View Full Version : Commodore 64 problems

September 9th, 2005, 11:02 AM

My son bought a Commodore 64 of a well known auction site :) It was supposedly tested and was working fine. We got it today, hooked it all up, and tried to use it. The cartridge games that came with it work fine, but the computing part does not work. We tried some of the trouble shooting stuff from within the book but still nothing. The screen is blank with no initial display or cursor. Just wondering if anyone had any ideas. This is for a kid who wants to play with vintage computers. He has all sorts of games for it that he purchased before he had the computer and is just a bit disappointed that it doesn't work.

We are unsure about the disk drive. It has a green light and the red light flashes when it comes on. Without the display or cursor I can't give it a command. How do we know if it is indeed working?


September 9th, 2005, 11:13 AM
hi momncrew,

One of the things that used to give me this kind of problem was the connector itself (the cable that goes from behind the computer towards the monitor (or TV adapter) and the tv adapter itself.

if you have a monitor (the 1702 or the 1902) you might want to make sure the cable that goes to that monitor is good...radioshack sells this kind of RCA type cable that you'd need.

a tv adapter might be what you need (depends on what you are using). if you are using a tv adapter make sure the TV is on channel 3 or 4 (as set on the TV Adapter).

If none of these work..post again and we'll see what we can do....if the game works, but not the computer part...it would be weird at best because the Game needs the computer's CPU to perform it's functions.

September 9th, 2005, 11:22 AM
Thanks for a VERY prompt reply.

We don't have the monitor. We are using a TV. We do have it set to right channel=right computer setting. There is a switch box that came with it that says TV/GAME. When it is on game you can play the cartridge game and everything is nice and clear and all.

We used to have one of these and I think we had a switch box/antenna splitter thing that said TV/Computer. Any difference??


Terry Yager
September 9th, 2005, 11:24 AM
The RAM could be bad, but still work to run a program from the ROM, couldn't it? (Just guessing, not really a C= person (flames to /dev/null)).


September 9th, 2005, 01:28 PM
hi CJ,

I'm not sure I understand, when you say that the cartridge games work, you mean that if you use a cartdrige it works fine, shows up on the TV and all?

But no cartdrige, nothing?


September 9th, 2005, 01:57 PM
Yep, if you plugged in a cartridge game (the Atari style ones that the C64 has a port for) they worked. The kids were playing a game. But when you just tried to run the computer the screen was clear (not fuzzy) but blank-no display, no ready, no cursor/prompt. We tried switching it off and back on, checked all cables, checked the book to make sure everything was hooked up right.

But now nothing is working. My son took it down stuck it all back in the box and later decided to try again. Can't even get the blank screen. We did get a new switch box (TV/Game) from Radio Shack and also put in a new fuse. Power comes on but still nothing. When we put the new fuse in we noticed that there was lots of corrosion on the metal parts inside. Plus it's pretty dirty-but my PC can get pretty dusty inside to so that may not mean anything. Well, anyway now I have a kid eager to play Commodore games and no Commodore to play them on. I hate to buy another at auction and get it and not have it work. The poor guy gave me his paper route money to buy this for him. Can't help feeling bad for him.


September 9th, 2005, 02:37 PM
well, if you were playing the cartridge games, it was hooked up correctly.
(I would try with out the drive plugged in just in case).
and would suspect that it was in mostly working order.

if it's no longer doing that, it doesn't sound good.

I wish I still had mine, I'd mail it to you. I gave it away about two years back.

It's not the same thing of course, but until you get the real thing working I've take a look at some C64 emulators for the PC.

you can *cough* get disk images and load them in the emulator just like the real thing.


seems cool.

September 9th, 2005, 04:42 PM
Yeah I think that corrosion might have been a bit too much for the good ole c64...

Terry Yager
September 9th, 2005, 06:19 PM

Where are you located? Pm me your snailing addy, and I'll send ya a 64 for shipping cost, courtesy of Discount Computer Resources, our local non-profit org which donates old computer equipment to deserving children. (I might even be able to get them to spring for shipping too).


vic user
September 12th, 2005, 03:57 AM
and Momncrew, if you take Terry up on his offer, I have a HUGE amount of C64 software, and I can copy and mail out some of it to you.

i can also supply you with a cassette drive, so your child can save programs to tape.


September 14th, 2005, 01:43 AM
Back to the question why a cartridge game might work but not booting into Basic; it could be the Basic ROM that is faulty, or maybe one of the CIAs if the cartridge does not require it. It may also be RAM problems. You say that the red light on the disk drive kept flashing when you turn on the computer, which to me sounds like the serial bus (thus the CIA) was broken. Ray Carlsen or someone else with a lot of C= repair skills may help you diagnose what is wrong, to possibly use it for spare parts even if you get another C64.

October 4th, 2005, 03:02 PM
Pulling a game cart out of a powered-up C64 can destroy the CIA chip(s).
It takes a few seconds for the capacitance in the power supply to decay.
That is why it more than likely worked originally with a game cart. (Explanation is below as to why carts can work and the CPU does not).
Snatching out the game cart under power, or under residual capacitance more than likely destroyed the CIA chip (s).

Blank screen is the most common symptom, and a failing PLA chip is
the most common reason. However, quite a few other failures can cause
it as well, such as a bad power supply (check with a known good
substitute), bad RAM chip(s), and in general, just about any other chip
in there because many chips share multiple data lines. If any one of
those lines is loaded down or missing a signal for whatever reason, it
can produce that symptom. To narrow it down a bit:
Turn the computer off and back on rapidly about five times. If the
screen ever comes up with flashing colors or all one color, the PLA is
suspect. Replace it to check. Try a cartridge, such as a game. It
essentially "replaces" some of the chips in the computer when it runs.
If a cart works, check the ROMs. The screen may have a normal border
even if the CHARacter or BASIC ROMs are bad. A bad Kernal ROM will
produce no border.
The internal RF unit outputs a signal that goes to the antenna
input of your TV. If the picture is snowy, suspect the RF modulator,
assuming the direct video output of the 64 is normal. If the computer
is "dead" but is getting power (red LED on), the modulator will produce
a black screen... darker than the blank screen of a failing chip in the
computer. A missing 9VAC (power supply problem) is a possiblility. Note
that the later C64C will still work without the 9VAC or if the internal
fuse opens. However, you will have no sound, the cassette will not work,
and the TOD clocks will not work.
See if any of the RAM chips (there are eight of them) get warm or
hot... feel each one with the back of your finger after the computer
has run for about 5 minutes. Shorted chips will get hotter than the
others. Note: bad RAM doesn't always get hot. See if the computer
resets the other components in the system like the drive and/or
printer. If so, try a "blind" disk command and see if the drive
responds.. try formatting a disk. If that works, you may have a bad VIC
chip (no screen display). Sometimes a bad SID chip will produce a blank
screen... pull it out and try the computer. It will run without it,
although you will have no sound, and a proportional mouse will not
work. The few large chips that normally run hot have a high failure
rate: in rough order... the PLA, SID, MPU and the VIC. Static zaps
usually take out chips like the CIAs. A shorted CIA can produce a blank
screen. Note that you will get the startup screen with the CIA's
removed. You can use that as a diagnostic.

October 4th, 2005, 09:09 PM
I'm officially impressed! :shock:

When I get another C64 I'm gonna ask you the questions if I have any!


October 18th, 2005, 06:59 PM
If you're still stuck reply and I can tell you exactly what chip is bad (don't remember off hand)...I went through the trouble of figuring it out on a 64 of my own a while back. The reason the cart works is that it's bypassing BASIC. This is a fixable problem and your RAM and processor are probably fine. Of course you may have to return to that well-known auction site and get another 64...for parts.

October 19th, 2005, 07:03 AM
It sounds like you have a failed CIA chip, but before you toss the unit...

First thing I always do is open the case, clean all contacts (tv tuner or electrical contact cleaner if handy, pulling them out and pushing back in if not). Any ICs in sockets I pry them part way out and push them back in (or sometimes just push them in, they've already worked their way part way out).

Just yesterday I took a PC to set up for a former foster child, and when I got there it wouldn't boot. I opened it and reseated the internal cables and it was fine.

November 15th, 2005, 01:56 PM
I've fixed a C64 or two over the years by opening them up and "blowing them out" with canned air. My friend's died after several years of use, and when we opened it up I found a huge pile of cat hair -- apparently, his cat used to sit between him and his keyboard! After a 2 minute cleaning, he was back up and running!

Over the past few months I've found half a dozen C64's in thrift stores. If they're under five bucks, I pick them up (I think I have four "spares" out in the garage, not including the 64c, 128 and sx64 I have hooked up). It's a bummer that your kid got a bad 64, but if you check your local thrifts you might be able to find a replacement one on the cheap.

c64 grief
May 7th, 2014, 07:06 AM
Greetings all! - new here and having C-64 issues!.

I am trying to resurrect a brown c-64 computer that i used 20+ years ago.

1) Power supply (i have 2 of them) is good and i have tried both of them.

2) Have (2) c64 monitors - 1 color and 1 "green" and have only tried the green mono-color monitor.

3) Have no hard drive connected.

4) Only have power cord from power supply connected to computer and video cable (have another as spare) connected to video input on monitor from computer.

I had this issue a very long time ago and i think i replaced one of the IC's along the back of the board and that solved the problem (I am sure that it was an IC). The other day i obtained a new video chip and installed it and have the same results as listed below.

A) Turning computer on i get a "green glow" from the green mono-color monitor.

B) I can wait hours and i get no display of text or anything else for that matter except as mentioned above.

C) I have read this thread and have re-seated chips with no change.

D) I have a cartridge program (communications software) that i have plugged into the back of computer and booted same with no changes.

.....So, what's my next step? Which IC would it be or, could it have been that i replaced years ago lined up against the back edge of the c-64 board?...ANY assistance would be appreciated!

May 7th, 2014, 11:39 PM
I believe the standard answer is the PLA chip, for which there are a couple different replacements, ranging from a fast EPROM (generally not recommended) to a SuperPLA or similar custom design. In the breadboxes, this is one of the first chips to fail, and if it does the computer won't boot up.

c64 grief
May 8th, 2014, 02:39 AM
I believe the standard answer is the PLA chip, for which there are a couple different replacements, ranging from a fast EPROM (generally not recommended) to a SuperPLA or similar custom design. In the breadboxes, this is one of the first chips to fail, and if it does the computer won't boot up.


...Thanks for the reply!.....This leads me to my next question; WHICH numbered chip is it as STOCK?

May 15th, 2014, 01:08 AM
The PLA is 906114-01 in socket U17.


If you search for C64 and PLA, you will get several discussions from the Internet about different solutions, pros and cons. You might be able to source an old stock or used PLA from a similar C64, but the risk is that it will fail suddenly. One standard tip is to power cycle it very quickly on-off-on for a few cycles and leave it on. Sometimes you can jumpstart it that way, which would be a crude way to identify that it really is the cause.

Here is Ray Carlsen's diagnostic article on the breadbox C64, chips and symptoms:

c64 grief
May 15th, 2014, 02:25 AM
.....Thanks for the reply......i Had since discovered the part number(s) and locations prior to your post...I just need to located a rather inexpensive chip to get this back on it's feet again.

June 10th, 2014, 07:21 AM
Hello, I just aquired C-64 with 1541 Floppy disk drive, and computer itself is working perfectly. But the disk drive doesn't work. I decided it has bad RPM and calibration. Is there any possibility to write down measurement program? The problem is that 50 km around me there is nobody who can borrow me another drive and I don't have program itself. Thanks!

Zippy Zapp
December 13th, 2014, 10:27 AM
.....Thanks for the reply......i Had since discovered the part number(s) and locations prior to your post...I just need to located a rather inexpensive chip to get this back on it's feet again.


Did you ever get this C64 to work and was it the PLA?


December 15th, 2014, 10:36 AM
I still have 1 1/2 tubes of PLS100 PLA chips programmed to replace the C64 PLAs. These are NOS PLA chips that were programmed and individually checked in a C64 before going back in the tube for storage. They are listed on my web site.


If you want to program your own PLA chip the code dump is publicly available on my web site in the downloads section. The MESS team used that code to improve the emulator and gave me acknowledgement for providing the PLA code. :)