Project VIC

[srowe]

After going over the board with a magnifying glass and a tactical flashlight, and finding no additional broken joints, I figured I'd have to get to checking a bunch of data lines. But, no. I pulled the 6502 and put it back in, and it started up normally. It seems to be reliable now. I'm going to keep it running, maybe overnight, and see if it's solid.

The sockets used are low quality, they only have a wiper on one side of the pins. Any contamination on the pins increases resistance and over time the wiper fatigues. I had something similar happen on the character ROM socket, I ended up replacing it with a better quality one.

[chysn]

The sockets used are low quality, they only have a wiper on one side of the pins. Any contamination on the pins increases resistance and over time the wiper fatigues. I had something similar happen on the character ROM socket, I ended up replacing it with a better quality one.

In this VIC, only two of the chips (6502 and one of the 6522s) were socketed (now three with the 7402). The single 6522 socket suggests that these may be aftermarket sockets, but who knows. It wouldn't shock me to see a single socketed 6522 be original to this machine.

And now for the result of the overnight test! Around 7:00pm (19:00) last night, I started this VIC-20 disassembling memory on a loop, and let that run for 12 hours. When I turned the monitor back on around 7:00am, the machine was still working. However, the display was black and white (here meaning shades of grey). There were little bands of color occasionally, but the color was mostly gone. I power-cycled the VIC and the color came back, only to go back to black and white about 15 minutes later. And then, without power-cycling, back to color.

It seems reasonable to think that this is heat-related, and I'm wondering if a heatsink on the VIC chip might be helpful. There's a mythology around heatsinks, and some degree of controversy whether they help anything on a VIC-20. So I'm interested in opinions of this forum. Could this be useful, or could there be other explanations for lack of color after hours of operation?

[srowe]

In this VIC, only two of the chips (6502 and one of the 6522s) were socketed (now three with the 7402). The single 6522 socket suggests that these may be aftermarket sockets, but who knows. It wouldn't shock me to see a single socketed 6522 be original to this machine.

I seem to recall the CR board I have only has one VIA socketed. The early two pin boards had all 24 pin and greater chips socketed. Over time fewer sockets were used, to save on cost I guess.

[chysn]

To be honest, I can't totally rule out my 1702 monitor for the video issue. It's in excellent condition, but it's still almost 40 years old. It was left behind in the basement by the previous owners of my house, which was pretty awesome.

Anyway, I'll continue the shakedown cruise and get more information.

Update: Yeah, it seems to be the monitor. If I leave my 1702 off for a few minutes with the VIC-20 on, then turn the monitor back on, it's black and white. I can bring the color back by unplugging the video composite cable from the monitor and plugging it back in. Weird. But the good news is, this VIC seems all right!

[chysn]

This VIC-20's been working well as a daily-use machine since my last post. It's reliable and stable.

However, the cassette isn't working. I know that there's power from GND and +5V on the cassette interface connector, because that's how I powered my logic probe. So I'll have to crack this machine open again and look for a signal on the motor control connection. This looks like it originates at the CA-2 pin of one of the 6522s, so I'll start there.

The cassette switch detection works as expected. Obviously I can't know much about the read/write operation until I get the motor control back to life.

Edit: Now that I think about it, it probably has nothing to do with the control signal from the 6522. It's probably a more straightforward power issue. The cassette never runs at all. It might be a few days before I can open it up and start prodding.

[Noizer]

Check the onboard fuse ...

[chysn]

Check the onboard fuse ...

Thank you, I'll definitely check out the fuse when I get the machine open. It was okay before.

But wouldn't an operated fuse manifest as complete loss of power everywhere? On the schematic it's wired in parallel to the power switch.

[chysn]

I measured DC output of 4.762V and .20mA at pins 1 and 2 of the cassette port, and 0.000VAC. This is nearly identical to the measurements on my functioning CR VIC (4.765V and .20mA). I also checked out two different C2Ns, to rule out weirdness with the datasette. One VIC drives them both, one VIC drives neither.

What the heck is going on here? Maybe the edge connector isn't making contact? It looks pretty good....

(Note: It's not just the motor that isn't going, the Save LED also isn't illuminating)

[chysn]

Check the onboard fuse ...

@Noizer called it! The 9V fuse is blown. Fortunately, these guys are easy to find in packs of five, and I should know on Monday if there's a next problem!

[chysn]

Update: The new fuse didn't blow, and a load from tape worked! I'm using this Project VIC daily now, and it has proven to be stable. It's my favorite VIC, even though it's slightly beat-up, because I fixed it myself. I don't have another VIC to fix yet, so I think I'm going to try my hand at refurbishing my original C2N. I might make a separate topic for that project if anything interesting comes up.

[Lechuck]

Thanks for the info, Chysun,

I followed your progress with great interest. Good to know you finally got another VIC fully functional. It has to be very rewarding when you fix it by yourself. I still have a pending fix for mine and only need to find the courage to try it . I hope to be as successful as you were...

Cheers

[chysn]

I still have a pending fix for mine and only need to find the courage to try it

I've found that the more VICs I have, the braver I get, when it comes to taking the soldering iron to one.

[chysn]

The Project VIC returns! Sort of.

I bought another VIC-20 this week. Again, it met my requirement of being both "untested" and "dirt cheap."

In this case, it started right up to the READY prompt. I won't lie; this was a tiny bit disappointing. But I'll be fine.

This is one is NTSC / 2-prong / U.S.A.-made / normal keyboard / gold function keys / gold badge. It's the only gold-badge (although not the only 2-prong) VIC-20 that I currently have. The plastic film is still on the badge. The case is in nice shape, with only the usual moderate and even yellowing, and none of the six hinge tabs are broken. All of the major chips, including the VIC, are socketed. Inside, it's gorgeous. Not a single dust bunny. The 6522s are both behaving themselves. Color is nice, pixels are crisp on a 1702, sound is good.

The 5V pin of the cartridge port has been cut, interrupted by a pair of wires soldered to each side, which end in a top-mounted 3.5mm jack. So some sort of cartridge disable mechanism. I'll either (1) remove this, and seal the hole, and repair the broken connection, (2) replace the jack with a toggle switch if I can think of a practical application for it, or (3) re-wire it and add a reset button there instead.

As to why this VIC-20 was so cheap, it's missing its "2" key. The shaft looks snapped off. If I press down on the pad, I still get "2". I knew this when I bought it, of course. But I wasn't going to look into replacements until its working status was known. Now that I know it's worth the time, I'm going to search this forum and others for key fixing ideas.

[srowe]

As to why this VIC-20 was so cheap, it's missing its "2" key. The shaft looks snapped off. If I press down on the pad, I still get "2". I knew this when I bought it, of course. But I wasn't going to look into replacements until its working status was known. Now that I know it's worth the time, I'm going to search this forum and others for key fixing ideas.

Replacing key stems is straightforward, once you've correctly identified the correct replacement part. There were three different keyboards used, the stems are all different. Replacement keys can be trickier to source, some are harder to find than others.

[chysn]

Replacing key stems is straightforward, once you've correctly identified the correct replacement part. There were three different keyboards used, the stems are all different. Replacement keys can be trickier to source, some are harder to find than others.

One issue is that the remains of the previous stem is broken off, and flush against the top. So that'll be interesting to try to get out of there.

But I found some promising files on Thingiverse, one of which is supposedly a top-row key, so I'll give that a shot.

A couple years ago, I sold a Commodore 64C keyboard, and now I regret that. It happens, I can't keep everything. But fixing this with 3D printing will add a thing to my skillset.