Help Needed Restoring a VIC20 to it's glory!

[CrashSerious]

First-- I'm doing this for the challenge and to bring something old back to life. Yes, I have probably spent more on it than buying a working VIC and I may have to spend more. Please don't respond with non-helpful comments like "you should just buy a working one." The goal here is to restore a bit of the era back to a working state.

I purchased a used VIC 20 a few months back and I have been trying to restore it to an operational state. I've been successful with my A500 and a A2000, repairing at teh componnent level, so I figured I would give it a try. It currently is partially operational, but it only displays a yellow boarder, white screen background and the cursor. (I couldn't get a image to upload, so I had to place it externally and provide a link. Also, I managed to get teh pic with teh cursor 'off' but it DOES blink)

https://ibb.co/b72BKp

When I type, I can see that it is accepting input-- but I cannot see the characters. If I type a small BASIC program, it seems to run like:
10 PRINT "FIX ME!"
20 GOTO 10
RUN

This clearly moves the cursor down the screen until the bottom, so it appears to scroll as desired. When I put a command in wrong, it appears to place the proper number of lines on the screen for a Syntax Error. For this reason, I don't believe the BASIC ROM is bad. Likewise, when I type the cursor moves the appropriate positions.

I have replaced the Character ROM from one that I bought from Dave's, the 8bitguy, website so I trust that it is good. (though I suppose it COULD be bad). I read several posts talking about garbage characters being displayed and inconsistently displayed characters. I gave it a try, this is 100% consistent if the characters are being generated, they are white on white-- invisible.

I have also replaced EVERY logic chip on the motherboard out of sheer desperation, yes-- all the 74xx series chips are brand new and the 555 reset timer as well. Since I had most around, and figured it wouldn't hurt to have the ones I didn't already have.

Literally the only chips I haven't replaced are:
Memory - My logic is that it's unlikely to be so consistent in it's partial functioning for that to be the issue.
the CIA Chips - I didn't think that would be the case. The keyboard works fine.
The 6502 - Well, it APPEARS to work and boot, the cursor flashed... she's doing SOMETHING.
The VIC - Actually, I did buy a VIC chip to test with, but the seller sold it as working and when I swap that out I simply get a black screen.

Thoughts??? Things to check? I'm at my wits end and literally no where talks about a yellow screen on the vic20.

Things I have at my disposal:
a 4 channel Rigol 1054Z (hacked to 100MHz) oscilloscope that I am familiar with
a 16 channel USB Logic analyzer
Solder, solder wick and pump, a mountain of solder, and a solder station
A willingness to try thought out, information backed, suggestions.

Things I don't have, but am looking for a reason to buy...
an EEPROM programmer/Reader
The Proper ROMS/Adapters to replace the KERNAL and BASIC ROMS (Maybe Character as well) (I haven't invested there because I can't find a lot of information on WHAT to buy)

Any help would be appreciated!

 

[mrr19121970]

tried any cartridges?

 

[CrashSerious]

I saw a post talking about cartridges working with similar (but not identical) symptoms. But, IIRC, that post talked about the character ROM being the issue. I've considered it, but have no cartridges currently. I may have to buy one to test that out, but then the question becomes--- one I really WANT or one that's the cheapest I can find.

 

[rootboy]

Have you seen this?

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

He covers blank screen problems as well as all sorts of other issues.

 

[CrashSerious]

I had seen it, which started me down the road of replacing logic chips.

Though, I'd 'lost' it-- thank you for reposting it. I now see the BASIC ROM and KERNEL ROM can do this as well. It's quite possible I need to figure out how to build some replacements. (I.E what chips to source, etc. Not how to physically DO it.)

Have you seen this?

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

He covers blank screen problems as well as all sorts of other issues.

 

[rootboy]

NP, hope you get it fixed!

BTW, +1 on the Rigol 1054Z. I have one as well, and it also has been "upgraded". Quite the nice little scope. I have a GQ-4X programmer, and it has served me well, but I've read of a lot of complaints about it online. YMMV. As for logic analyzer(s), which one?

 

[demolition]

Since BASIC seems to be running but with chars being invisible I can't see that it could be a problem with the 6502 or CIAs. My initial focus would be the 74 chips (which you replaced), the character ROM (same), the VIC and the color RAM. I guess some stuck bits in the color RAM could cause this kind of behaviour.

If you do not have any spare RAM chips, then you could try swapping some of them around to see if the error changes. I can't remember which specific chip holds the space for color RAM, but I'm sure someone else could tell you (or you could derive it from the schematic).

I also fully understand the urge to fix a broken machine. I find it boring to just buy a working machine. I spend most of my time fixing stuff and when I am done with a machine, it usually doesn't see much use as I move on to the next machine that needs fixing. Actually using them is often less fun than figuring out how to fix it..

 

[CrashSerious]

NP, hope you get it fixed!

BTW, +1 on the Rigol 1054Z. I have one as well, and it also has been "upgraded". Quite the nice little scope. I have a GQ-4X programmer, and it has served me well, but I've read of a lot of complaints about it online. YMMV. As for logic analyzer(s), which one? :smile:

The one I have is the cheap USB Saleae analyzer, but for what I need, the software is sufficient and it's helped me fix a few things.

Since BASIC seems to be running but with chars being invisible I can't see that it could be a problem with the 6502 or CIAs. My initial focus would be the 74 chips (which you replaced), the character ROM (same), the VIC and the color RAM. I guess some stuck bits in the color RAM could cause this kind of behaviour.

If you do not have any spare RAM chips, then you could try swapping some of them around to see if the error changes. I can't remember which specific chip holds the space for color RAM, but I'm sure someone else could tell you (or you could derive it from the schematic).

I also fully understand the urge to fix a broken machine. I find it boring to just buy a working machine. I spend most of my time fixing stuff and when I am done with a machine, it usually doesn't see much use as I move on to the next machine that needs fixing. Actually using them is often less fun than figuring out how to fix it..

Actually, I now wonder about the BASIC ROM as well, thanks to teh link rootboy pointed me back to:
UE11 901486-01 ROM BASIC
Startup screen with borders but no characters. Disk commands do not
work, but game carts still work.

I found that as a by product after thinking about writing a ROM. I was looking for replacement EEPROMS, and I found this:

http://hackjunk.com/2016/05/07/vic-20-replace-basic-and-kernal-roms-with-27128-eprom/

Realizing they are on the same pins for the most part I started looking at continuity between them,all but pin 20 have continuity. (CE) Which is correct according to the schematic. Then I checked the signals at the scope. and on the BASIC ROM (UE11) I found it to be constant high-- which is odd, I would expect it to be toggling like the Kernal ROM's pin 20. Both seem to feed of one of the 74138's I replaced, UC5, the BASICROM specifically off pin 9 of this chip. I'm not certain yet if it is supposed to be enabled High or low, but always High seems suspect.

It's possible I suppose it's stuck booting. I'll check more tomorrow, but I don't see anything on the schematics (which are pretty terrible to read) that could have failed pulling it high constantly.

Either if you able to confirm a waveform on pin 20 of UE11?

 

[rootboy]

Sorry, I don't have a Vic-20, otherwise I would. But nice find on the hackjunk site, making your own ROM sounds very doable.

 

[demolition]

Well, I have a VIC but no oscilloscope around unfortunately..

Regarding a new ROM, I think it should be quite simple to just replace the ROM with a 2732 EPROM. AFAIR you just need to swap two pins. I replaced the kernel ROM in my VIC with a JiffyDOS version programmed in a 2732 that way.

 

[Bryce]

My bet is that the character ROM has failed. Try probing a data pin of that ROM with one channel and the /CS pin with the second channel of your scope. Trigger on the /CS pin going low to see if the data pins are outputting anything when they should.

Bryce.

 

[demolition]

I guess if you have any other 24 pin ROM around (doesn't matter what it contains as long as it contains something), you could put that in place of the character ROM. Obviously a BASIC or KERNEL ROM would translate into some weird characters, but at least it should not be invisible any longer. If you see a change, then it is very likely that you just need to find a good character ROM.

 

[Bryce]

Cool idea.

Bryce.

 

[CrashSerious]

Thanks! Knowing that chip works, lead me to its data sheet. (http://ee.hawaii.edu/~sasaki/EE260/Labs/Datasheets/m2732a.pdf) I'll read through that and try to determine if CE (E) needs to be high. (I think data is only available when it's low from the "read" section, but need to digest it more later) I was hoping for an EEPROM version so I didn't need a UV eraser, but Those appear to be cheap if I screw one up.

Well, I have a VIC but no oscilloscope around unfortunately..

Regarding a new ROM, I think it should be quite simple to just replace the ROM with a 2732 EPROM. AFAIR you just need to swap two pins. I replaced the kernel ROM in my VIC with a JiffyDOS version programmed in a 2732 that way.

 

[CrashSerious]

The good news and bad news....

The good news is I have a fully working VIC20 now. The bad news, is it's one I bought ($60 for a beautiful no yellowing machine with a datasette and 3 games... couldn't pass it up) and to bring closure to this saga. I was able to chip swap and found that the VIC chip in my non working one in fact DOES exhibit the behavior in the working machine. So it was the VIC chip and teh one I bought off AliExpress to replace it had the exact same issue! Now to just find one here or on ebay to get it running.

The bright side is, I now have a slew of extra chips; all the logic chips (x10 or x20), a working character ROM, and a working BASIC ROM. Plus I actually purchased a EPROM Programmer and some EPROMS to play with. So, in the end it's a win.

SO--- for future searchers... if you see this... it's the VIC Chip.

 

[rootboy]

Excellent!

As for the extra parts, this is how the sickness begins...