View Full Version : Serial not working after inserting cartridge with power on

May 10th, 2006, 03:21 AM
Well, the subject says it all. The serial port on my c64 stopped functioning after a cartridge (Fastload) had been inserted while the power was on. My 64 reads other cartridges but when I try to access the floppy drive, my c64 just hangs on the "searching for". There is no activity on the floppy drive but the drive still works on other c64's. So, the issue is with the c64 itself. I checked out the schematics and it seems that U22 may have been toasted or U2 damaged. Any ideas? I'm sure this is not the first time a cartridge has been put into a c64 with the power on. Hopefully, this is a common issue with a known remedy.

Any help is greatly appreciated.

May 10th, 2006, 03:58 AM
U2 is one of the 6526 CIAs, and it is possible that you fried one. They may be a bit hard to come by, at least loose chips. U22 seems to be a 4164.

Cartridges probably should never be inserted or removed when the power is on - some people are hesistant to do it to joysticks, video, serial connectors too but I think the risk of short circuiting the wrong pin is less than when inserting a card edge connector.

May 10th, 2006, 04:33 PM
I foundthis infor but it's for the vic20. I'm not sure if it will apply to the c64

Startup screen normal, but drive access problems ("SEARCHING FOR.. "
forever). One or more joystick positions don't work. No cassette or user
port access. Can produce blank screen if shorted (remove to check).

Startup screen normal, but drive access problems ("SEARCHING FOR... "
forever). One or more joystick positions don't work. No cassette or user
port access. Can produce blank screen if shorted (remove to check).

All of the joystick positions seem to work fine. So, I'm still not sure what the problem is. Perhaps it's not getting a clock signal.

May 11th, 2006, 05:39 AM
6526 is the follow-up to 6522, and possibly used slightly differently.



May 11th, 2006, 06:01 AM
I'll start digging through that info. Thanks! I have the newer (smaller) mother/circuit board BTW. I'll post the actual make as soon as I get a chance to open it up again.

May 13th, 2006, 04:14 AM
Thanks for the links, carlsson. That info is great!

Well, I ventured back into my 64 again. It’s actually a C64C. Here are the board specs:

C64E N
HKC S/N 290961
WW 25 /87/88

PCB NO 252311 Rev 4.

Since just about all of the serial signals danced around U22 (7406), I was fairly certain that was the source of my issues. So, I swapped it out with a new working (I tested it) 7406. Unfortunately, this did not remedy my issue. I used a data probe to make sure the 7406 was working properly while running in the C64 board. Everything seemed pretty normal.

So, I looked at U2 (6526 CIA). I tested pins 2-9 and 34 since they deal with the serial port. I checked everything just after a fresh boot. Then I rechecked after doing a LOAD”$”,8 . These were my findings:

Pin 5 (ATN OUT) - After Boot=LO, After Load=HI
Pin 6 (CLK OUT) – After Boot=HI, After Load=HI
Pin 7 (DATA OUT) – After Boot=LO, After Load=LO
Pin 8 (CLK) – After Boot=LO, After Load=LO
Pin 9 (DATA) – After Boot=HI, After Load=LO
Pin 34 (RES) – After Boot=HI, After Load=HI

Things that I found odd are, the CLK OUT seemed to be solid. I tested it with an analog multimeter and the needle didn’t seem to move at all. CLK OUT being a clock signal, I expected to see some fluttering/fluctuation. Is it possible that the clock is running too fast for any kind of needle response to occur? I have not checked it out with an oscilloscope yet. Do any of those readings seem odd to anyone else?

Also, as shown above, the RES remained HI at all times. Even after typing the LOAD”$”,8. I did not see any kind of blip or falling edge to the HI signal. Again, it is possible that the reset is a falling edge reset and was too fast for me to detect. Does anyone know if RES should be HI all of the time? When should it go LO?

So, does anyone have any input? I’m not looking forward to swapping out the 40 pin CIA, but there really isn’t much else between the serial port and the MPU, other than a few diodes and resistors. BTW: just for kicks I swapped out U6 (MPU) since it is in a socket. The MPU is fine.


May 13th, 2006, 11:54 AM
Buy another C64C and cross your thumbs? :p You've come much further than I would ever do.

Terry Yager
May 13th, 2006, 02:25 PM
I'm glad you said it, carlsson. I didn't wanna think I was the only one dumbfounded by the prior post(s)...


May 14th, 2006, 05:10 AM

It was U2 (6526 CIA). I replaced it with a 6526 CIA from a non functional C64C board I had available. I used U1 from the spare board rather than U2 since the two are identical IC’s and U1 isn’t routed directly to any ports, unlike U2, which manages the user and serial ports. Therefore, the likelihood of U1 being damaged is less than that of U2.

I was a bit concerned with heat damage since this IC has already proven to be sensitive. So rather than desoldering the IC, I ended up clipping it from the board and soldering forty little legs consisting of leads saved from trimmed resistors, diodes, etc. which were scrapped from past projects subsequent to installing those components on PC boards. It wasn’t pretty, but this method minimized the amount of time necessary to heat each pin of the IC. Actually, it appeared quite humorous before I trimmed it up. I’ll share an image. It should be good for a laugh.


Thanks for your help, carlsson. If I hadn’t found those C64C schematics while rummaging through your extensive C64 archive, I may not have been successful. Thank you!

The reason I didn’t want to, simply, give up on this particular board is due to the fact that I had performed several modifications to it which I would rather not repeat. I will add images of those modifications to the hacks/mods (http://www.bigmech.com/misc/hacksmods.html) section of bigmech.com (http://www.bigmech.com). In addition, while resolving issues on my own, I always end up learning something.

I hope this thread may be of some help to anyone who has a C64 with serial port issues, or anyone who may have made the same mistake of inserting a cartridge into a C64 while the power is on.