Repairing an old VIC

[fuzzybad]

Hey guys, I'm new to this forum but have been in the Commodore community for a long time. My first computer was a C64, but have recently discovered her older sibling, VIC-20

Anyway - I acquired an older VIC (PET keyboard model). Serial #64944. Cosmetically, it's in great condition. Looks like it was stored a long time and has never been worked on before. Problem is, it has an issue which appears to be memory-related. It boots fine, shows the proper amount of memory, and I can type simple BASIC commands. However, if I enter a simple program -- even a one-liner -- it either locks up or displays garbage on the screen after RUN or LIST. Some game cartridges work fine, others work partially with graphical corruption. For example, Radar Rat Race has junk on the screen although it is playable.

I'm thinking that it's probably a case of bad RAM, and I have ordered replacement 2114 RAM chips, which I intend to install in sockets. Just wanted to check with you guys before I break out the soldering iron. Also, I have tried swapping the major chips with known good replacements, and it didn't help. (I swapped the ROMS, 6502, VIC, etc. I didn't swap the VIAs yet, because they are stuck in their sockets pretty firmly and I didn't want to risk damage.

Thoughts?

Thanks,
Jas

[eslapion]

@fuzzybad
The symptoms you describe makes me suspect you're perfectly right about the faulty SRAM.

Just a word of caution: the traces on these older VIC-20 are very fragile. Boards weren't manufactured in the same way at the time of the VIC with PET style keyboard/VIC-1540/VIC-1541.

I hope you get it back in good shape. I myself own a similar VIC with serno. 8729.

[fuzzybad]

@eslapion thank you for the feedback. I will proceed with the RAM replacement and let you guys know how it turns out. I am aware of the fragile nature of older circuit boards, but I do have quite a few year's experience with a soldering iron. I intend to clip the leads from the RAM chips, then pull each lead out individually. It will be time consuming, but should be safer than trying to remove the chip in one piece. I once salvaged a CIA from a C64 which was soldered in place. That was a PITA..

By the way, are you the one producing the new VIC multicart? I ordered one and am looking forward to checking it out!

[srowe]

If you've got the ability to burn an EPROM I wrote a memory test utility that I used for debugging problems on my VIC.

[fuzzybad]

Unfortunately I don't have access to an EEPROM programmer. Thanks though.

[fuzzybad]

Good news everyone!

I successfully replaced the bad 2114 RAM chip, which solved the issue with being able to enter and run programs. Color and sound output look good. Cartridge port works, joystick works, keyboard works but has some issues, and Datasette works.

For the curious: I diagnosed the bad 2114 by piggybacking a replacement chip on the existing RAM. Once the bad chip was identified, I used a Dremel tool to remove the leads from the chip body, then desoldered and removed each pin individually. The new chip was installed in a socket.

Now that I'm able to test the computer further, I discovered a couple more issues that I could use some advice with:

1) IEC Serial Port doesn't work - I tested with a known good drive and only receive a 'DEVICE NOT FOUND' error. I'm thinking it could possibly be a bad VIA, broken circuit trace or bad solder joint.

2) Keyboard issues - I completely disassembed and cleaned the keyboard (down to the circuit board) yet some keys have to be pressed multiple times to register. And sometimes a single keypress results in multiple characters going to the screen. Additionally, there is a strange behavior with the left shift key. For example, if I press Left Shift-4, a '4' is printed probably 90% of the time. Left-Shift-5 prints a '%' every time. The Right Shift key does not have this issue, and prints the proper character every time. Could it be a VIA issue?

What do you guys think?

[srowe]

1) - Yes your symptoms could be a faulty VIA. Are yours socketed? If so swap them and see if the symptoms change.

2) - How did you clean the keyboard contacts? I use an eraser followed by denatured alcohol. My keyboard has little conductive pads on the keys, I've rotated/swapped them around to even the wear out. The keyboard is pretty good now but I still get the odd repeated key.

[highinfidelity]

Once the bad chip was identified, I used a Dremel tool to remove the leads from the chip body, then desoldered and removed each pin individually. The new chip was installed in a socket.

That's an interesting method of which I've never thought about. I might try to use it next time!

Concerning the keyboard, I also think you don't have a hardware problem. Looks like the contacts are uncertain. Has anyone ever tried to clean the keyboard contacts with a contact cleaner sprayed on a cloth and then passed over the PCB and the bottom of the keys?

[joshuadenmark]

The rubber contact pads on the keys are very fragile and should be cleaned by rubbing it on the surface of a piece of white copy paper or similar.
Use of solvent on these pads will destroy the surface and the switch will fail.

[highinfidelity]

Use of solvent on these pads will destroy the surface and the switch will fail.

Oh my! Thanks for telling me, I had no idea.

With "white copy paper" you mean a normal xerox sheet for office use, do you?

[joshuadenmark]

Yep, rub it gently on ordinary Xerox paper

[fuzzybad]

I swapped the VIAs with known good ones and there was no change in the condition. Any other ideas? Perhaps a support 74xx chip might be bad?

Regarding the keyboard, I wiped the circuit board down with rubbing alcohol and cleaned the rubber key pads gently with a very mild mix of dishwashing soap and water. I'm hoping that with usage the keys will get more responsive. If they don't perhaps I'll try the copy paper method.


Thanks,
Jason

[srowe]

I swapped the VIAs with known good ones and there was no change in the condition. Any other ideas? Perhaps a support 74xx chip might be bad?

The serial bus output lines go through an 7406 inverter, UE3 or UB4, depending on PCB revision. That could be defective but as it's also used for the /RESET line I'm not certain.

[fuzzybad]

I just tried piggybacking a 7406 chip on top of the one in my VIC. It didn't help.

I'll study the schematic some more and see what's attached to the serial bus lines. I see there are a couple of transistors on the board just to the left of the video section. I wonder if possibly a bad transistor or capacitor could be to blame..

[fuzzybad]

Great news, I replaced the 7406 chip and it resolved my serial port issues.

Still need to figure out some way to get the keyboard working better, some keys are ok but others have to be pounded on repeatedly.

While I was under the hood I replaced the primary capacitors, and noticed it gets incredibly hot inside the case after about 30 minutes of use. Side effect of Commodore putting the voltage rectifier on the main board, I suppose. I'm going to put heat sinks on the main chips but I'm not sure how much good it will do..