Commodore 64 (C64) Graphical Corruption Repair

A short video covering the repair of a Commodore 64. The text shown by Basic on boot was duplicated and badly positioned. Games showed even worse corruptions in the sprites.

[@GadgetUK164]

Ironically, the C64 was telling us what was wrong - RAMSIC lol

[@DirtyBob7777]

how did you get it so clean on a flat screen? Also I have a c16 that has text problems. It boots but the text starts to turn completely black. is that the PLA chip going bad? or some random cap going bad? thanx

[@c128stuff]

Nice to see Blue Max in your video :-) one of my favorite classic games.

[@FastFeet]

Love your C64 Videos... I would love to see some C64 Power Supply Videos.... hard to find.

[@argy111]

Gadget..... sometimes....just sometimes...I give credit where it's due. you my friend are a bloody genius. had a c64 board...completely dead..replaced everything...EVERYTHING!! even the 2x 74ls257 right at the beginning.. ended up with black garbled characters and shimmery bits.. today I decided to watch over your video...this video again.. 74ls257...both of them replaced....heartbeat and singing... you sir are a star. if I could hug you right now... well you know..pretty emotional after on and off 6 months of scratching my head... thank you!!

[@GadgetUK164]

+Jason Birch (argy101) Thanks, glad you got it working =D How come it didn't work when you swapped the 74LS257's right at the beginning though, was it because both had failed?

[@argy111]

GadgetUK164​ I can only assume they were faulty on replacement. Something I forgot is,and I think sometimes we're all a victim of is.. Just because something is new, doesn't necessarily mean it's going to work first time. They were not from my normal supplier so chances are...qa wasn't on the list of priorities . Anyways here's to a learning curve and a half.

[@3DPDK]

the 4066 is a CMOS bilateral switch. Basically a voltage controlled switch that allows the VIC to share the data bus with the CPU. the 74LS258 (257 on some machines ) is a quad multiplexer which allows 2 different inputs (x4) to output on the same bus - it is used to control which chip is using the address bus - in the case of C-64, the pair of 258s specifically used to address RAM locations. The question is, if this was the culprit then how did the program run? My guess is that the 258 wasn't completely bad but marginal on one of it's address lines. One of the RAM chips may be marginal also. The combination caused an addressing problem in that RAM only, obviously screen ram for both BASIC and the ML game. Changing the 258 upgraded the voltage on the effected address line but the RAM may still be marginal. The program was stored on a different "bank" and that RAM was strong enough to tolerate the lower voltages. Expect to see this problem again with this unit sometime down the road.

[@GadgetUK164]

+3DPDK I don't think the RAM was marginal since it has been used extensively since without issue, but it's certainly a good assumption to make! Thanks for clarifying the 4066 - It's a hell of a long time since I was involved in electronics, and what you see in my videos is just me getting back into something I last practiced over 20 years ago. With my health condition as well I tend to make a lot of very obvious mistakes in my videos.

[@3DPDK]

GadgetUK164 Same here. My electronics career sidetracked into a professional life at sea about 25 years ago. I've recently rekindled my interest in electronics when I bought a Rasberry Pi for Christmas last year. I remember much about the C-64 because I did some heavy hardware modifications to mine as certain chips began to fail.

[@load463]

Gadget I took your advice I only had one other fuse left and that was in my other good breadbin that board is a really early board with everything socketed and I didn't really want to start opening it up so I bridged the c64c fuse holder with wire and switched it on the wire didn't glow or get hot neither did any of the ic components and the funny thing is the machine worked perfectly in fact its so strange . possibly found the cause ?? the four socket extension the PSU was plugged into made a crackling sound when you touch or slightly move the PSU plug so I wonder if the extension has been sending a surge down to the PSU/c64 . Would this cause the fuse to blow ?

[@GadgetUK164]

Yes, its possible for bad connection at the PSU side to cause a fuse to blow. As the connection makes and breaks you get small current surges which over a short period of time can cause it to pop.

[@cloerenjackson3699]

For some reason, the wah wah funk guitar from the No Second Prize title music on the Amiga is playing in the background. :-)

[@HuntersMoon78]

Does get a bit repetitive though.

[@TheMadmagik]

I noticed something here in the video - look at RAMSIC - and the navy seals text for the credits etc - the letters are not corrupted themselves but for certain characters, the wrong character is displayed. If you work out the binary for B and R it may give you an insight into exactly what was wrong with the address bus...... Same in the navy seals text - ORION displayed correctly at the bottom but OCEAN and some other text was scrambled. It points me to stuck address bit.

[@RetroMarkyRM]

Great vids, am enjoying working through all your c64 ones...btw any idea of what i can do with my plus 4 that powers on but shows a black screen? I know normally its the TED or CPU is the culprit but is there a way with a multimetre to check the chips first to confirm which is dead? (im a multimetre noob so if you can explain exactly how on an analogue one I'd be grateful)

[@GadgetUK164]

It's a bit tricky but I guess you could do something similar to what I did on the video I covered on looking at faulty chips for the C64. ie. put multimeter on resistance mode, take the chip out of the system, and measure resistance between ground and vcc (or vdd) pins on the chip (you will need to find the pinouts for TED and the CPU first). Also measure resistance between gnd and the data lines (d0 to d8 etc), and address lines to gnd. Do the same from VCC to data lines and VCC to address lines. Basically all the address lines should be approximately the same (and the data lines should all measure like each other) - if you find one that is very low resistance where all the others are high resistance, then that connection probably is the issue.

[@SMASHBROS03011973]

thanks I will try it .ur upload videos are very useful for all .

[@load463]

ive a c64c thats started blowing fuses and obviously no sound and the only other c64 i have (other than my working breadbin ) is another broken breadbin and i dont think you can use the 65 series sid to test in a c64c board can you ? i dont have a 85 series sid to use to try

[@GadgetUK164]

Does it blow a fuse each time you swap it out? Try removing the VIC or SID and then power it up. That could help you work out which component is drawing too much current. The other thing you can do is temporarily bridge the fuse with wire, power it for a few seconds and see what is getting hot - something will be drawing a lot of current and heating up in the process. You shouldn't use the 6581 in a C64C, the C64C powers the SID with 9V, whereas the breadbin powers the 6581 SID with 12v, so whilst you might not damage the 6581 in a C64C it probably won't work as the voltage isn't heigh enough. You could buy a nano swin SID for around $15 on eBay - that can work in either a breadbin or a C64C.

[@TheCaitlinlopez]

Is not heat is paint solvent or another chemical like thi

[@MarkTheMorose]

Blue Max and Bruce Lee, two more old favourites of mine. In Bruce Lee, a second player controls the green sumo wrestler, but I think all you can do is 'bellow', I don't think you can actually punch or kick, could be wrong. Both were also on the 8-bit Atari, and Bruce Lee I think got a Spectrum version. I think the same company who did Bruce also did Conan and Zorro games, so there may be similar game mechanics. Oh-oh, now I'm thinking of old C64 games again... Falcon Patrol, Hover Bovver, Fort Apocalypse. OK, stopping now!

[@GadgetUK164]

Yeah, great games lol! I've never checked out Conan and Zorro but I will now =) There's a charm in these old games that makes them great fun to play. I love graphics and sound on those games as well, despite the limitations there's something special about them.

[@TheMadmagik]

Sorry for being a smart arse, but address bit A4 was being pulled high. Probably internal short on that 258

[@GadgetUK164]

+MAGIK It was actually a problem with A6 or A7!

[@diademmichael]

Great videos on a classic piece of history! I'd like to get your thoughts on an issue I'm having with my 64 breadbox. When I turn it on it shows "30784" in the top left corner of the screen. When typing, odd characters appear and eventually "@@@@" show up in a different location than the cursor. I've swapped out the PLA, CIA's, Kernel, Character and Basic, VIC and SID with no change in behavior. Beyond this the chips are not socket-ed so I was hoping you might have an idea before I start unsoldering them. 1984 Assembly 250425. Thanks again for the great videos. Michael

[@GadgetUK164]

Your fault sounds like it is perhaps caused by the same fault I had in this video! It would be useful if you could either upload a video or some photos of the screen? 74LS258 is my guess. Before you desolder it and replace it that way, you could get a new 74LS258, bend the pins inwards slightly, and slide it 'over the top' of the existing 74LS258 (making sure that the pins are snuggly gripping the chip below. See if behaviour changes, if it does, then it's going to be that chip. That isn't test isn't always conclusive - sometimes the behaviour might not change when the chip is faulty, but often it will change behaviour enough to provide that clue.

[@GadgetUK164]

Might also be the 4066 connected to the VIC - but not sure that would cause the same behaviour. Possible though. You could also try the same piggy back test method there before you comit to desoldering anything.

[@diademmichael]

Thank you for the suggestions. I'll give those two chips a shot using the piggyback as an initial test. I have a still shot of the screen and inside of the board, but that doesn't give you much. I'll take a video of the malfunction after work and upload. My board has two HCF4066BE chips in U28 and U16.

[@diademmichael]

My board is a bit different in layout and chip numbers. The 74LS258 area you replaced is occupied by a 7709 chip. There are two 74LS257AN chips a a few chips over in U13 and U25. Pics here: app.box.com/s/f9kyq2npo024z8s7t9vfr4oabsvjcxso app.box.com/s/gfq4e2dr58k84832xnfpn7w5qnob467q

[@GadgetUK164]

+diademmichael The 7709 can be replaced with a 74LS258!

[@benbale6963]

Very nice work, Compliments! Sorry, i have a question: I'm trying to fix my Commodore 64 with Assy No.250425 and replacing the PLA the computer work but with a defect: the welcome screen is displayed, but the text reads: ***** CKIK@KRA 20 BASIC R2 ***** 20K RAI SYSPAI 38911 BASIC BRAA RAA@Y. █ When i write a command (for example PRINT "HELLO") the computer understands and executes the command but most of the characters that I type on the keyboard are wrong in the render video. Power Supply is ok, POKE/PEEK commands work, Restore button work, BASIC ROM (U3) is ok, KERNEL ROM (U4) is ok, CHARS ROM (U5) is ok. In your opinion what would be the fault? Thanks for any answers!

[@GadgetUK164]

See the video I did on graphical corruptions for C64 - chance are it's either your VIC chip, or the chip I replaced in that graphical corruptions video.

[@benbale6963]

Thank you so much for the reply! I'll try :)

[@benbale6963]

Hi, I wanted to thank you for the advice you gave me! Today I got the parts and I tried to fix it. The defective part was the U26 and now my Commodore 64 is reborn!!!! Grazie!!!! :)

[@cor1128]

Thanks looking forward to it . ik deed A full re cap of mij A1200 the caps where leaking sorry to say i deed not make A video of it

[@FlashGordonMurr]

I friggen love this channel. It so fascinating for someone who knows very little about computer repair. Brilliant. Had a question, my c64 which is in great shape works no issues. However I notice when I play games I get on a certain part of the screen tiny bit of graphic distortion. Games work fine sound etc. it happens to all games so I am sure it's maybe a graphic chip? Or maybe a power supply issue? Any suggestions? I thought it may have been the 1571 sd device but it happens when I try a cartrage game also. Also tried monitor and crt tv same distortion. Is there anyone who repairs these still?

[@GadgetUK164]

That kind of thing can happen with a bad connection on the VIC chip. That was one of the reasons I added heat sinks and a fan to my VIC. I had exactly the problem you described and found after taking out the VIC and cleaning up the pins and then reseating it, it would be OK for a period of time (maybe a week or month), but it kept coming back. I noticed the chip was getting extremely hot, and that influences "chip creep", where the VIC basically just starts making a bad connection again. Try reseating your VIC to see if that fixes it. The other possibility is a glitchy PLA or something.

[@SMASHBROS03011973]

hi there what's the right chip to solve overheating lost ghapics. 😕

[@GadgetUK164]

+SMASHBROS03011973 Probably the VIC. Sometimes taking the chip out, cleaning up the pins with very light sand paper and re-fitting the chip in the socket can sort that kind of problem. Ultimately if that works it sounds like you need more cooling on the VIC.

[@GadgetUK164]

+SMASHBROS03011973 But it could also be the PLA, or one of the 2 x 74 logic chips connected to the VICs address bus.

[@StRoRo]

And yet again I find an old video . I got a C64, I get the blue boot screen but the text is corrupt, and I'm getting 4 cursors, the keyboard characters are mixed up.

[@GadgetUK164]

Haha =D Does it look like any of these screenshots here:- derbian.webs.com/c64diag/ That website can be useful in working out quickly many common faults.

[@StRoRo]

Looking like it could be the U14 logic chip or the 74LS258 as I think its better known as. I asked on the Lemon64 site and was told garbled screen repeating every 64 bytes is a problem on the A6 line, which does go through the 74LS258. Now I need to find where to get one, are they quite generic?

[@GadgetUK164]

+StRoRo Yes, just search EBAY - bound to be on there! Probably is that chip. Could be VIC chip if not that.

[@binarybox.binarybox]

@@GadgetUK164 Useful link, thanks.

[@RetroMarkyRM]

hey inspector. i gotta 128 with bad char rom. do you know if it's possible for me to burn one on my eprom programmer? (finding it hard to source one!) thanks mate

[@GadgetUK164]

I think the problem is the pinouts are slightly different from most modern EPROMs. You might be able to use a 2732 (If you can find one). The alternative is to use something like a 27C512, and 'adapt' its pinout, either with a socket, or by bending pins and using wires to re-route the pins that are different. I would contact 'Dave Curran' over at Tynemouth Software, I bet he can provide you with a spare, or possibly an adapter!

[@RetroMarkyRM]

thanks gadget. tynemouth it is then. ;)

[@RetroMarkyRM]

thanks, looks like he can provide / make an adapter. Also he's gonna help me with my 5 octave keyboard to sound expander ribbon cable :)

[@GadgetUK164]

Awesome =D

[@DirtyBob7777]

how do you get it so clean on a flat screen?

[@GadgetUK164]

Part of it is the C64 revision and the capacitors (this unit has been fully recapped), but the other part is the TV itself - not all LCD TVs are equal with regards to upscaling of composite signals. This TV is a 720p Phillips ambilight from around 2006 / 2007.

[@cor1128]

your video are great learning A lot. love the C64 but i like the amiga more wil you doe some repairs on amiga,s ?

[@GadgetUK164]

Thanks for the great feedback =) Yes, I will do more Amiga videos at some point. I've got a 500 with an Agnus problem and an Amiga 500+ with corrosion damage which will be uploaded later.

[@zedexspectrum4266]

I Was Under The Impression That You Can`t Mix The 8 And 6 Series Chips, The 64C Board`s Were Of A Lower Power Than The Bread Bin.

[@GadgetUK164]

That's certainly true of the SID, and the VIC, but not the CPU. As far as I know it's possible to swap RAM, 74 logic, CPU, and ROMs (although an adapter would be required as the C64C has two of the ROM chips merged into one). I don't think the CIAs are interchangeable between the C64 and C64C either. EDIT: And the PLA is very different in the C64C as it contains other 74 logic in the same package, but in theory (with a tonne of work) it's likely possible to adapt the C64C PLA to the C64.

[@zedexspectrum4266]

Ive Got backup for my socketed first revision bread bin that i suspect has a dead pla because it boots to a shimmery multicoloured display i may have a go at resurrecting it now ive found your videos.

[@GadgetUK164]

If the letters are shimmering different colours it's probably the color SRAM. Let me know how you get on!

[@zedexspectrum4266]

its not so much the letters its the whole screen area,its just jumbled colored garbage drive.google.com/folderview?id=0BxusiBO3qBxGaC1jLVM2UHJROWc&usp=sharing Please let me know your thoughts.

[@GadgetUK164]

Wow, that's very strange! I think my first guess there would be the VIC chip, or the surrounding circuitry there - the stuff that's usually very near the VIC chip. If you've got the model with the shielding can around the VIC there's normally either a single MOS chip nearby (I forget the part number), or 4 or so 74 series chips - My guess is it could be one of those, as this looks beyond the sort of fault you would get with CPU, PLA, RAM, ROM etc, clearly a clock / video issue imo. Check the voltages before you swap anything out.

[@discoHR]

Blue Max.

[@GadgetUK164]

+discoHR Hehe, cool game isn't it =D

[@lactobacillusprime]

Cool another revived machine!

[@GadgetUK164]

Thanks =) I've subbed to your channel - you've got some great vids I've yet to check out later =)

[@lactobacillusprime]

GadgetUK164 Thanks for subscribing. I have a C128 to revive myself. Something's wrong with the keyboard - I'm getting another broken machine and will probably end up splicing the two together. Still mustering up the courage to do so! :P

[@GadgetUK164]

lactobacillusprime Interesting to see how you get on with the C128! If you've got a keyboard fault and it's not the actual keyboard I would suspect the CIA - 6526 (U1). Typically you can find a block of keys just doesn't work.

[@lactobacillusprime]

GadgetUK164 All keys work - it's like the cursor down key is stuck in C128 mode, those keys are not used in C64 mode. The keyboard has physical issues. So I am planning on swapping that out with a working keyboard to see if it is a physical keyboard thing. I think it is. Got a 15 euro spare machine coming may way :P

[@GadgetUK164]

Ah OK. Sounds like probably dirt on the keyboard contacts then.

[@mrsnaglepops1876]

wel done m8

[@GEORGE-jf2vz]

Bruce Lee is an excellent game.

[@meruginger934]

Assy 250425...like my Board Thx for this vid

[@webwarp2023]

Hi GadgetUK164 can you tell me if you repair C64 computers ? I´m living in Portugal If i ever need your help you can repair my C64? grettings

[@GadgetUK164]

Hi, sorry I don't do repairs for other people - not enough time, also suffer from ill health as well =/

[@GeoAnas]

GadgetUK164 Sorry to hear that...I wish you all the best !