Vic 20 black screen :(

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Sonic
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Vic 20 black screen :(

Postby Sonic » Sun Dec 24, 2017 9:54 am

Hello
I've bought a very sick Vic 20 which is very yellow and has 2 keys broken off. My main issue when i power it up it does not have a picture its completely black.Any suggestions what to check first ?

Thank you
Sonic

norm8332
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Re: Vic 20 black screen :(

Postby norm8332 » Sun Dec 24, 2017 10:30 am

What kind of power supply plug is on it? It's either a two pronged connector or a multiple pin round din connector. The latter being the CR (cost reduced) version. Power supply voltages is always the first thing I check. You can also reference this repair doc: http://personalpages.tds.net/~rcarlsen/cbm/vic20/vic20.txt for ideas. There are a lot of causes of a black screen. It is a process of elimination. Initial Prime suspects are the Kernel ROM and VIC. But I would start by carefully re-seating all socketed chips.

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Floopy
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Re: Vic 20 black screen :(

Postby Floopy » Sun Dec 24, 2017 12:38 pm

BSOD
(Black screen of death) :mrgreen:
-Floopy

Sonic
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Re: Vic 20 black screen :(

Postby Sonic » Mon Dec 25, 2017 9:58 pm

It seems i have sound when i plug in a game cartridge but no video.. So i assume the CPU is working ? So my VIC is dead !!! :x :cry:

norm8332
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Re: Vic 20 black screen :(

Postby norm8332 » Mon Dec 25, 2017 10:39 pm

You cant assume the VIC is dead yet because it is responsible for sound output also. How are you connecting it to the tv? Is it the CR version (see above) or the older type VIC-20? It's harder doing a diagnosis without knowing.

Sonic
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Re: Vic 20 black screen :(

Postby Sonic » Tue Dec 26, 2017 12:09 am

norm8332 wrote:You cant assume the VIC is dead yet because it is responsible for sound output also. How are you connecting it to the tv? Is it the CR version (see above) or the older type VIC-20? It's harder doing a diagnosis without knowing.

I have a din cable (from the Vic 20) going to composite video to 1084s monitor.
The Vic 20 M/B ASSY 250403 / FAB NO 251040--01 Rev D

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Mike
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Re: Vic 20 black screen :(

Postby Mike » Tue Dec 26, 2017 3:05 am

Sonic wrote:The Vic 20 M/B ASSY 250403 / FAB NO 251040--01 Rev D

That means you've got the CR variant, which needs the PSU with the DIN plug plugged in on the right side of the computer.

The video chip in the VIC-20 reacts quite sensitive to a slightly malfunctioning PSU. In that case, it tends to stop video generation but still works when a working PSU is connected. Do you own a multimeter for checks and know how to use it?

It seems i have sound when i plug in a game cartridge but no video...

This could simply be iffy cabling of the video signal. The VIC-I chip inside generates both video and sound, it's very unlikely that it still produces sound but no video anymore.

Sonic
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Re: Vic 20 black screen :(

Postby Sonic » Wed Dec 27, 2017 4:28 am

Yes i do own a fluke multi meter & i know how to use one.. Checked both voltages from 2 different PSU and both measured the same voltages..
As for the cabling i tried with 2 cables and i still get no picture but with sound.. This has got me stumped..

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Mike
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Re: Vic 20 black screen :(

Postby Mike » Wed Dec 27, 2017 5:48 am

Sonic wrote:Checked both voltages from 2 different PSU and both measured the same voltages..

You should check the ripple on the +5V supply by setting the fluke to AC. That ripple should not exceed, say, 0.1 Volt.

i tried with 2 cables and i still get no picture but with sound.. This has got me stumped..

You could build an own cable by combining a DIN-RCA breakout adaptor and a AV cable like I did here (this one also carries a separate chroma from a S-Video mod). Other than that, you'd need to buy a known working VIC-20 and test the presumably non-working VIC chip in it. Also, the video circuitry around the VIC chip could be the culprit, but then you need a scope and a good read of the schematics to further diagnose this.

Sonic
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Re: Vic 20 black screen :(

Postby Sonic » Wed Dec 27, 2017 7:46 am

Thanks Mike I'll give those suggestions a go

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J.E.E.K.
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Re: Vic 20 black screen :(

Postby J.E.E.K. » Wed Dec 27, 2017 6:36 pm

Recently I got a VIC 20 with the same symptoms. After checking all the voltage stuff ...

Take a closer look to the black screen on power on if the screen is turned on very bright. If the VIC chip is still working (kind of) you will see some vertical ghost patterns all over the screen in character intervals. They are trembling around as long the CPU is trying to execute the reset procedure, coming to an end after a short while (probably the CPU get stuck on some point).

In my case it was the KERNAL ROM which was gone. Checked it with a analyzing ROM replacing the original one. Others told me, this isn't unusual. ROMs seem to be more sensible in this regard then expected. Got the ROM replaced by a conversion socket with an EPROM ... :D
Game cartridges didn't work either because the module procedure in the KERNAL had not passed yet.

Sonic
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Re: Vic 20 black screen :(

Postby Sonic » Tue Feb 13, 2018 4:06 am

Fixed !!!!

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J.E.E.K.
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Re: Vic 20 black screen :(

Postby J.E.E.K. » Wed Feb 14, 2018 4:08 pm

Sonic wrote:Fixed !!!!

Fine!
Was it the Kernal ROM?
Or what else?

Sonic
Vic 20 Drifter
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Re: Vic 20 black screen :(

Postby Sonic » Mon Feb 19, 2018 6:45 am

J.E.E.K. wrote:
Sonic wrote:Fixed !!!!

Fine!
Was it the Kernal ROM?
Or what else?

A dead 6560-101


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