Stubborn C64 Repair

Desoldering Station ZD-915*:

TIME STAMPS:
0:00 Intro
1:53 Sponsor Message
2:18 First Inspection
4:24 Dead Test
7:08 Measuring Voltages
9:06 Finger Test
9:27 Shorted Kernal ROM
10:31 Broken RAM
14:00 Multiplexers?
16:00 Wonky Address Lines (A Red Herring)
18:46 PLA?
20:10 More Broken RAM?
21:40 CPU?
24:29 Even more broken RAM!
26:15 Comparing with a good C64
27:06 Replacing more RAM
28:10 U14 Logic? BASIC ROM?
29:30 Replacing ALL the RAM
30:23 Reset Circuit?
35:06 The Solution!
36:22 Final Tests
39:01 What was broken?
40:53 Reassembly
41:23 Thank you & Good Night

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Thanks for watching!

This video was sponsored by PCBWay.

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#JanBeta #Commodore #C64 #Repair #PCBWay

[thebyteattic]

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!

[alerey]

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

[LUNATIC]

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

[tharkthax]

Best thing to watch on a Sunday afternoon 👌

[davidczepanski]

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.

[jaycee]

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

[Wallygjs]

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.

[willyc]

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

[darkstatehk]

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

[lotharschwab]

"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 :)

[Sephnroth]

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!

[awilliams]

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

[al.d]

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

[stephenbruce]

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.

[JamesPotts]

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

[zuimelanieforno]

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 😎

[hansoak]

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

[rdh]

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!

[Wikcentral]

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

[gertsy]

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.