C64 Scrambled Video Repair
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- Опубликовано: 2 авг 2024
- Troubleshooting and repairing another Commodore 64 board (ASSY 250425) with a weird scrambled characters/flickering lines fault. // Kindly sponsored by PCBWay - Get your first PCBs free! - www.pcbway.com/
The IC puller I'm using is a Jonard Tools EX-2, available from various electronics resellers.
Here's some handy resources I refer to when troubleshooting and repairing C64s:
The Pictorial C64 Fault Guide is here: derbian.webs.com/c64diag/
Ray Carlsen's troubleshooting and repair guides are here: personalpages.tds.net/~rcarlse...
You can get the .bin files for various C64 diagnostics cartridges from World of Jani (in case you want to make your own or try in an emulator): blog.worldofjani.com/?p=164
Here's the GitHub with Sven's diagnostics harness: github.com/svenpetersen1965/C...
Time stamps:
0:00 Introduction
1:43 Fixing bent Keyboard Connector
2:42 Testing Voltages
5:23 Populating the Board
8:17 Initial C64 Test
9:33 Jupiter Lander Distorted Picture
10:45 Fixing the Cartridge Port
12:31 About the Ultimax Cartridge Mode/More Fixing
18:45 Scrambled Character Issues
21:03 Broken VIC-II?
24:09 Power Issues
28:34 It's not the VIC-II!
34:36 MOS7801 Broken?
35:49 U14 Logic Broken?
38:17 More Testing
39:28 Replacing U14 74LS258 Logic
43:51 More Testing, More Cleaning
44:50 Another Logic Chip? 74LS373
49:32 Thank you and Good Night!
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#JanBeta #Commodore #C64 #PCBWay 
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Support tech: What kind of computer do you have?
User: A Cgeegdgre 64... with Basac.
Support tech: Huh?
"hopefully you are not bored by watching me repair commodore 64s" - certainly not :o)
Same here, i cant get enough of watching this kind of video's :)
The more time a diagnosis takes, the more we learn. Thanks for taking the time to show us how to approach this kind of problems!
Certainly not getting bored watching you fix C64’s or any other thing come to that. Keep it up 👍 it’s great to see.
As an owner of several C64 machines (two breadbins which both need fixing and two C64C models which do work fine) and long-time fan, user and (crap) programmer of such, I always appreciate your videos on these. I know it's an old upload but that doesn't really matter in this day and age, does it? We all get to learn something. ☺
It's actually nice to know WHY you replaced that logic chip. Knowing what it does is very helpful for identifying problems.
The PLA can also cause all kinds of intermittent weirdness when it's on its way out... first you'll get random weird stuff happening with cartridges (pretty much what you get when replacing a PLA with a ROM). I started noticing that my Datel Fastload 5.1A (plus freezer) cartridge doesn't work right on several of my C64s, I think one is unusable, like it goes to Basic but whatever you type in only gets you READY. Like you type LOAD "*",8 and you get READY instantly without the machine even accessing the drive, many machines crash when accessing the drive and the others appear to work, but after loading a program, it turns out it only pretended to be loading it, the memory is still blank.
I thought my Fastload cart went bad, but it's working fine on all my short board C64s and some of my long board ones as well. It's just like an early warning system that your PLAs might be going bad in the next few years.
I'm fixing lots of old arcade machines and found that sometimes, chips don't really fail, but their output gets slurry, like rise and fall time increases and if it's a timing critical chip, the machine first develops intermittent faults and then stops working.
But this is almost as rare as chips outputting signals on their input pins (no bidirectional pins!) - THIS was a nightmare to troubleshoot because a) the chip works, the outputs make sense for what's being input and b) you're not expecting this failure mode!On fixing
I also got my Parts-C64 down to zero parts. But that fixed like 6 or 7 otherit C64s, so task failed successfully!
(btw sad stuff: You can't desolder SID chips. The first one hamfisted younger me basically ripped one out, that one still made some noise, but most channels gone, some waveforms only working on some channels, filters whacked up. The second one I had a solder sucker and desoldered it. The result: No filters working and one channel very quiet. The third one I had a really good solder sucker, knew to heat the legs on the parts side not the solder side while sucking and let the chip cool down every 4 or 5 pins and still - no filters working. At least all channels there. Next time I encounter a C64 with a soldered in SID I'm gonna fix the machine no matter what. Even though I have a desoldering station like yours now)it
btw I just checked my Plus/4 with the MT RAMs and it still works... if this machine still works in 10 years, it might be worth a lot of money because SUPER RARE WORKING MT RAMs.
Verdammt gute Arbeit , ich bewundere dein Durchhaltevermögen und die Geduld ich hätte die Kiste ausm Fenster geschmissen und geschrien .Super Video , von mir ein Daumen Hoch.👍✌
Great job Jan :) Early in the video when you showed the wrong characters on the BASIC screen, I was suspected some sort of flip flop or something causing the wrong characters to be fetched due to bits not being set or something when addressing the Character ROM, glad to see that seemed to be the solution! I would never have known which chip it was though lol
Never bored watching C64 repair videos! Not only are they interesting, they can also give the rest of us insight into problems we may be trying to diagnose on some of our machines. A few years ago I was trying to diagnose a cartridge port problem, and eventually I found that from the factory, one of the pins was never put through the hole. It was just sitting on top of the motherboard and barely making connection so the problem was intermittent. Pin 6 if I remember right.
M = 77
Y = 89
I = 73
E = 69
Q = 81
A = 65
Bit 3 fails.
I was about to say the same thing! in each case, the same bit is flipped, namely 00001000.
I came to the same conclusion because each digit is shifted by 8.
Y becomes Q for example.
This is, in many cases, a better way of troubleshooting than swapping ICs "just in case". Even without an oscilloscope, debugging video hardware is possible by just looking at the screen output. As already pointed out, all characters on the "BASAC" screen have bit 3 pulled low. So either the video RAM or the interface to the character ROM is at fault. Nonetheless, Jan's video is very instructive.
Got half way through the video and thought the same that a bit was getting flipped when the vic was reading the character rom
I was going to suggest PEEKing screen memory to see if the character is being read incorrectly, or if the RAM has "forgotten".. The fact that it's a single bit change suggests a single ram chip, there are no buffers on the data bus to fail, and any address trouble would affect more than a single bit. Also connection trouble at the VIC chip would be affecting color-ram access, which doesn't seem to be happening.
The letters from H to O, X, Y, and Z were getting changed. Somewhere along the line the 8's place bit data line was going low.
I remember many decades ago when I had an Apple ][+, I poked a memory location and it acted like a bit wise AND mask with all ASCII characters going to the screen, and it produced symptoms just like what you’re seeing here, with the incorrect characters appearing on screen. In fact, it looks like bit 3 is being held low; this would turn a “Y” (=90, or 0x5A, or 0b01011010) into a “Q” (82, or 0x52, or 0b01010010). One way to be sure is to just hold down the “Y” key, and see if you get all “Q”s (some sort of data bus fault), or both “Y”s and “Q”s (a RAM fault).
I’ll never get bored of C64 board videos. Always watch them.
49:57 ... But it sometimes crashes randomly. [Randomly crashes into a rock]
Intermittent problems are the most difficult to troubleshoot and the more difficult the more swear bad words come out and then you might find yourself trying to have a conversation with the board your working on like its going to engage in said conversation. I use the same chip extractor its actually a god send. Its been a while since I have worked on a C64 Board. Commodore used that same power switch in a number of devices and finding a direct replacement is a PITA because there are many which are similar but not quite the same and a few years ago someone actually in Germany had a bunch of them and I picked up a few just to have on hand.
OMG what an episode! Another great repair (and good nerves' proof as well) by Jan Beta! :)
Addendum:I also noticed it was turning “I”s into “A”s (BASIC -> BASAC). “I” = 74, 0x4A, or 0b01001010, and “A” = 65, 0x41, 0r 0b01000001, which would require 3 bit errors; bit 0=1, bit 1=0, and bit 3=0. This would turn a “Y” into a “P”, not a “Q” (“P” = 81, 0x51, or 0b01010001) so maybe it is a RAM fault, after all. Weird. 🤓
P is charcode 16, Q is 17, you add 8 (bit 3 of address bus), and it’s Y. So bit 3 of address to char rom grounds when board heats up. If the logic chip does charrom addressing, or the flaky connection was nearby, that may have fixed it
Looks more like a timing violation to me. Setup/hold issue.
I love your repair videos, you always manage to diagnose whats wrong and know exactly what to do to fix it. You are like the Louis Rossmann of the Commodore world.
I have a C64 with a very similar issue - shifted characters, Jupiter looking wonky - and I suspected the same exact chip as Address line 6 seemed to be stuck to zero - and that address line goes via the 373. However I tested it with a scope and it seemed to do its job - I'll check that again, thank you Jan, I'll mention your help in my repair video! ;)
That board definitely needs a re-mask if you can do it. The more bubbles you have in the mask the more likely you're going to have more problems down the road. Now if this is just for testing then no problem. I was going to say that usually when have to rock a connector or a cartridge you are usually looking at the mechanical characteristics of a connection. Solder cracks always cause such a pain when troubleshooting. Excellent video!
Thank you Jan! I always enjoy repair works!!
I think it is safe to say that I am not bored since I'm still here watchin' ya learn as ya go along or demonstrate somethin' about retro computers. Now if only it didn't take 3 chips to fix a problem on a commodore 64 that would be a bit better but that's these computers.
Well done Jan, that one really tested your patience! :)
When you take out the logic chips you can test them with the tl866 eprom burner or you can always piggyback them.
Thank you I really enjoyed this video and look forward to more commodore 64!
44:35 I would have been pissed if my C64 would have done this when I was a kid as I liked to play this game and often loaded it to listen to that sound track.
Incredible work!
you never gave up and now we have more easy our retroworks! thanks, very much thanks!
Another great and entertaining troubleshooting video Jan! Thanks for such great content ! Keep it up with commodore :)
Another great video Jan! Keep it up 😎
Thanks for great content as always Jan... Nice to see you on camera more as well.. btw..I was thinking that you should definitely start throwing in some random impersonations of Perifractic from time to time... He always does the 'hi ... It's Jan Beta' over on his channel.... I think some payback is in order!!! (Love both of your channels) :) thanks again for being awesome Jan..
A piece of cheese would be more reliable than an MT RAM chip
Get bored watching, nah it inspires me and makes me want to work on something.
By the way, you should get screenshots of the fault and submit it to the Pictorial C64 Fault Guide page with a description of the problem. They can always use more fault examples. I have one on there for a bad trace!
"bored of watching you repair C64" will never happen
Saw a very similar error at Adrians Digital Basement, where he had a stuck bit on one of the RAM chips.
Letters here were also shifted by 8.
If it's something you have not seen before can we go deeper and look at what the chip is doing ?
Watching and trying to learn, I have 4 machines in different states. From fully working to graphical artifacts on them.
You should see "The Pictorial C64 Fault Guide", u26 faults are on there but your fault looks slightly different, maybe this is a new failure that has not been categorised on there (mixed up characters)
Always a pleaseure to watch :)
Hey Jan. Just found your awesome channel. Do you think freeze spray would have helped you find that marginal 74ls373?
It might have changed behavior I guess so the answer is maybe. I don’t often use freeze spray but it definitely helps diagnosing marginal faults sometimes. :)
Gute Arbeit!
If you have a Minipro of some kind, it supports testing common 74 series logic IC's too. I've used this recently when fault finding on Amigas. Very handy :)
I was just going to suggest that also. I have never come across a logic chip that can change state (fail) with slight temperature difference . Interesting.
aaaaah my favourite videos!!!!
Poor keyboard connector! Great job fixing that elusive bug!
I was saying to myself that it was just text mode that was messed up. Jupiter Lander is a text mode game. This was a tough fix. Nice job. Also, after having that game as a kid, I never realized that, that was an improper name for that game. They should have picked a different planet. You can't land on a gas giant.
I love c64 repair videos too.
I typed in a word processor on Dragon 32 - on the Real Dragon it says sn error and on the emulator XRoar it says fs error when i run a small program (a word processor) thanks please help. thanks.
ooh cool, I have one of these long boards that only have two RAM chips, it also has the 6581R4AR SID chip and for some reason it came in the doorstop C64 case (the second model that usually has the short board in it)
Have you thought about using freezer spray? It can help to locate bad chips very easy.
Jan, I love your videos but please cut long episode like this one into two parts. I forget to come back to watch the rest of the videos. Your great, thanks
Most people ask for fewer multi-part videos. I like this format. I mostly watch in 2x speed.
Because as we all know, PCB stands for Problematic Character Bits....doesn't it?
whats on the green 5 1/4in disk? do a video on it, data recovery maybe? :)
Are you maybe doing some Amiga 600 or 1200 videos with repair and modification? Hope soon a new video popup on my screen 😊
So does this mean the first U14 was actually okay?
Character ROM would be my first guess. Many were too slow and used to cause "sparklies." You probably have one that has degraded over time. Next guess would be the PLA but man that sure looks like the character ROM issues we used to have back in the day. Commodore even had us solder a bypass cap on the VIC to reduce the issue but I would just get a faster EPROM version of the Character ROM burned and problem solved.
I just stumbled over this video. My boss has an old C64 with graphics problems after trying to replace the keyboard. Any ideas where to look? I would first try to undo his modifications to the keyboard connector, hoping that his wiring created some issue on the bus, but beyond that? I never had a C64, but as far as I know the graphics chip did a good job at hiding its DMA access to the memory without slowing down the CPU. It's easy to see that this hidden DMA may get out of step when there is some issue on the bus and then the video chip gets wrong data and displays weird things.
Never give up 👍
as memory is bank switching shared between video and cpu, it looks like signal bleed going to the ram
Thank you!!!
Seemed like one where you had to really think about it. Quite nice to see a difficult one where you get to the end. Although still another small issue to go?
Yes, I believe there are still some sockets not making good connection or something like that. It works for hours and then sometimes just randomly crashes. Really confusing to troubleshoot!
@@JanBeta Time to get the scope out?
Hope your brother Jurgen is not too hungover this morning Jan.
Your not the only one who noticed the similarity.
Should just replace the single-wipe sockets anyway. Sockets are the easiest to desolder.
In cases like this one, freezer spray comes in handy.
The whole video I was thinking its a Multiplexer, ITS A MULTIPLEXER!!! (It wasn't a multiplexer). Nice repair
isnt it a problem with all those bubbles on the backside of the board ? it looks rather unhealthy ?
They didn't think you were going to look underneath.
I replaced a faulty 74LS373 in an old synthesizer recently, to fix a memory access fault.
I would like to repair C-64's but I have some physical issues that makes that challenging, so I very much enjoy watching your channel. You should consider a PayPal account - I'd donate money for parts and such.
I've never seen so many crumpled traces!
A 74LS373 was the problem? I have a very old computer and use that one a lot. Well actually, these days I use 74HCXXX parts, because the processor is one of the earliest CMOS processors. Some of my older ICs are TTL (74XXX, or 74LSXXX) and now 42 years old. They seem to be almost indestructible. I'm used to treating my ICs like raw eggs because early CMOS can easily be destroyed just by handling them too casually. But TTL? What destroys them when they just have been sitting in their socket or soldered onto the board for a few years?
certain vendors can be problematic.. for example Fujitsu brand ones are often faulty
@@jaycee1980 Usually it's the timing variations between brands that cause the issue and it's due to crappy design (Ie not doing proper timing budgets and the like). I've had to fix many issues like this on designs due to Engineers not having enough experience or were rushing the job and not thinking things through. All permutations have to be analysed. The original designer of the c64 admitted he rushed things and had to do many kludges. A slugging cap here and there to meet timing or fix other issues.
Very likely it's the PLA that has degraded and the timing is out of spec. You could potentially slug the clock to to the '373 with a small capacitor (~5pF). Maybe the 373 got out of spec too.
The PLA is fully tested and working in several other C64s, also tested this one with a replacement. The main issues were definitely caused by the 373.
So the reality is that the PLA may be out of spec too. You might have a situation where it has become marginal. Just because it works in other systems doesn't mean it's in spec. But if your fix works then so be it. You may have increased the timing margin by putting in a new 373. Try the 373 in other systems too and that will tell you. Maybe you already did this ?
I Never get board
Jan Beta - we need new videos 😁
Saw this once before many years ago, i swapped out so many logic chips until it was fixed ;[
Keeping that green floppy right next to that big-ass loudspeaker... won't it get demagnetized?
It’s a broken floppy, no worries. Also, the magnetic field from speakers is often overrated in that respect. You would have to put the floppy directly on top of the actual speaker (where the static magnet is) to see any effect I guess.
Just for fun, since you socketed the bad chip, you could have put the old one back to further confirm the fix. :-)
Should see if it has the same issue with a disk based game as it does with the cartridge game
No, it didn't. It had different issues for different versions of the Jupiter Lander cartridge, too. I can only assume that the timings were slightly off for the cartridge.
New friend here hope to see you around friend
**Jan** the CBM 64 start up screen looks normal if you suffer from word blindness or dyslexia
Please don't wear your ESD Strap while the power is on. You could ground yourself out through your heart. Not a good ending.
its a bad power supply
MT = Micron Technology
Another victim of adam
First
Want to be RUclips friends? 😆
Sorry, I very bad English and translate. I not know much of Commod(or?)e. I hear of power y heat y chip words in video. I live Japan and hire house repair person before of coming Winter. Someone house next door in Shibuya send me search video for house renovate idea. Does this commode thing still work as house furnace? yes? House contractor tell twig-chan can find alternative to gas heating and install commode power supply box to heat small apartment yokoshitsu. twig-chan was told possible of changing from gas to Commode heating of house yokoshitsu in Winter. Contractor spell word commode not Commodore and twig have many confusions and may visit bad video. twig has house of very small yokoshitsu, and compact, so need warm when next of Winter comes. Anything for keep of feet and bottom warm. Make many translate by google