functional VIC-20 but no video

[mjj]

Hello everyone! I'm new to the forum. I recently bought a bargain VIC-20 which, as expected, doesn't work. The VIC-20 was my first computer so it has a special sentimental value for me, as it surely does for many of you.

The board is a 250403 (VIC 6561-101). Here is the video output I get:



If I blindly change BG/FG colors the screen flickers so I know that at least the 6502, the keyboard VIA, RAM and the BASIC and KERNAL ROMs are functional.

On a CRT tv + modulator + RF cable I get a similar result:



On pin 2 (color) I get this signal:



While on pin 3 (luminance + sync) I get this:



I don't know the details of composite PAL signal so I don't know what I should look for on those signals. Intuitively I can see how the different levels of luminance (cyan / white) are being generated and I can also see two differentiated amplitudes in the color signal.

I've measured the electrolytic caps in the VIC cage but they are all above nominal ratings so I put them back on.

I've seen a very interesting thread here with lots of posts of pin 2/3 outputs but unfortunately all the linked images are gone

Any idea what to try next?

[mjj]

Small update: today the VIC behaved differently and displayed this:



Which means now there is HSYNC but the screen still moves downwards so VSYNC is somehow missing.

While on that state, I'm also able to change BOTH the background and foreground colors by poking 36879. For example, if I poke 36879, 25, the entire screen becomes white and if I poke 36879, 8, the screen goes black despite the fact that the bootup cyan + white screen actually looks like blue + black...

Anyway, I've taken benefit from this primitive 1-bit communication to peek a couple of values from the CHAR ROM and verified it works correctly with something like this:

Code: Select all

POKE36879,8:IFPEEK(32768)=28 THEN POKE36879,25

I've taken a few more shots of pin 3 (sync + luminance) and the signal definitely differs from yesterday:

5 ms/div:



100 us/div field blanking (I don't think VSYNC should look this way):



20 us/div end of blank / first line:



5 us/div detail of "VSYNC" pulses:



I'm mostly convinced the problem is in the VIC chip but I'll appreciate any feedback.

[eslapion]

AFAIK, the video signal, when the 'load' is attached should be 1v p-p (from sync to white level) and your scope indicates in excess of 2v p-p.

You should adjust the white potentiometer (plastic) near the 6561-101. Don't touch the red (metal) variable capacitor.

[mjj]

I tried adjusting the trimpot to no avail. Also note that the last shots I posted were directly out of pin 3, ie. luminance + sync BEFORE mixing with color info so the actual signal out the DIN connector has a lower Vpp.

I ended up ordering a new VIC. I think the fact that the VSYNC is missing and the chip behaves differently in a random fashion gave it away.

Thank you!

[mjj]

Quick note to let you guys know that I've got a replacement 6561-101 from Aliexpress. Just got it in the mail and my VIC-20 is back to life!!

[joshuadenmark]

Great news thanks!

[mjj]

For future reference, this is what PIN 3 output (LUMA + SYNC) should look like on a healthy 6561:

BASIC welcome screen, one frame @ 2ms/div:



Detail of vertical sync @ 100µs/div: