Glen M
Experienced Member
I've been working on this most of the evening and its driving me nuts. The system seemingly works fine, it boots to basic and it will load and run software but the screen colours are off and in games the graphics are garbled.
I've attached an image of the basic screen, taken from direct capture.
Following a guide I found here - http://dunfield.classiccmp.org/vic20/v20diag.txt this suggested UC2 as a possible cause. Sure enough pins 1 and 2 didn't look right on the logic probe (always high but pulsing slowly) so swapped that out but no different. Looking at the schematics (http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/vic20/index.html) pin 1 of UC2 is actually the colour signal generated at UC6. All the inputs to UC6 look good so swapped that out and no difference. The only other component in the chain of the colour signal is a 74LS125 but you guessed it swapping that makes no difference.
I know its not the VIC chip itself as I've tested that in another machine.
My next thought is RAM but as you can see in the screenshot the system is reporting all 3583k free, looking with the logic probe there is nothing stuck high or low on the RAM. Leaving the machine running for several minutes doesn't change anything and no chips get overly hot, well the custom chips get warm, the rest of it is all cool.
One query, does the VIC-20 have colour or video ram? The schematics don't make it clear although I found a block diagram that does state video ram. Beyond that any ideas what might be wrong here?
I've attached an image of the basic screen, taken from direct capture.
Following a guide I found here - http://dunfield.classiccmp.org/vic20/v20diag.txt this suggested UC2 as a possible cause. Sure enough pins 1 and 2 didn't look right on the logic probe (always high but pulsing slowly) so swapped that out but no different. Looking at the schematics (http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/vic20/index.html) pin 1 of UC2 is actually the colour signal generated at UC6. All the inputs to UC6 look good so swapped that out and no difference. The only other component in the chain of the colour signal is a 74LS125 but you guessed it swapping that makes no difference.
I know its not the VIC chip itself as I've tested that in another machine.
My next thought is RAM but as you can see in the screenshot the system is reporting all 3583k free, looking with the logic probe there is nothing stuck high or low on the RAM. Leaving the machine running for several minutes doesn't change anything and no chips get overly hot, well the custom chips get warm, the rest of it is all cool.
One query, does the VIC-20 have colour or video ram? The schematics don't make it clear although I found a block diagram that does state video ram. Beyond that any ideas what might be wrong here?
