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Commodore VIC-20 problem BLACK SCREEN

A

Automan

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Jul 29, 2016
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hi, I just found a old commodore , but switch power on do nothing , i checked the main power fuse, the internal fuse all good , still no red led on , i change the led and finally see a bit o life, but still no screen , if i connect to RF modulator only lines and static, with Scart cable , black screen and nothing else , the chip after a couple of minutes became hot but not too much , i just reseated all the socketed chip but still nothing, anyone can help me?
thanks a lot
(sorry for bad english , italian here!)
 
I had this happen to mine after I put in an internal memory expansion. Turned out to be the RESET pin being held to ground from my custom reset button (which I added at the same time).

However, in the process of finding that, I did a ton of research, and there are a variety of causes for blank screen. Anything from a dead VIC chip to fried memory/memory controller chips, to a dead CPU or ROM.

If you have spare chips, or another working VIC-20, you can swap the socketted chips between them one at a time to find out what's not working. Start with the CPU, move on to the VIC, and then the ROM.
 
sadly no spare IC or motherboard, anyone sell one for spare parts? how i identify cpu , memory or other IC (the VIC is under a small metal cover to shield RF signals is all i know) thank you
 
As your first post identifies potential problems (possibly) with the power supply - have you checked to see that you have a correct source of +9V unregulated and +5V regulated from the internal power supply? The term "a bit o life" is not something that electronic components will recognise (!) They either have the correct voltage to function or they don't. The LED is a 'consumer' check that the power supply is "ON" - nothing more. As I have found on numerous previous occasions, the LED can be lit on these things but the correct voltage supply is not actually present and the electronics fails to operate as designed.

Have you got a multimeter to test the voltages?

Have you got the schematics (from e.g. http://zimmers.net/anonftp/pub/cbm/schematics/computers/vic20/index.html) and do you know how to read electronic schematics? If you do, this will help...

One potential problem with 'old' computers is the potential health of the electrolytic capacitors. They sometimes go short circuit (usually with a bang if they are large enough!) and sometimes go open circuit - so they don't perform their intended smoothing function.

You can measure the voltage across capacitor C1 (4,700 microFarads). This should be round about +9V d.c. (Incidentally the component IDs and values will depend upon the specific 'issue' of the VIC-20 - I used the schematic I found at http://zimmers.net/anonftp/pub/cbm/schematics/computers/vic20/vic20-left.tiff as my initial reference. If you can identify exactly what revision and/or assembly number your VIC-20 PCB is - this would help significantly).

You can measure the voltage across capacitor C3 (100 microFarads). This should be round about +5V d.c. (but could vary from +4.75V to + 5.25V).

If these voltages are correct - I would measure the voltages across the power supply pins of all the ICs next. Again, the schematics show which pins these are. This check is trying to rule out a faulty PCB track starving an integrated circuit of the power supply it needs to operate correctly.

This doesn't definitively prove that the capacitors are OK - but gives us a better feeling. You would (ideally) use an oscilloscope to monitor the power supply rails - but I assume you don't have access to any test equipment like this?

Once you have done this - and confirmed the measurements are correct - we can proceed to the power on reset (ICs UE6/UE3).

Dave
 
Last edited:
Automan said:
sadly no spare IC or motherboard, anyone sell one for spare parts? how i identify cpu , memory or other IC (the VIC is under a small metal cover to shield RF signals is all i know) thank you
Click to expand...

Since you're italian like me, you can take a look at this page on my site I wrote a long time ago, based on Ray Carlsen's guides.

http://www.verrua.org/vic20/ --> See: "Riparazioni"

Unfortunately the black screen is the most common issue, and the hardest to fix because (almost) all components can lead to the black screen.

Nowadays some component are quite hard (and/or expensive) to find, like the VIC processor (6561 since you're VIC is surely a PAL version).

Spare parts apart, you need some experience and some equipment (multimeter, probe, solder iron, etc.).

If you have no skill in repair, my suggest is to find a working VIC on eBay or so on. Don't trust to people saying "it worked when I used it some years ago but"... choose a 100% working one, guaranteed!

if you want, you can write me in PM (italian, of course ^_^ )

Giovi
 
A burnt out LED on a dead system is usually a good sign of a past power supply problem.

But in a VIC20, I'd not expect a power supply self-recovery.

Also, there's really only two ways to have an external power supply problem: Either someone plugged in something they shouldn't have, or, lightning.
 
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