Fixing broken VIC with garbage display

[Nick]

Hi,

My original VIC died *years* ago and I only recently replaced it. It worked for about 2 weeks and then POOF, it died too. Fortunately, I'm a big boy now and I can get another one without crying to mom. But I'd really like to fix the dead one.

Here is the problem: when I turn the VIC on instead of getting the ready screen, I get garbage (random colors, random characters). The border is setup fine though. The first thing I did was "re-seat" all of the socketed chips. Then I checked for bad solder connections on most chips. Then I tried plugging in extra memory thinking maybe it would move video memory to a usable area. I started thinking maybe the 6502 was fried but I tried some game carts and found that some of them work just fine. So it's not the 6502, VIC chip, Kernal ROM, page 0 RAM or color RAM that's causing the problem. I removed the BASIC rom and noticed that I get the same garbage screen so maybe it's the BASIC rom? I hope not - can't replace that chip.

I read in this forum something about "freezing" the chips to find a bad one. Can someone please explain what this means. Any other suggestions would be appreciated. I can solder but I suck at de-soldering.

Thanks!

[Leeeeee]

but I tried some game carts and found that some of them work just fine.

By some of them work I take it you mean those that don't need you to type SYSnnnnn to start them?

I removed the BASIC rom and noticed that I get the same garbage screen so maybe it's the BASIC rom? I hope not - can't replace that chip.

Yes you can, with an EPROM and another socket.

I read in this forum something about "freezing" the chips to find a bad one. Can someone please explain what this means.

It means just that, each chip in turn is frozen with (usually) a freeze spray and sometimes this temporarily 'fixes' the fault, then you know which chip it is. However this is not the 'cure all' technique some claim.

Any other suggestions would be appreciated. I can solder but I suck at de-soldering.

Well if you want to try freezing it and have a big chest freezer that is -20c or better stick the PCB in there for an hour or so then quickly connect it up after removing it. If it works and then fails you'll see the bad chip as it will be the first one that is condensation free.

Alternatively run it for a while and feel for a hot chip, and by hot I mean 'oh look that's my fingerprint burned on there' hot.

but I suck at de-soldering.

That's usually how it's done 8^)=

Lee.

[Lord Thag]

I think I can help you. My new vic did the same thing, and I spent an hour swapping IC's with another unit with a bad power switch. The two chips that were bad on my unit (which had a fine border/background but screwed up characters and graphics) were the chip under the little silver square RF shielded area in the center of the motherboard (just below the disk and monitor connectors), and the chip directly below the RF shielded area. Since I'm at work, i can't give you the exact chip names.

I replaced both of these and the system worked great. You may only have to replace one. I would start with the one south of the RF shielding.

[Nick]

Thanks for the tips. I tried the freeze trick and that didn't help any but sure did make a mess. Gotta let that VIC dry out for a while.

Since the problem occurs right away I don't think it has anything to do with heat. Some chip is fried. I just need to figure out which one. Since some carts work but Basic doesn't it must be something that Basic needs that the simple carts don't. My guess is that it's memory. Carts do use memory of course, but Cosmic Cruncher type games probably don't need very much memory. Obviously some of the memory works. The screen works just fine with the simple carts. It's too bad I don't have some kind of programmable cart. Then I could put some simple assembler code on it and do some memory diagnostics. Something to think about.

Unfortunately, I'll have to put this off for a while since I don't have a bunch of replacement parts to try. I've already ordered another VIC and *if* it works then I'm not going to mess with it. I may look around for a cheap defective VIC though. Those 2114 RAM chips cost about $2.50 each at Jameco and there are 10 of them. I *may* pull all of em, add sockets and new RAM and just cross my fingers. I suspose a fully socketed VIC board would have some extra value. Especially if this is a common problem.

Oh well, for now I just give up. I'll wait for my replacement and hope that one doesn't die as well.

[Lord Thag]

If that doesn't work let me know. I have a parts Vic that has working RAM. I'd be willing to work out a trade. I need an old two-prong power supply for a Vic I picked up, as it's supply is DOA. Let me know.

[Leeeeee]

Thanks for the tips. I tried the freeze trick and that didn't help any but sure did make a mess. Gotta let that VIC dry out for a while.

I didn't think it would but it was worth a try. Freeze spray makes a similar mess just not all at once, condensation is mostly harmless though.

Since the problem occurs right away I don't think it has anything to do with heat. Some chip is fried. I just need to figure out which one. Since some carts work but Basic doesn't it must be something that Basic needs that the simple carts don't. My guess is that it's memory.

Memory is the most likely culprit.

Those 2114 RAM chips cost about $2.50 each at Jameco and there are 10 of them. I *may* pull all of em, add sockets and new RAM and just cross my fingers.

If you are really good at de-soldering, and you have the sockets, socket all the RAM and use the colour RAM as the test socket. Any bad RAM chips will instantly show up by scrambling the colours on carts that otherwise work.

I suspose a fully socketed VIC board would have some extra value. Especially if this is a common problem.

Not really, the more unhacked a Vic is the more it seems to be worth, of course working/hacked is worth more than broken/unhacked. 8^)=

If it turns out it is the RAM and you don't want to spend $25 on 2114's and your soldering is up to it (turns out some people aren't as errr .. confident as I am with an iron) I believe you can replace the whole lot with a 6264 8K RAM chip and get a permanent 3K expansion into the bargain.

Lee.

[Leeeeee]

I need an old two-prong power supply for a Vic I picked up, as it's supply is DOA.

It's just a 9VAC 3A (27W) transformer, not hard to find.

Lee.

[Boray]

If you try typing on the keyboard, can you see any change to the screen? If that's the case then it could be the character rom.

/Anders

[Nick]

If you try typing on the keyboard, can you see any change to the screen? If that's the case then it could be the character rom.

The "garbage" display is made up of various valid characters. Most are graphic characters but some text chars as well. The colors are also random. Nothing happens when I type at the keyboard. I tried blindly making some pokes to produce a tone and that didn't work either. Just to see what would happen, I removed the Basic ROM (left the Kernal in place) and it behaved the same way.

I suspect the garbage is the result of the screen not being initialized (cleared). Simple game carts (i.e. Cosmic Cruncher, Avenger) work just fine. I figure it is either a RAM chip that plays a critical role in Basic booting up and the simple carts don't care about OR the Basic ROM itself could be bad. Since RAM is generally easier to blow than ROM I suspect it is the RAM. Unfortunately, for now I can't do anything to fix the problem. If I knew it was one RAM chip, I could easily replace the one chip. But I'm not ready to replace all of the RAM chips only to find that it doesn't fix anything. There is some risk is removing chips.

I did order two more VICs from eBay though. One supposedly works (will have to wait and see) and other I paid only $0.99 for because it has a key missing. I bought the broken one on purpose. When I get the broken one, if it turns out that it works, then I'll swap the keyboard. If it doesn't work then the first thing I'll try is swapping the Basic ROM (since it's socketed). The next step is to start swapping RAMs. It would be easier to use new RAMs than to remove than to reuse old ones. But I'd hate to buy RAMs if I don't need them.

It did occur to me that I could take a variety of single chip static RAMS and replace all 5K with one chip. I have some 32K x 8 RAM chips around. It would be messy and I'd rather not do it. But it can be done. Ideally, if I had the parts, it would really help to make a custom "ram tester" cart. That would be the most helpful in this case but I'll save that idea for some other time.

Does anyone know where to get a "blank" protoboard that has the same connector as the carts? That way I could try connecting any type EPROM as a cart.

Thanks again for all the suggestions.

[carlsson]

As you probably know, there are at least two major revisions of VIC-20, the original one with ten (?) 0.5K chips and the cost-reduced one with two 2K chips and two 0.5K chips.

[Boray]

I would guess it's either the Basic ROM or the Kernal ROM. Because cartridge games sure uses the internal RAM as well!

/Anders

[Leeeeee]

I would guess it's either the Basic ROM or the Kernal ROM. Because cartridge games sure uses the internal RAM as well!

Bad guess then as most of them don't. Most cart games use page 0 and page 1 and possibly some from page 2 and 3 but very few use the 4K from $1000 to $1FFF for anything but the display. For that to work only two of the eight (or one of the two for a CR Vic) RAM chips in that 4K need to be working.

If any one byte from $1000 to $1FFF fails during the BASIC cold start memory test then the test dead ends in an infinite loop and never gets as far as clearing the screen. So I'd be suprised if it isn't RAM or a RAM related failure.

Lee.

[Boray]

Also check Ray Carlsen's repair articles if you haven't:
http://personalpages.tds.net/~rcarlsen/cbm/vic20.txt

/Anders

[Nick]

Thanks again for everyones advice.

I received my $9.99 replacement VIC and my $0.99 "for parts only" VIC today. So now I have one original VIC with 9V AC power supply that is very clean, complete in matching BOX (same S/N) but is usless because it displays garbage; one working but dirty and yellow "cost reduced" VIC with a C64 type power adapter and RF adapter; and another working cost reduced $0.99 VIC that has a missing keycap.

I was hoping both of the VICs I received today would be the "old" type VICs with the two connector 9V AC apdapter so I could fix my "broken" VIC but that wasn't the case. I'll probably try swaping the Basic ROM later but I doubt that will fix anything. Ultimately, I'd like to fix that "original" VIC because it's the same type I had when I was a kid and I like having the matching box. BUT for now I will stick with the one that works and worry about fixing the broken one later. I'll keep the one with a missing keycap for replacement parts ($0.99 + $6.00 shipping = cheaper than a replacement 6502). Time to move on and start posting topics about the VIC that works.

****************

Before I conclude this thread I want to point out another HINT about what's wrong with the broken VIC. When I first starting using it, it seemed to work fine: Basic seemed to work ok; most carts worked ok; but some carts didn't work and I just assumed the carts were defective. Then a few weeks later Basic stopped working and only a few SIMPLE carts worked. Now that I have a working VIC, I noticed that ALL of my carts work. Thus, even though there was a problem that prevented some carts from working, Basic still worked. Eventually the problem got worse and Basic stopped working and more carts didn't work. This still suggests it's either RAM or a RAM related latch/demultiplexer chip that's fautly.

Sooner or later I hope to fix the problem and post the results. But it's interesting that even when the VIC *appeared* to work, some carts (such as VIC-Forth) didn't work, yet when tested in a 100% working VIC those carts work just fine! So the moral of the story is: if some carts suddenly stop working but Basic and simple carts still work then your VIC might be on it's death bed - NOT the carts.

(whew.... time to end this thread and start taking about working VICs)

[HermanVIC]

I had a screen full of random characters and non functional keyboard.

I swapped out the VIC chip inside the metal enclosure and got my screen back and Basic function in the minimal system 3583 bytes.

Herman