Restoring a 2001 chiclet

[Chandler1]

Hi. First post...
I've just bought a project 2001.
Fired it up, and there are several problems

0. Wobbly fuse holder. Jammed it up for now but will replace
1. Only 2356 bytes(or thereabouts) available
2. Vertical lines on monitor. Constant lines, not each character
3. It freezes after 30 seconds to a minute-no keyboard input accepted.
4. Sticky keyboard

It's going to be another week before i can play with it, but meanwhile, all suggestions gratefully accepted!!

 

[crock]

First (and easiest) place to start is to very carefully lift and re-seat all the socketed chips. On such old machines, oxidation forms on the pins and sockets. This could explain items 1 (the ram chips), 2 (character rom) and 3.

The chiclet keyboards are easy to disassemble and clean.

Rob

 

[Chandler1]

Ok. I've wired up a new fuse holder, and it no longer freezes

I lifted the ram chips and reseated. No change. 2385 bytes. I'll try again next week.
I reseated the character rom, and the screen cleared of the vertical lines.

Now the whole screen jumps say 4 times every 10 seconds.

Still, I'm happy with progress so far!
Thanks to Rob for the tips.
More later.....

 

[crock]

Can you explain 'jumps' ? Can you hear an audible click from the monitor? If so, the video board might need attention. I had a video board on a 2001 that did this and it turned out that the years of accumulated dust on the board was enough to cause a spark to track from the flyback transformer to a mounting screw. A good clean sorted it out. It could also be dried out caps which again, are not too tricky to replace. Just be careful when working on the monitor as it can give you a nasty kick.

WRT to the RAM, the PET kernal moves up through the address space until it finds a location that doesn't test good and sets the free memory accordingly. 2385 + 1025 bytes for zero page, stack and work area puts the failure at byte 3410. Which board do you have and which RAM chips? If it's a static RAM board, you can try swapping the respective RAMS around to identify the culprit. If you put a bad RAM in the socket for the ZP address range it will probably not boot at all. Try swapping the pair at I4/J4 with the pair at I5/J5 and report back.

Rob

 

[Maverick1978]

Nice that yours is counting ram! I think my character rom is bad - just displays the same character pattern. Reseating chips didn't help, though I can't help but wonder... would it be safe to pull the chips, spray them and the sockets with contact cleaner, then try again? Maybe the oxidation is enough to ensure that they won't work? It's been stored in a shed for the 6 months I've had it, but who knows how/where it was stored for the previous 25-30 years prior to my getting it.

Note that I haven't had or taken the time to start troubleshooting mine hard-core, but the contact cleaner thing just occurred to me, and I can't help but wonder if it would be that simple....

 

[dabone]

I would VERY GENTLY clean the legs if they are oxidized. I use a pencil eraser and brace the chip legs against the corner of my desk so that they are supported during the cleaning.

What kind of motherboard do you have?

later,
dabone

 

[crock]

Contact cleaner is OK but a bit messy as some leave a residue. I sometimes use it for chip sockets which are not easy to clean.

 

[Maverick1978]

What kind of motherboard do you have?

I took a look yesterday and didn't see any defining ID's on the board so far as p/n or revision. What should I look for? Sticker on the back says Model PET 2001-8, SN 28517. My chassis has the spot for the tape player and membrane keyboard, however it's got what looks to be a 3rd-party mechanical keyboard that rests on top of the chassis (keyboard has a plastic tab on one edge that fits into the chassis cut-outs to hold the keyboard in place).

I've also an 8032 PET with a full keyboard. Any chance that would be compatible? - my 8032 is missing the fuse assembly and the power supply has been completely disconnected from the unit. Working condition on it is unknown. I always wanted a PET, but they're rather large... I'm not sure how long I want to keep these - but I'd definitely like to get them restored before making the decision to offload them.

 

[crock]

I took a look yesterday and didn't see any defining ID's on the board so far as p/n or revision. What should I look for?

A six digit part number, something like 320008. The 2001's had at least 5 different motherboards depending on what types of ROM and RAM they were for. Failing that, you can infer it from the RAMS and ROMS themselves. Even easier, just take a decent quality picturer of the board and post a link.

I've also an 8032 PET with a full keyboard. Any chance that would be compatible?

From memory, they keyboards are electrically compatible but if you have the 'business' keyboard on the 8032 (with numbers instead of symbols on the top row), the mapping will be a bit wonky.

Rob

 

[Chandler1]

Can you explain 'jumps' ? Can you hear an audible click from the monitor? If so, the video board might need attention. I had a video board on a 2001 that did this and it turned out that the years of accumulated dust on the board was enough to cause a spark to track from the flyback transformer to a mounting screw. A good clean sorted it out. It could also be dried out caps which again, are not too tricky to replace. Just be careful when working on the monitor as it can give you a nasty kick.

WRT to the RAM, the PET kernal moves up through the address space until it finds a location that doesn't test good and sets the free memory accordingly. 2385 + 1025 bytes for zero page, stack and work area puts the failure at byte 3410. Which board do you have and which RAM chips? If it's a static RAM board, you can try swapping the respective RAMS around to identify the culprit. If you put a bad RAM in the socket for the ZP address range it will probably not boot at all. Try swapping the pair at I4/J4 with the pair at I5/J5 and report back.

Rob

I swapped i4 with j4, and i5 with j5
Still 2385 bytes

I think I've seen a program that tests each memory area, then reports which is failing. I'll try that some time next week. I have spare memory once I've found the broken chip, any suggestion which chip now I've performed the swap? Many thanks!


Meanwhile, wrt the screen, the whole display "twitches", in that the whole display moves a fraction of an inch, then returns to where it was. The characters are all fine. I couldn't hear any noise.

With an old tv, I'd normally tap it to make it stop, but I don't want to risk bashing my pet!

 

[MikeS]

Well, it was admittedly a little ambiguous, but I think what was meant was to swap the pair of chips in column 4 with the pair in column 5; exchanging row I with row J won't tell us anything.

Actually, I'd take the pair in column 4 and swap them with the pair in column 8.

Or better yet, swap the two chips in column 4, one at a time, with one of the two video RAM chips near the back. One (or both) will probably put some garbage on the screen; that will tell you which chip, not only which pair.

The monitor could be lots of things, possibly a bad cap; check the connections at both ends of the video cable first. Gently tapping the various components on the monitor PCB with a long insulated stick could indeed find a poor connection.

 

[crock]

Well, it was admittedly a little ambiguous, but I think what was meant was to swap the pair of chips in column 4 with the pair in column 5; exchanging row I with row J won't tell us anything.

Yes, that was my intent.

Actually, I'd take the pair in column 4 and swap them with the pair in column 8.

I intentionally did not suggest this as it was not clear which board the OP had. If it was a 2114 static board then column 8 has the rams for the zero page, meaning it would likely not boot at all if you put duff ones there.

Or better yet, swap the two chips in column 4, one at a time, with one of the two video RAM chips near the back. One (or both) will probably put some garbage on the screen; that will tell you which chip, not only which pair.

But that is much better idea...

The monitor could be lots of things, possibly a bad cap; check the connections at both ends of the video cable first. Gently tapping the various components on the monitor PCB with a long insulated stick could indeed find a poor connection.

I would think bad caps is a good bet. It's about 2 hours work to dismantle the monitor and replace them.

 

[MikeS]

...If it was a 2114 static board then column 8 has the rams for the zero page...

By golly, you're right! I've never seen one of the 2114 boards but that they'd reverse the locations never would have occurred to me; thanks!

 

[Chandler1]

http://www.commodore.ca/manuals/PET_RAM_test.htm

I think I'll do this.
Lets me test my system out and I'll only have to pry out one bad chip (I hoped just one)!

I'll need to find a cassette first, so I can save the program on to it....if my tape deck works...

 

[Chandler1]

Ok.
I've swapped i4 with i5, and j4 with j5 an I've now got 3409 bytes free! Progress!!
Oh, and the board is 320008

So, do I now know that a chip in I5 or J5 is bad?
This is exciting! ( sheltered life?) lol!
Thanks for the help. Shall I try more shuffling, which pairs...
I need to do the spot killer mod too-all in time.

 

[Chandler1]

Oh Heck.

SO while I typed up the post above, I left the machine on. it froze up.

I switched off, then back on again and the screen is full of characters. I'd say random, but they are the same every time (2nd and 3rd letters are HU)

Also, the memory ram and basic rom chips all get hot (i'd say row J first), as does the mainboard power connector, and the cassette deck hums like the motor is on, but the spindles aren't moving.

Have I killed my pet?

Can I remove memory Ram (6550?)chips all together, and just put in a pair at a time? I have tried reseating the memory ram, the video ram and the basic rom-no change, but I'm a little nervous now of removing these 35 year old chips repeatedly...

Any ideas?

 

[Chandler1]

Is the garbage screen due to bad video ram? I tried changing one video chip with a memory one, which changed the characters but didnt fix it.
Shall I keep trying memory ram until the screen clears?

 

[Chandler1]

I just tried pulling most of the memory ram out of the board (leaving 4 in place in I/J1/2) and its still the same, then I tried putting one of the ram's Id pulled in the Video ram, and the screen totally blanked. So it looks like at least that memory ram is dead. Should I do this for each of the memory rams I have to test them?

I assume a dead ram would give a blank screen, and a working ram would give at least the screenfull of PETSCII characters.

THis is so frustrating. I had a working pet, with low memory, and just by swapping memory ram its died! I'd left it running for 30 minutes, and typed and ran a simple program, so it all looked good except for the memory-probably just one chip dead, and now its toast!!

If I can't find the issue (which may be memory ram or basic rom or or or or or...) then would the PetVet work?

What else needs to function for the PetVet to run the pet (i know I can take out all the rom and memory ram, do I need two working VIdeo rams?)

Thanks to all.

 

[crock]

I feel your frustration, it's something I have experienced many times. It is not uncommon to turn on an old machine and find that it works, only to then find it dies a few minutes/hours/days later. All hardware is slowly decaying, even when switched off. Heat and oxygen slowly act to breakdown insulation, dry out capacitors, oxidise contacts etc. What often happens is that the heat and stress of being powered up after many years is just enough to push some components over the edge.

It is of course possible that it was the process of swapping components that did something but I think it's unlikely if you were careful. After all, it was working immediatly afterwards and it only stopped after 30 minutes. This suggests that something else has gone over the edge.

Now you have clarified that you have a 320008 board, that tells me that you have 6550 RAMS. The original test of swapping I&J4 with I&J5 and getting more memory tells us that one of the chips in column 5 is duff. If it still worked I would have swapped them individually back again or swap them with the display RAM to find the exact culprit. It's critical to have good RAMS in the leftmost sockets as they hold the zero page and stack. Having the othes missing I think would at least allow the machone to boot. To answer your earlier question, yes the ROMS and RAMS do run quite hot.

I assume a dead ram would give a blank screen, and a working ram would give at least the screenfull of PETSCII characters.

No. A blank screen is one full of spaces (0x20) so it needs good RAM. If the rams are present, but uninitialised you get garbage, as it renders whatever values the static rams have (maybe someone else here knows what a 6550 typically has on initialisation?) There is many causes of a garbage screen of which RAM, ROM and the CPU itself are just three.

Did you re-check the voltage rails from the regulators? Check all 4, the voltages should be no less than 4.9V and no more than 5.1V. If these check out good, I would reseat all the chips again. Just heating up after many years could have made them shift in their sockets a fraction.

Rob

 

[Chandler1]

Now you have clarified that you have a 320008 board, that tells me that you have 6550 RAMS. The original test of swapping I&J4 with I&J5 and getting more memory tells us that one of the chips in column 5 is duff. If it still worked I would have swapped them individually back again or swap them with the display RAM to find the exact culprit. It's critical to have good RAMS in the leftmost sockets as they hold the zero page and stack. Having the othes missing I think would at least allow the machone to boot. To answer your earlier question, yes the ROMS and RAMS do run quite hot.

No. A blank screen is one full of spaces (0x20) so it needs good RAM. If the rams are present, but uninitialised you get garbage, as it renders whatever values the static rams have (maybe someone else here knows what a 6550 typically has on initialisation?) There is many causes of a garbage screen of which RAM, ROM and the CPU itself are just three.

Did you re-check the voltage rails from the regulators? Check all 4, the voltages should be no less than 4.9V and no more than 5.1V. If these check out good, I would reseat all the chips again. Just heating up after many years could have made them shift in their sockets a fraction.

Rob.


Thanks Rob-Do you mean RIGHTmost sockets? (I1 and J1)? I thought I'd read that somewhere else.
I'll check the rails later, maybe later tonight or this week..., do you know off the top of your head which legs that is? I'm old and forget such things...

oh yes, and when it froze up, I think the cassette motor started up too, that is, it worked fine, then froze, then the motor started, then I switched it off. Does that suggest the PIA chip (facing towards the tape deck) needs attention?