Commodore C64 (C64C) Disk & Tape Loading Fault (Repair)

A short video covering the repair of a Commodore C64C.
Thanks to some technical knowledge I picked up from one of RetroGameModz earlier videos this system was fixed pretty quickly. Had I been a bit more logical with this repair I could have spotted the cause in a few seconds.
Don't work with boards on the carpet like this! You do risk ESD damage to chip!
RetroGameModz - / retrogamemodz

[@GadgetUK164]

Don't work with boards on the carpet like this! You do risk ESD damage to chip! Worth pointing out that the diodes need to be schottky diodes! I swapped them out earlier. It's better to use schottky diodes due to their fast switching speed.

[@maftyus]

You, my friend are a lifesaver - I'd been going round in circles swapping CIAs around etc. Checked the diodes and 2 had shorted. Swapped them out with 2 from my spares board and now everything is working! Top Marks! PS, Love the channel

[@GadgetUK164]

That's great! Glad you fixed it! =D

[@9ElevenGamer]

"Does whatever a Spider can" Nicely done, it's always great to see an old system revived.

[@larsenmats]

I got a C64 from a friend which he had bought from ebay and never tested. It would not detect my SD2IEC when I tried it. However, it would work with the datasette. I inspected it and didn't find anything. So put it aside. That was until I watched this video. I ran down to my retro room to inspect these diodes. First I meassured the values on my working C64s. Then I meassured on the broken motherboard. Lo and behold, turned out two of these diodes was shorted. At closer inspection I could see one of them had burned away that white coating on the component leg. I found two of these diodes in an old modem PCB I had laying around. I swapped them with the two burned ones on the C64 motherboard and now the SD2IEC is detected and working! However, the SID chip is making some crackling sounds and don't play music correctly. So it probably suffered from this shortout (whatever had happened to it). I have tested this sid chip in other boards and got the same fault. Also tested 100% working sid chips in this board and it was working perfectly. So thanks for your videos man! I really appreaciate them!

[@GadgetUK164]

Awesome!!! Thanks for sharing your success story - glad you were able to fix your C64 =D

[@RetroMarkyRM]

Great fix. I didn't know about the diodes on the 64 so will add the knowledge to my database :)

[@GeoAnas]

Very clever repair ! Well Done !

[@bwack]

Good find. I've been working on a video this week on a similar problem, but the cause was very different. I found that some disk drives add 1k pullup on the SQR line (The "1541C" drive does, but not the other 1541 revisions).. Thats 1k in parallel with the 3k3 pull up on the C64 that you mention in the video. An output driver (inverter) in a 3rd party brand datasette didn't like the extra 1k pullup (or extra 5mA for every disk drive attached) and couldn't bring the voltage down. 6.5mA was a lot to sink for a single I/O line back then, and so it was for the MC14584 in the 3rd party datasette. I swapped it with the one the Commodore datasette uses (74LS14) and problem solved.. but anyway it is curious that Commodore added 1K pullups to the SQR line. Some computers like the SX64 and VIC20 doesn't have pullups on the mainboard on the SQR signal, and I suppose it was a late patch to prevent floating input on the CIA chip (on VIC20?).. and also, the SQR line was never used as intented.

[@GadgetUK164]

bwack I hope you do a video on that - it sounds really interesting stuff! I always find it interesting when they make revisions and add / remove stuff etc to deal with design flaws.

[@bwack]

GadgetUK164 made the video. uploaded it when you asked. :) I've only done videos for a month and I'm quite novice at doing the videos.

[@GadgetUK164]

What's your KZbin name? I cannot see a link to KZbin on your Google+ page. Google+ is a pain isn't it?

[@PMCRetroGamer]

absolutely love your videos. im no good with electrics but watching your videos im learning as i go along! and as you mentioned i watch and subscribe to retro game modz as well! keep em coming!

[@GadgetUK164]

+Phillip Croxford Thanks =) RetroGameModz has some awesome videos - he's a professional but I am sure you've gathered that already ;)

[@gergelysinko5093]

It seems I have the same problem as well. No device detection at all. Some of the same diodes are shorted as well. Hopefully I can also fix this by simply swapping them out.

[@DisgracedSoul]

Hey just an update I found the problem with mine, turns out it wasn't the board at all I got a new one to do some comparison tests and the results were the same!!!!! Looked inside my datasette looks like it's had water damage/been stored in a damp place and one of the traces had corroded through. Little blob of solder later and it worked :) needed to realign it though (trial and error method :( ) but it's now working!

[@stephenfalken]

Hey mate. As always a great video. In the past I have spoken to people who have been foolish enough to plug the PSU into the serial port, I wonder if that is what had happened here to blow all those diodes...

[@GadgetUK164]

+Andy Mason You are probably correct! It was likely caused by something like that!

[@chelsona2574]

life saver!!

[@1msx2go]

Interesting & enjoyable watch there. I've got to watch more of these vids... always good to know how these consoles & computers work & seeing what they look like inside. I'm hopless at soldering let alone messing around with chips n' wires. So I daren't do anything for fear off setting the building ablaze :) I hope you don't mind me asking...have you ever repaired any Mega CD's & C16?

[@GadgetUK164]

1msx2go I have repaired a couple of Mega CD's - faults tend to be either the SMD capacitors or failing laser. Never looked at a C16!

[@mindluge]

i have a 250407 rev.B board giving me the "Searching for *" error and i know it's not the floppy drive and am trying to trace the fault, but i'm not good with a multimeter. it's my understanding that the problem is either the first CIA at U2, the 7406 at U8, or the clamping diodes. i've cleaned the power connectors and the serial connector with ISA and contact cleaner. do you know what the location is of the diodes i would need to check on this board? thanks and best wishes.

[@GadgetUK164]

Test connectivity between the serial port and either side of the diodes you see onboard - there should be a block of them somewhere that measure as a short to the serial port connections.

[@NotHimJim]

Interesting sir!

[@ToshioSpoor]

+GadgetUK164, thanks for this video +bwack pointed it out to me and I had the same issues with clamping diodes but on a breadbin...

[@bwack]

Looking at the datasheet for the 6526, the A mark is the frequency range. A is for 2MHz, and none is for 1MHz. The IC only has 5V power supply pins. no c64/c64c issue there I think.

[@GadgetUK164]

bwack Thanks, very interesting! I wonder what the 1 / 2Mhz stuff is all about - is the C64C 'more capable' regards comms speed or something?

[@bwack]

GadgetUK164 I don't think so... I only know that you can mix 6526, 6526A and 8521 as you please with the C64s. I was surprised when I opened my breadbin from 1983 and it had both 6526 and a 8521. Been looking around and the 2MHz IC's (6526A) is used in SX64 computer and the 1571 disk drive.

[@GadgetUK164]

I didn't even know about the 8521! Someone should create an FPGA or CPLD replacement if possible. Something mounted on a 40 pin DIP board that can just plug in. The number of working CIAs will diminish over the years.

[@bwack]

Sounds like a good idea. CPLD is probably preferable because it doesn't need external flash. Correct me if I'm wrong I'm not updated on the FPGA technology.

[@GadgetUK164]

I don't think all FPGA need external flash, they just have much more capability. I am not that up to date on FPGAs myself. It does look like the CIA just has some shift registers and stuff that could probably be replicated in a CPLD without too much effort.

[@MarkTheMorose]

Could those components fail if someone plugged a serial device in while the computer was on? (And who says the C64 is no good for web use...?)

[@GadgetUK164]

MarkTheMorose I guess its possible from static electricity but I wouldn't expect those diodes to pop like that =/ With something like that I would expect the voltage to be pretty high for a period of time I think. Static could do the job if the voltage is high enough though. I wonder if the original 64 had clamping diodes there on the serial port? I must check that soon!

[@Trevorodunne]

Somebody tried to plug in the mains power cable into the serial port

[@DisgracedSoul]

I have the same issue of tapes not loading. Im not getting a continuity beep across the diodes and dont want to go as far as replacing chips just yet. Is it possible the 151j capacitor in front of the serial port is the problem? It has a smooth black bit on the top that the others on the board dont have

[@GadgetUK164]

Hi, it's very unlikely to be the cap! Most likely to be the CIA, or the diodes.