Commodore 128 Repair Attempt Part 1

[@bozimmerman]

Learning good troubleshooting is never a waste of time. I wish there were more videos exactly like this one.

[@HeyBirt]

Putting the capacitor in backwards would damage the capacitor itself, and perhaps cause a larger current draw from the power supply for a short period of time. I would not suspect that anything else would be damaged as a result. Since you have the proper voltages present on the board I would first suspect that in the process of the board rework a trace was damaged or a solder bridge created. Check for proper voltage supply to each chip first as that is easy and quick. As I recall you mentioned some of your waveforms were 4VPP? If so that is too low (I may have misheard or misunderstood though). As someone else mentioned the boards first boots up in Z80 mode and then control is transferred to the 8502. I remember hearing Bill Heard talk about this process and why they did it that way but don't remember the details. If the Z80 is booting up then looking into what the 'transfer' process to the 8502 is might yield a clue.

[@Zhixalom]

I love that we get to see you working on it whether or not you actually succeed. And I think you absolutely should do that collective repair streaming thing for this C128 board. Any 8-bit which has avoided becoming trash for this long definitely deserves it. In either case you have just gotten yourself another Patron :)

[@alexboehm7171]

Jan, these atempt to repair videos are the best! Frustrating for you but all of us keep fingers crossed and watch with alot of popcorn 😃

[@JanBeta]

Ha, thanks! Unfortunately this repair is VERY time consuming (something I don't have anyway at the moment) so it might take a while until I can get back to the repair. Got some very good tips from folks in the comments/on Twitter so I'm still optimistic. ;)

[@AndyDavis007]

Thanks, Jan. Always appreciate your efforts and honesty. This is great. I now feel the impetus to finally set up my workbench and power up the C128 I picked up at a consignment shop and see what does or doesn't happen. 😎

[@Hacker-at-Large]

In my limited experience, I’d be looking at the logic chip “upstream” of the capacitor that you smoked. Those 74LSxxx chips are easy to burn.

[@dangwoot]

it's always fantastic when your voltages are "coming" everywhere on the board XD

[@EduArana]

Do not give up man! finally you will get it. Give a mayor checkout for corroed lines on the bottom and on the top of the board. Watch for corroed lines near the c55 cap. Try to remove nicely the z80 and put a socket on place, then swap with another z80. You have already check all the important things. Maybe is some dataline corroed. Good luck!

[@stephenbruce8320]

Well the unit worked until you started future proofing it so I would start checking out the solder traces to make certain that you have good continuity in the area you previously worked on. If I remember correctly everything starts with the Z-80 so if something were wrong with the Z-80 starting up cleanly the cartridge port would not work. This might explain why your Dead Test cartridge may not have started. The Z-80 starts first then passes control over to the 8502. The Z80 and 8502 operate as serial co-processors but not parallel. So I would focus checking the areas and associated solder traces to make sure they are in good working order and then I would look closely at the Z80. If you have the SAMS COMPUTERFACTS for the C128 they have test points and expected wave forms which might be helpful. I offered you service manuals previously my offer still stands. I know its frustrating but think about it for a minute as frustrating as it can be at times nothing is more joyful then when you turn that power switch on and things come alive.

[@FireballXL55]

Hi Jan I wish you did not have the scope AC coupled earlier, I know that most of the signals were 4V but with AC coupling you could not see if the swing was from 1V to 5V or 0V to 4V or any value between. Rich

[@RDJ134]

Seen the whole video without skipping, its freaking awesome and cant wait to see you fix this puppy :)

[@JanBeta]

Thank you! Hope to get it fixed some time. I got some really good suggestions in the comments to this video. :)

[@Ichinin]

You do not need a program to play sounds, in the C128 mode you can just type PLAY "CDEFGABO5C" and it should play a scale, can try it out in Vice to hear it.

[@Ichinin]

You can try removing CIA+SID chips, also the Z80 chip, i do not know if that will help but... The Dead test should flash N number of times indicating what the problem may be, if you cannot see that maybe the monitor doesn't stay in synch long enough. Also, you can try the Ray Carlsen C128 troubleshooting guide here: ftp://www.zimmers.net/pub/cbm/documents/repair/troubleshooting-c128.txt

[@Trevorodunne]

Brilliant

[@danielmantione]

Removing the Z80 is a bad idea: The Z80 becomes active before the 8502 becomes active and I remember something that a working Z80 is also necessary in order to be able to activate C64 mode.

[@RacerX-]

Ichinin my understanding is it will not boot at all without the Z80.

[@SkuldChan42]

Bill Herd actually said in more than one talk that the Z80 boots first - because of bus contention issues. It decides based on key combos disks inserted, or carts inserted what OS boots.

[@discoHR]

Z80 is responsible for booting. Swap it. I had one which didn't boot and replacing the Z80 fixed it.

[@leonkernan]

That was my first thought too.

[@Dkentflyer]

Really good video Jan, its really interesting to see the fault finding process in action, really hope you can work out the problem.

[@JanBeta]

Thanks! I try my best. Unfortunately it's going to take a while until I manage to continue because the process is so time consuming...

[@glenndoiron9317]

Agree completely with FourX2k3, pin 34 appeared to be ~2V peak to peak. Something is dragging that data line down, yank IC's connected to that data line until it returns to 4-5V like the other data lines. Since the defective IC is loading the line down, do NOT attempt to prove it is bad by installing them in the working unit.

[@game_whisperer1606]

Hi Jan. I see already some good comments regarding the capacitor that was once incorrectly placed. My immediate thoughts though are with the modulator. With another working board to play swaps with I know I would try swapping the modulator. Also I would try attaching a datasette and issuing the load command. Since the Z80 is soldered I’d try that last but that’s another swap check. Also can you force a reset on the CPU by grounding the line for an instant, you might kick it into life. Enjoy.

[@danielmantione]

No, it certainly isn't the modulator. If it was the modulator, the luma signal on the scope would look different (because of characters on the screen). You see the syncs only now. Further, the VDC didn't give a picture either, which doesn't use the modulator at all. What is going on here, is that the VIC-II and video circuit works correctly, but the VIC-II never gets initialized by the KERNAL: The computer crashes before the VIC-II gets initialized. Therefore the picture remains black. A very likely cause of this is bad RAM, but because the dead-test cartridge didn't work (it needs neither RAM nor ROM to flash error codes), I suspect RAM is not the cause this time. It could be a broken PCB trace or CPU problem, more analysis is needed here.

[@GadgetUK164]

Hey! I noticed you had the scope on AC coupling when checking data / address bus - worth checking you are on DC coupling for anything digital, although you could see the levels OK on that setting there - it can just give you a cleaner signal making it a bit easier to read. Other things you could try - since your scope does 2 channels, check how the two CPU clocks look side by side to make sure you've not got a contention issue. Have you double checked all your replaced caps to make sure you dont have a bad connection or missing ground or something as part of the recap? Also - when you swapped out the bridge, are you sure nothing shorted there - eg. DC to ground or AC to DC etc? I guess the good news is that you've tested lots of socketed chips there - most of the rare ones etc. So at least its probably going to be something minor like PLA, RAM, or some TTL.

[@GadgetUK164]

Looking at schematics there I see various TTL chips as well (eg. 74LS32) I would have a proble around the vanilla TTL there (138 etc), and look what's going on with chip selects too.

[@JanBeta]

Yeah, I realised that DC was the suitable setting for the job a bit late... I'll definitely work through your suggestions (actually making a list to go through at the moment). Thank you so much. Really hope to fix this. :D

[@GadgetUK164]

I feel your stress - It's certainly no easy to fix fault. Could be the Z80, RAM related to the Z80, some address decoding issue etc etc. I had a similarly difficult issue with the BBC the other week. But if you test say the 74HC32 (just a simple OR gate) - if you see the correct behaviour with scope or logic probe, you can at least mark that as OK etc. Eventually you will end up with few chips left if you've not yet found the fault.

[@philrod1]

Can you do a poll for what everyone thinks the eventual fix will be? I'm guessing a TTL chip associated with the Z80. I'm basing this off my decades of experience knowing almost nothing at all about electronic engineering.

[@philrod1]

Dude! It's bed time. I've been looking forward to this video, and you go and release it at this time of night! ☹️

[@MrSdrugo]

My last repair on a 128 I had a broken ram chip and a partially broken 8502. Incredibly the ram gave problems even with the bank switching by swapping the resistors. Also 74ls257 are a common problem. So I would go with a complete ram resocket and test. Test cartridge also helped but could not identify the partially working cpu, that was spotted cause I had a 1mhz clock where should be 2. That’s why some games worked and others not and why I could not load from tape.

[@JanBeta]

Ah, thanks. Interesting partial fault with the CPU. I'll watch out for that, too. Checking the logic ICs and RAM also makes sense of course.

[@joel_ricci]

The machine is not booting, PLA and MMU verified working, so the CPU - Glue Logic - RAM - ROM train is where I would put my money at this point.

[@Four_X]

One of those data lines for the 6510 looked a bit sketchy on your scope. Pin (34) when you probed it looked at a far lower TTL level than the other data lines, this may be normal but you might want to investigate it more as it does look a little suspicious. If I had to guess what's wrong, it a problem with the processor or one or more dead memory IC Jan.

[@danielmantione]

Well spotted! This is indeed suspicious and it is a good place to start further research.

[@anks888]

I saw that aswell

[@davesclassics]

Great video, Jan! Looking forward to part 2. I'm in the middle of doing a repair video too. =)

[@JanBeta]

Thank you Dave! Hope yours is going to be more of a repair than mine (so far at least...). ;)

[@bwack]

Hi Jan Beta, you checked voltages, but did you check 9VUNREG ? When a electrolytic capacitor is powered in reverse, it becomes conductive (and also bad if you not replace it afterwards I think). It might have taken the full-bridge-rectifier with it, or it still keeps dragging the 9VUNREG down. (Just looking at the schematic here. I have no experience with this machine). Edit: Unlike the C64, the C128 has no fuse on the 9VAC on the motherboard. I suspect the full bridge rectifier takes the bump if something happens (like reverse cap). Strike that I can only see it used on the cassette port ? The 9VUNREG also helps powering the 7812 regulator, but I think you measured the 12V..

[@JanBeta]

Hey, thanks. I will definitely check the 9VUNREG (I can't remember if I did) and the rectifier. The 12V are spot on I think. And yes, I think the 9V are only used for datasette in this machine. :)

[@SeltsamerAttraktor]

I recently fixed a stack of 8086 boards and every time some glue logic IC was at fault. Usually the bus drivers. But without my DSO I would not have been able to identify the fault.

[@Collectorcast]

I don't have a 128, but if it's enough like the 64 you may want to check the multiplexers for the RAM. I had a dead 64 doing the same thing and both the multiplexers were dead. Cheap and easy to replace.

[@AaronHuslage]

I think it might be that a logic chip may be having an issue. If it's not booting, the Z80 is probably not accessing memory (RAM or ROM) correctly. If you have a logic probe you can possibly see if the data lines are transitioning properly too.

[@wimwiddershins]

Great video Jan. Now you have 2x C128. I'm suspicious of either the video modulator or the logic upstream of the big capacitor. Keep on it, I'm sure you will get it working again. :)

[@kim1jo]

Nice video. I just repaired a C128 with a similar error, and it took me a long time before finally finding the problem. Every step described in the Commodore Diagnostic Instruction and Troubleshooting Manual seemed to pass, but then I noticed that the signal level on one of the data lines was off. It turned out to be a bad ram chip. I would follow FourX2k3's advise, and investigate that suspicious data line

[@JanBeta]

I will! Thank you Kim. :)

[@josephnealescratchcards]

Awesome video as always

[@TzOk]

Which supply rail was the capacitor you soldered wrong? If it was a filtering cap, all it could do was to short this rail, this shouldn't do any damage to anything except the capacitor itself. Another option to check is if you hadn't damage any vias when removing all capacitors. You should check if there is a pass between both sides of this board, by each pin of the capacitor.

[@SianaGearz]

Unfortunately these capacitors don't go completely short, they turn into resistors; and in turn, RAM for example is prone to be knocked out solely by supply noise.

[@bwack]

Black screen, and processors run? Check luma and chroma and power at the modulator area. might just be something around there. Measure directly on the 8 pins of the modulator.

[@JanBeta]

I will check again. I think I got signals on the DIN socket but no picture. I doubt that it's only the video circuitry because it doesn't react at all to the dead test or blind commands (no sound, no activity on connected peripherals). Thank you! So hope to find the fault. :)

[@robwebster7406]

Hope you get it sorted soon 👍

[@officermeowmeowfuzzyface4408]

Just send it over to Retro Recipes. He's certainly skilled enough as an electronics engineer, isn't he?

[@Asriazh]

Maybe it's easier than You think. So You put a polarized capacitor in the wrong way around. I could not catch it during the last video, but did You reuse the same capacitor that was soldered in wrong and put it back in the right way around this time? Usualy with that abuse, capacitors blow up. I wouldn't be surprised if it would at least be damaged. So much that it stops the Commodore 128 from starting up. If that's what You did ^^;

[@bwack]

I am thinking the same, but the 12V regulator measures fine and it needs 9VAC..

[@EdwinNoorlander]

Do you have a logic analyser? That you can check the boot sequence and compare with the working one.

[@TheDefpom]

Are there circuit diagrams available for these, if so point me to them and tell me the cap number.

[@danielmantione]

Yes, Commodore computers are very well documented. Take a look here: www.zimmers.net/anonftp/pub/cbm/schematics/computers/c128/index.html

[@dominikschutz6300]

The answer of Stephen Youndt sounds reasonable. But you could also take out the chips and build a C64 reloaded and ZX Spectrum and reuse the PCB for a toilet or something XD

[@pauldeane8369]

I'm troubleshooting a dead board and comparing results with a working board. one thing i noticed, on the user port, the 9V lines are only giving me just under 5V (~4.7). But I have that result on both boards. I see in the video you have almost 12V. What am I missing? The 5V line reads about 4.8.

[@SledgeFox]

This is so interesting, thank you very much!

[@JanBeta]

Oh, glad you like it! Hope to get this fixed sometime. ;)

[@FernandoelChachi]

Jan maybe it isn't a good moment to ask, now that your C=128 isn't working, but I was thinking about the kernal switcher that you use in your C64 (I use it too). Do you think that it can work with the C128 too?. I'll like to see if you add it once you fix this great Commodore 128. I'm meaning that I will like to see where do you connect things and so to make it work there.

[@JanBeta]

The version I used in the C64 won't work in the C128 I think because the ROMs are bigger. There is a version for C64Cs though that might work. I'll look into it if I find the time!

[@redcray0n]

Maybe you could socket the remaining chips on both of your C128 so you can test those in the working board.

[@WX4CB]

silly question, but when you're using the scope, why didnt you ground at the edge of the board at the front instead of having that long ground lead to the cartridge shield ?

[@kokodin5895]

i had similar nightmare with sanyo rubycon lamp balast for dlp projector i start to test evert singe transistor in known working unit and everything worked and in the end i gone crazy with hot air and replaced one resistor and puf working projector don't give up sometimesit isas simple as ded rom chip i would unmodyfied it from german mod and go over schematic with continuity tester first then i would check if data lines are not shorted or stuck

[@LeftoverBeefcake]

Today I learned that hopefully my 128s will stay fully working until I'm nothing but dust in the wind. And also to always double check the polarity when replacing capacitors. ;)

[@artur3edd]

Have you tried to hook up a disk drive? Does it pass the reset sequence correct?

[@KonjonoAwesome]

I agree with replacing the Z80, or the ROM that holds the Z80's code.

[@MrGeekGamer]

Can I ask what desoldering station you use? I've been trying to do it with solder wick and it just frustrates and burns!

[@PureAudioTones]

Hi Jan, did you check all the new capacitors are ok or not?

[@OriginalLictre]

This may seem obvious, but when you discovered that the one capacitor was in backward, did you remove it to replace with one in the correct orientation?

[@rlgrlg-oh6cc]

I think the cap that was installed backwards should be replaced. Also, just want to point out that even though it was backwards, we can't say for sure that was the reason why it's no longer working. There may be an un-related issue.

[@JanBeta]

I replaced it with a fresh one after I found out about my fault. I agree, this might be something completely different, too. Teaches me to check after each replaced component... :/

[@retroelectrons2]

Good TRY! helps me know thinks don't always go well.

[@FrankColonier64]

This happened to me when recapping on my A1200. It was a bang but I was lucky again. My favourite computer is running. Instead of Kerkos I have built in standard Panasonic-Elcaps. I proudly posted photos in a Facebook group and after a Shitstorm I was banned from the group. It must have been an Amiga sect I told them to join. Anyway, the Amiga might run for another 30 years. Who will survive whom, I don't know. Now I am waiting for my V2 of Apollo and then I will render again with Lightwave. From a very good friend I get the original Babylon5-Station 3D-model. Gruss aus Gladbeck, Frank

[@balloonsystems8778]

The data and address bus scope traces did not look ok to me. You should try a working C64 to compare what a running bus should look like. You should see rectangular waves.

[@SeltsamerAttraktor]

Because they look so messy? That is normal. As long as you don't have a storage oscilloscope, you are going to see the activity in real time. A non storage scope needs a repeating waveform to display it properly. As long as the machine is idle the activity is going to be repeating but in a long complex way composed of sub patterns. Some patterns repeat more often than others while also other things happen at other times but less often so, which causes these ghost lines.

[@balloonsystems8778]

Yes - messy == bus timing error. Try it yourself if you don't believe me.

[@Ketma73]

Daft question, but did you swap out the capacitor you initially put in the wrong way? Comparing the vids it looks identical in installation.....have you tried a new one or the one from the working board...

[@JanBeta]

Oh, I replaced the capacitor with a fresh one after I found out I put it in reverse. I installed it with the arrows pointing to the negative side on top this time to make it more visible. ;)

[@grommdk]

have a c128 board with the same problem - only black screen in both 128 and 64 mode. have tested socked chip OK in my working c128 board, and i cant make it work - i realy hope you´ll find "the holy grail" :)

[@grommdk]

All D bus and A bus seems to be fine, same as clock too.. the 80 colums cable could be an option, but havent got one...

[@psyolent.]

hey bud, i'd piggy back a memory chip one at a time mate as they seem to be the first thing to go bang matey, my 64c had dead ram and dead cias

[@jotdot]

some times new caps are bad

[@iam5085]

Diods?

[@canadiansaabfreak6530]

The primary line of this video: *there's something else rotten*

[@gillesgeeraerts5751]

Adam Goulart If I remember well, Jan lives in the north of Germany, so we are not so far from the kingdom of Denmark :-)

[@markp7129]

As much as I enjoy your videos I wish you would go easy with that brush whilst cleaning. I’m sure it’s a special antistatic item but you seem so violent whilst doing it.

[@JanBeta]

Oh, sorry. It indeed is an antistatic brush and I had to scrub a bit to get some old flux residue off. The board also has some slight corrosion that I wanted to clean a bit.

[@bayareapianist]

I had a C128 back in1988, 30 years ago and I killed it. I became an EE engineer later and fixed the boards to support my education. One thing I learn was the VLSI chips die very easily by electric shocks or ESD. Measure the current going to the borod if it's too high feel the chips. If they are warm, most likely dead. The best is a thermal imaging camera if you have one. My hunch is that you killed one or more 4164 DRAMs because they are very sensitive and usually don't have high voltage input protection. I didn't watch the whole clip. Just FF to the end!

[@danielmantione]

Bad RAM certainly gives symptoms like this. However: The dead test cartridge did not work. Now, the dead test cartridge does not need a single byte of RAM (or ROM) in order to flash an error code on the screen. Because the dead test cartridge didn't work, this is an indication that something else is going on. Perhaps a broken PCB trace or so.

[@bayareapianist]

Daniël Mantione Bad RAMs keep the data/address lines to constant 0 or 1. The best way to find out if this is the case as I said is to measure the current and see if there chips are using current to hear up. Bad/broken lines can be checked with a meter.

[@danielmantione]

What you write is correct, but you can see in the video on the oscilloscope pictures that the bus lines are pulsing. Further, a bad RAM can keep an address line low or high, but only on the multiplexed address bus. The multiplexers that generate the signals on the multiplexed address bus are one way. This means that the normal non-multipexed address bus (to which the CPU is connected) remains usable in case of RAMs that keep lines low or high. This means that the dead test cartridge is not affected by RAMs that block the multiplexed address bus. RAMs are directly connected to the data bus and can block it completely. Therefore they can cause situations that result in a black screen and the dead test cartridge does not work. Therefore you should check wether the data bus is pulsing in such cases. I think Jan did this.

[@o_-_o]

ohhh, what a sexy brand; where can I get LIPS monitor?

[@PMCRetroGamer]

Socket the processors and try them in the working board

[@MrTonaluv]

Where is the graphics booster? You took it out! There's your problem.

[@LastofAvari]

Killed C128 just in time for another repair series. ;)

[@transistorbaluba]

i hope you did replace the wrong-way capacitor, before troubleshooting.

[@JanBeta]

Yes, that was the first thing I did after I realised I got the polarity wrong.

[@allan.n.7227]

BETA max :)

[@danielmantione]

Jan, did you receive my e-mail?

[@JanBeta]

Yes! Sorry, didn’t have the time to answer properly yet...

[@TheZerohimself]

I would think you were wasting time testing 80 column mode, as you get no audio output.. even if a video chip was borked, you would *probably* still get sound.. (i am no expert).. I would get a schematic, and look at everything upstream from that cap you put in wrong...

[@redherring3110]

zerohimself yes I would want to have a good at the upstream it might be capacitor mishap did damage something I just feel there is a short somewhere

[@TheZerohimself]

Def replace that cap you put in backwards!

[@LudwigVonBremen]

na Jan...? mal wieder scheisse gebaut wa' ? -> i'm just kiddin' ;-) mag Deine Videos

[@JanBeta]

Ha! Ja, da ging so einiges schief... Aber ich habe es ja inzwischen glücklicherweise wieder hinbekommen. ;)

[@God-CDXX]

the 8 bit guy .. try him

[@gettingpast4391]

Biden FU

[@MartinPHE]

Lets see what DID YOU DO to make it this way. What is the cause and effect of these actions. A. banging, pushing around on bench, flexing thru-hole component board. B. ripping out unsoldered parts. C. Improper replacement of parts. D. Improper mfg. voltage/specifications E. Poor soldering technique Aka. scratching/severing mask, traces, cold joint. Humm surely you didn't think you could switch the cap around the correct way and it'll still work properly. Sending it off does not help you learn anything or having others tell you what's wrong. So there is a short list of things to look up. BTW spraying the board then using a brush is not cleaning anything, your just smearing it around and ripping the cap off the strip is a good way to damage it. Not all caps are the same, ignoring voltage, temperature. little things like ESR, ESL play a role in high frequencies (RF). Think of this for next time, instead of eating the whole jar of peanut butter too see if you allergic to it, take a small bite and see what happens. Aka. replace a part turn it on. Some things are more forgiving then others so slow down, read datasheets, schematics, service manuals then you'll understand the limitations of things you can or cannot change from Mfg specs. If not then just adhere to the " NO USER SERVICEABLE PARTS INSIDE" label. A+ for the chuckles.

[@atchertha]

Wow. What a dick.

[@RacerX-]

Martin P wow! Congratulations, you won the putz of the thread award. Next time lose the attitude and try constructive criticism. You just come of sounding like another know it all turd.