Resurrecting my Vic

[Berlioz]

Any idea what's wrong with my picture? This Vic has been in storage for many years, and now I'm hooking it back up to get it back to its old self. When I turn it on, I get nothing but a grey screen with a blue/green border. No text. Turning it on with a cart in has the same effect.

Is something fried in there?

[carlsson]

As per Ray Carlsen's text, it sounds like the Basic ROM but then many game cartridges should play anyway.

http://personalpages.tds.net/~rcarlsen/cbm/vic20.txt

[Spirantho]

First off - Hi. First post here.

Secondly, I'd say check your CPU above anything else. If you're getting a border and background correctly it sounds like your VIC chip is ok, and your RAM probably is too otherwise you'd get corrupt text on the screen.

You might also want to check that your CPU is getting a clock signal - if it isn't, then it won't do anything so it won't be running your ROM - hence no text.

I'm not so hot on VIC hardware so my information is limited.. does anyone have a ROM disassembly anywhere? That might help....

[6502dude]

If you're getting a border and background correctly it sounds like your VIC chip is ok

- not necessarily

, and your RAM probably is too otherwise you'd get corrupt text on the screen.

- actually defective ram could cause problem of no characters on display.

You might also want to check that your CPU is getting a clock signal - if it isn't, then it won't do anything so it won't be running your ROM - hence no text.

- clock is good, otherwise Vic-20 would never get to the point of intializing registers in 6560 and creating a border display with correct colours.

does anyone have a ROM disassembly anywhere? That might help....

Richard Cini posted rom disassembly years go online.
But why bother? This is a hardware problem.

[Spirantho]

If you're getting a border and background correctly it sounds like your VIC chip is ok

- not necessarily

Not necessarily, no... but in my experience if a video chip goes kablooie you know about it - a completely black screen usually. In this case it sounds like it's probably ok. Probably, not definitely.

, and your RAM probably is too otherwise you'd get corrupt text on the screen.

- actually defective ram could cause problem of no characters on display.

Yes, it could... but again in my experience it's unlikely.
The VIC uses its memory for the frame buffer doesn't it? So if the memory is knackered you'd usually get a screen full of "@" signs or something, at least if it's like the PET (and I think I've seen this on a +4 or something).

You might also want to check that your CPU is getting a clock signal - if it isn't, then it won't do anything so it won't be running your ROM - hence no text.

- clock is good, otherwise Vic-20 would never get to the point of intializing registers in 6560 and creating a border display with correct colours.

Fair enough. If those are set by the ROM then the clock is good and the CPU is at least functioning partially.

does anyone have a ROM disassembly anywhere? That might help....

Richard Cini posted rom disassembly years go online.
But why bother? This is a hardware problem.

Why bother? Because looking at the ROM disassembly you can see at what self-checks the ROM does. We know it's failing somewhere between the initial registers to the VIC chip being set and the actual BASIC prompt, so it may help to know exactly what the ROM checks. If, for example, it checks any of the support chips and halts when there's an error, we know something else could be at fault too.
When troubleshooting hardware I find it's best to know the most you can about the hardware and the software.

In this case the first thing to do though is to reseat the CPU and VIC chips and see if that fixes it.


A little addendum: In a thread about black screens just below this one in the forum, I found this:

Jogi once told me every VIC-20 will produce a empty border screen if only the VIC is still in the board

I'd defintely check the CPU and its clock signal then, if that's correct.

[a1bert]

Because looking at the ROM disassembly you can see at what self-checks the ROM does.

Self-checks: None. There is no room for it. In the C64 the only thing checked is determining PAL/NTSC by checking what raster lines appear in the VIC-II so that the right RS232 dividers are set. In VIC20 there is no such code AFAIR.

What happens at boot: (from memory)
1) init stack pointer, init peripheral chips
2) determine functioning BASIC RAM start and size
3) relocate screen depending on BASIC RAM location
4) prompt

If you get the correct border colors, the 6502 should be okay and KERNAL ROM at least partially functional. Stack or zero-page RAM can be bad, or KERNAL or BASIC ROM's fail.

If any cartridge game works, RAM is probably okay. If some cartridge games fail (but some work), it's probably BASIC ROM (or maybe KERNAL ROM).

Anyway, more information is needed.