VIC-20 was working, now dead unit

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BlueBadger47
Vic 20 Newbie
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VIC-20 was working, now dead unit

Post by BlueBadger47 »

Greetings all!

I recently bought an early model (2 pin PSU) VIC-20 from a thrift store. The unit was sold as "untested" but I figured it was worth a gamble. Long story short, I was able to get her up and running. I purchased a PSU and video cable from eBay and was happily playing around with BASIC. Everything seemed to be working fine, just one odd thing was I had to leave the PSU plugged in for a few minutes before I could plug in the VIC and turn it on. If I plugged in the PSU then plugged in the VIC right away it would not power on. When I let the PSU sit plugged in for about five to ten minutes then plug the VIC in, the VIC will start right up

I purchased a copy of Avenger (VIC-1901). I followed my usual procedure of plugging in the PSU then plugging in the VIC a few minutes later. The VIC booted to Avenger and I played a game or two. All of a sudden the VIC shut off and would not turn back on. Switching on the power just resulted in a dead unit and now power LED. I unplugged the VIC and set everything aside.

Tonight I got some time to work on it. I checked the PSU with a multimeter. It is outputting between 9.5 to 10 volts AC. I did a visual inspection of the VIC and the fuse does not appear to be blow and there is no obvious damage not the mainboard. I plugged in the VIC, waited a few minutes and tested pins on both the user port and cassette port. I referred to the VIC-20 user guide and tested pins 5 and 10 on the user port, which should have +5 VAC and +10VAC, but both showed readings around 0.003 VAC. I also tested pin 2 on the cassette port and got similar results.

I've reviewed Ray Carlsen's repair notes and it looks like the rectifier or voltage regulator are likely culprits here. Is there any other testing I can preform to narrow it down further? What sort of repair/replacement options are available? Am I even barking up the right tree at this point?

I'd appreciate any help you all can provide. This is my first Commodore computer and I was looking forward to using it and eventually picking up one of the recent memory expansion/ROM carts.
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mrr19121970
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Re: VIC-20 was working, now dead unit

Post by mrr19121970 »

the bridge rectifier gets so hot that it can desolder itself. check and reflow the joints.

check the input A.C. and output DC voltage.if you don't have a.c. going in then check the switch. if you have over 5v coming out, then next check VR1 and then the big smoothing capacitor and the fuse. remove the fuse. clean the contacts with alcohol. squeeze the contacts as they get loose over time. reinsert the fuse.
r.cade
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Re: VIC-20 was working, now dead unit

Post by r.cade »

Another simpler problem could just be the power switch. Surprisingly, I've had these go bad in more than one VIC.

Check with a multimeter that it's actually switching.
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BlueBadger47
Vic 20 Newbie
Posts: 4
Joined: Fri Feb 02, 2018 8:59 pm
Location: Florida, USA
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Re: VIC-20 was working, now dead unit

Post by BlueBadger47 »

Thanks for the suggestions. I'll look into the different components next time I have a chance to work on the VIC.

The next couple of questions may sound silly, but I want to make sure I know what I'm doing so I don't further damage the unit.

1. How exactly do I test the switch with the multimeter and what values will I be looking for?

2. Same question with the bridge rectifier.
Imperious
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Re: VIC-20 was working, now dead unit

Post by Imperious »

I suggest You look here for the proper userport pin descriptions. Pins 10 and 11 are the 9 or 10v AC, Pin 2 is 5v dc.

http://www.ntrautanen.fi/computers/hard ... c_user.htm

When testing the switch leave the unit unplugged, and because it's a switch the contacts when closed should be as
close to zero ohms as possible. depending on your multimeter below 2 ohms is ok.
BlueBadger47
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Location: Florida, USA
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Re: VIC-20 was working, now dead unit

Post by BlueBadger47 »

I just got a chance to work on the VIC tonight.

I checked the switch with my multimeter. With the switch closed the reading was holding between one and two Ohms. I also checked the fuse and got a good continuity check.

I looked for schematics and descriptions but was unable to pinpoint the bridge rectifier or voltage regulator. I've included a picture of the mainboard. I would appreciate any help identifying the correct components.

I also took the mainboard out of the case, to see if there were any damaged solder joints, but there is a gold shield attached to the board. Upon closer inspection it appears to be soldered to the board. I don't recall seeing anything like this in videos or documentation about VIC-20 restoration. I did see a few solder joins through cutouts in the shield, but none of them appear to be damaged.
Mainboard overview
Mainboard overview
Shielding on back of mainboard
Shielding on back of mainboard
Visible solder joins
Visible solder joins
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