VIC-20 Video Problem.

[jdgabbard]

Hey guys, I recently came into possession of a VIC-20. Today I plugged it to test it, and was created with a short black screen during the memory test, then a composite image as below. Shortly afterwards (3-5 seconds later) the video did a slow creep into the second picture. Then another 3-5 seconds later it lost sync with the TV and the video was lost completely.

 

Trying this again after about 10 minutes provided the same results: short black screen, composite video for about 5-7 seconds, then video distortion, then I powered off the machine. I wouldn't think it is the 6560, since I am getting video. But seems like it is loosing horizontal sync. But that is just my guess. Any gurus out there experience this problem before?

 

I appreciate any pointers.

 

Links to images of the video:

 

http://retrodepot.net/wp-content/uploads/2018/07/img_5858.jpg

 

http://retrodepot.net/wp-content/uploads/2018/07/img_5859.jpg

 

 

[carlsson]

Have you tried with a different TV, preferrably something older? Your TV detects the signal as 480i, though I think the video really is 240p, plus that the frame rate might not be exactly 60.000 Hz. That would explain if the TV goes out of sync, if it misreads the signal. Which other systems are you using the same TV with for comparison?

[jdgabbard]

Thanks for the reply. The TV gets used infrequently with various systems. A IBM 5160, TI-99/4A, SNES, NES, Genesis, a second VIC-20 (works fine with the TV), and various other computers.

 

I have not tried it with a different TV. But that may be worth trying.

[carlsson]

Well, if it works with a second VIC-20 that should eliminate the TV being a problem.

 

I think there is one or more capacitors in the video circult that gets charged, and perhaps is the cause of the problem.

Edited July 15, 2018 by carlsson

[jdgabbard]

Yeah, its not the TV. However, I will say that I do get a little bit of a rolling interference (very light, but noticeable). At first I thought it might be an issue with the White and Dark trim pots, as Ive had a similar problem, and adjusting those worked. However, that has not been the case with this VIC.

 

I am figuring either a resistor or a capacitor has gone out of spec, but without knowing a ton about the video circuitry of the VIC, or commodores in general, Im hesitant to poke around until I can study the schematic. Im hoping someone has seen this before and has a starting point to suggest.

[jdgabbard]

Out of curiosity, does anyone know what the voltage at pin 2 of Q4 should be? According to the schematic HERE it should be 9v DC. However, from doing some reading, I am hearing that the cassette port specifies 6.5v DC for the motor. This doesn't seem right at all since it would end up being around 8.6v DC after passing through the transistor.