Help troubleshooting / character G. definitions

[javierglez]

Hello, I have a partially working VIC20 which seems to display correctly the ROM characters as soon as they aren't reversed. See the attached picture.

It's not a VIC chip fault, as it performs correctly in another VIC.
I checked a couple of cartridges and they run fine.

I can't promptly check the char rom bc, although it is socketed in this computer, it's not in the other VICs I have right now.
For starters, I don't know if the reverse chars definition is stored in ROM or it is somehow inverted. I guess it could be some auxiliary component.

Now that I'm looking at the picture again, I see that sometimes it is as if it where overlaying two characters together.

[Mike]

Javier,

I don't know if the reverse chars definition is stored in ROM or it is somehow inverted.

the character ROM contains two full character sets, the uppercase/graphics one and the lower/uppercase one, both with 256 characters and including the reverse characters in their respective upper half.

For a start, dump the character ROM from one of your working VIC-20s with:

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SYS57809"CHARS.PRG",8:POKE193,0:POKE194,128
POKE780,193:POKE781,0:POKE782,144:SYS65496

with that file, on your suspect VIC-20, simply do a

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VERIFY"CHARS.PRG",8,1

If VERIFY just prompts you with "READY.", the character ROM is okay, and the error is somewhere else.

If you get a ?VERIFY error, then the character ROM is possibly defective, but see below.

Now that I'm looking at the picture again, I see that sometimes it is as if it where overlaying two characters together.

Is the picture stable or do the characters flicker?

In any case, there is not any glue logic involved between VIC and character ROM: the ROM directly 'sits' on the VA bus and it is selected with VA13=VA12=0. An error that only involves the inverse characters would be one that affects VA10. VA10 is 0 for non-inverse, and 1 for inverse characters.

What type of mainboard is this (2-prong or CR)? Do you own a multimeter and know how to use it?

[javierglez]

Thanks Mike, the picture is stable, I plugged in an LCD TV, I'd say no flickering at all.

Good idea to peek the character ROM. Yes I peek the same weird character definitions the VIC displays.

This may be a good chance to use the socket in my Stack Storeboard or the EPROM reader I bought form Aliexpress and I haven't used yet.

Yes I own a multimeter and a logic probe too, although I don't have much experience using them.

I'd say the motherboard looks clean.

It's an early VIC, SN around 12000, I got a black screen on arrival, I thought it was dead, I probably didn't plug it correctly. Today was the second attempt.

(edit - it didn't occur to me check the lowercase set - there the character definitions are almost empty - white spaces)

[JonBrawn]

Before you do anything too exciting, power off then try gently easing each of the socketed chips up slightly from their socket (small flat blade screwdriver at one end and then the other - don't go wild, just enough to get them to move in the socket), and then push them back in with firm pressure at both ends of the chip. Then power on and try again.

[javierglez]

The BASIC interpreter runs OK and I tried a couple of game cartridges, for the moment it's just the PETSCII graphics which look odd. I haven't checked the datassette/drive yet, though.

I already removed the chip, it looks in good condition, maybe the socket or some trace are bad, but considering the VIC chip is OK, there's no glue logic and the empty lowercase set, the charset ROM is my first suspect now.

I'll check the voltages before trying to dump the ROM. I guess the Storeboard should be compatible as it is from the same era, and it expects 2k / 4k ROM chips. The Aliexpress reader is another story I guess.

[javierglez]

It was the character ROM, although at some point a pin inside the socket had bent inwards too much, and address line 9 had lost continuity. I think this was me and not a previous issue. I managed to restore the socket pin by prying with a sewing needle.

Btw there's a picture of a VIC20 booting without character ROM