London u.k

Jan, I feel your pain. Been there too. :) Very happy to see another C64 saved. And as always, another great video.

Apparently, you didn’t hear me yelling “it’s the PLA” half way through. Lol Another great video. Thanks, Jan!!

Well done Jan! I would have given up. It's a surprise that some chips seem to work in another machine; that would confuse any repairer. And that reset signal source coming from a different pin on a diff rev was another gotcha. Always good to have a known good machine and chips even with the expensive digital test equipment.

Did I somewhat enjoy? I enjoyed the video a lot! Thanks, Jan!

Great job. Well done for hanging in there.

Now I know what a wonky signal looks like. Great video Jan!

Always love a C64 fault find video.. great work Jan..

Awesome job as usual Jan.

I always enjoy these videos, even if the investigation may seem frustrating. You didn't give up, and got the best result: a c64 was saved. The middle finger made me lol :) Thanks for sharing the journey!

You get a thumbs up for giving the C64 the bird! Great repair work. Always love watching you repair C64s

Well... That reminded me of Noel's latest C64 video... And don't forget: If it shows a black screen, it is almost always the PLA! ;-)

No need to apologize 😎Your videos are always great to watch 😊

Very well done Jan, thanks for showing the failures as well, that's what we learn most from.

Thanks again Jan! :)

That wasn't an easy task but you managed it. Congratulation! Might be a satisfying feeling. The misleading slow rising signal, which seems to be wonky, could be a result of a (normal) tri-state high impedance condition. When no component drives that signal actively it goes slowly to high (or low or something between). And sometimes such a signal form indicates a real fault. Thats digital business.

"Today in the lab..." I see what you did there, watching Fran too it seems :D

Really enjoyed this one, a lot of great tips on troubleshooting a very odd problem!!

Thank you, great to watch!

Jan,hai dovuto tribolare su questo C64 ma hai vinto tu.sei il migliore.alla prossima amico,ciao.

Love seeing that new Keysight DSO on the bench! It's a ton easier to read on video :D

Wie immer: Ein super Video und ein Genuss zu schauen 👊🏼💥😊

When you see a slow rise time like that, it's usually a sign that the bus is not being driven at all... the slow rise is often due to a pullup resistor. Sometimes this can be normal.... but typically not on an address bus! However in this case the address bus is split by muxes, because the VICII also generates addresses for the RAM. I know how you feel when you wanna give a stubborn system the middle finger all too well, i've had a few 500+'s with Varta damage just like that :)

Great repair, great video. Congratulations one more time. The short chip legs thing was not of this world !

Grande, legendary Jan Beta! 👍👍👍

great video, fascinating trace through diagnostic process, and like others i lol'd when you swore and when you showed it the finger :)

I enjoyed the Mission: Impossible theme. :)

I was shouting at the screen REMOVE THAT PLA AGAIN! Haha, one can get caught up in these repairs! Well done.

Hi Jan, really enjoyed it, it’s sooo annoying getting trolled by a fault you accidentally caused yourself. All’s fine in the end :) have subscribed!

I have really enjoyed watching this.

Great video as always!!

What a nightmare Jan. well done for sorting it. Cheers Graham

Wow this one was a real slog for you..... glad you got it fixed in the end

You have the patience of a saint jan, well done chap. :)

29:00 Whistling "Mission Impossible" made me chuckle :)

Thanks for the exciting thriller. Murphy gets us every time ... :) Nevertheless: the patient is alive!

36:33 I felt that sigh in my soul. Great job.

Jan has the patience of a saint !!

Very helpful and informative video for me.

hahaha... 29:00 "Mischen impossible" ... when all replacements do not work.

You bring Another C64 back to Life!! Great👍👌

well done / Gut gemacht! Danke Jan

Whew... that made me tired just watching! You have the patience of Job, sir! Well done! :)

Great video 🙂👍

Spit my coffee all over my laptop... lol (29:28)

i was guessing leaking capacitors... but short ic-legs.. never crossed my mind. Well done! i was watching it to get my time going (3dprint a key for my paper-dispenser) it just bendt it self... so.. i made a hole int the dispenser, and placed a splint.. problem fixed.

Great save :)

Nice work.

Never give up....Never Surrender!.....Sometimes we have to walk away to come back another day! :) Kudos Jan Beta....!

Tip for quick-testing DRAM: If you suspect one chip is bad, you can piggyback a known good chip right on top of it. Bad DRAM usually don't drive its output, so the known good chip which is tucked on top takes precedence.

Was great watching this and the Atari ST renovation

we always love yours video

From mistakes you learn the most.. and in the end a working c64.. bravo again Jan...is it possible to check if the short pins make or don't make contact on the board?

Great video, looks a lot like my efforts, with same frustrations. Maybe it would be helpful if you did a series in which you scope every chip ,one at a time, on your functional computers. This way he people who do not have a working 64 to aid in our troubleshooting , could benefit from seeing what the proper waveforms should look like. I have a scope, but I’m not sure how the waveforms from each component should behave in a machine that is not sick. That old make my scope readings meaningful. Thanks again!!

Another great video....

Yeah I wonder if it would have been worth triggering on a bus control line (no idea about the C64 bus architecture - been years since I looked at the schematic). Otherwise when the bus is tristated you will get junk. Could also trigger on a bus control line and watch one of the data lines to see if the data line is stuck. I'd love to get a broken C64 to fix and play with but they are expensive here!

Awesome video, I enjoyed the process and was really surprised that the address signal was normal, that looked terrible (Sawtooth binary anyone).

Now this is cool! I know this exact C64 from somewhere ^_^

I love it, that even with your experience you can get to the wrong slope of analysis! Just fixed up a my first dead c64 from ebay, somehow I love fixing things now...damn..bought ebay empty with electric utils and stuff

What would be great is to have the scope graphs of a working one saved, maybe in a program or database.

good job!!!

muchas gracias Jan por tan ilustrativo vídeo aprendo mucho con ellos buen trabajo. Podías hacer un vídeo de como se construye un FE3 para el vic 20?

Part way through I was wondering 'will he end up socketing and replacing every chip on this machine?' In the end he was nearly there. The only way this would have been more agonising for Jan is if upon reasembling the machine it suddenly stopped working again. At that point C64's might fly (most likely out of a window).

29:30 I think a lot of us have been there giving the middle finger at stubborn c64 repair! 😁 Can you please comment what was beeped out on 33:28?!😄

Jan Beta, the C64 Wizard!

With all of the people repairing C64s over many years, I'm surprised someone hasn't created a document showing the normal 'scope traces at various key test points on working C64s of the various types.

You... Did It.

What I've been working on the long board ones, I first check the fuse. It it's ok, it's usually the PLA (like 80% of the time), then CIAs and then RAMs. With short board ones, it's never the PLA, but usually CIA's and then the RAMs.

Points for effort and not giving up.

Misled by readings from the oscilloscope - and by way too short pins. Wow ! Aber jetzt haste ja (fast) alles auf dem Board gesockelt :)