PET 2001-8 BS, 320008

[jucole]

So I got a PET the other year from Ebay (the PET was our first family computer); it was sold as faulty and so I decided to have a go at repairing it. I did a few things to it and then just ended up getting a RAM/ROM replacement (ROMulan v4.1) to get it up and running. I didn't really do much of testing at all of the main circuit - i just took all the chips out tested the logic voltage rails for 5v and put all the chips back in.

I fixed up some cassette stuff (see pics) and left the ROMulan in, as it was up and running. But I'd really like to fix it properly, hence joining this forum and this post.

As I recall the ROMulan diagnostic said it 2 bad ROMs; I remember swapping out each of the RAMs into the video RAM slots and the screen looked like similar garbage each time - so I thought the RAMs were possibly ok.

The PET works when I had PET RAM and PET ROMs switched out.

The PET also works when I had RAMs switched out and only the 2 bad PET ROMs switched out.

But the PET wouldn't work when the I tried to use the PET RAM and bad PET ROMs switched out.


here are some various pics of the PET so far.


So i will continue my repair quest to try and find out why the PET RAM doesn't work and report here and maybe get some replacement ROMs to put in.


Watch this space... lol

 

[Nivag Swerdna]

I would try divide and conquer... let's consider ROMs first...

We can come back to PETTESTER but for now try running VOSSI... that is Bank 1 i.e. 0001, ensure that F ROM is ROMulated F Jumper ON, 9AB, C, D and E all OFF.

What do you see?

PS
Having a complete set of working original 6550 RAM would be a minor miracle.

 

[daver2]

Ignore my post.

Go with Nivag’s recommendation first.

Dave

 

[jucole]

Hi Nivag, sry for the delay; H3 remains constant but H5 checksum will keep changing every few seconds.

 

[jucole]

Basically H3 and H5 are suspected dead! unless they are crappy sockets? or something else.

 

[AndyG]

Hi all, been a while away for the forum … too many Teams meetings during the day to want to stare at a screen in the evening (and got distracted with an Apple II) .

it is not surprising that you have found some of the roms are faulty.… this version of board is notorious for failing 6550 and 6540 IC’s…. If they haven’t failed now they will likely do so in the future. I would wager therefore that some of the ram is faulty too. Finding replacement 6540 roms are nearly impossible unless you have a second board. From my experience pop out the roms and ram chips and run off the adapter board. You can put the roms and ram chips back in for display purposes. If you want to use the remaining good roms, then Retro innovations do supply eprom adapter boards but it depends on what you want to do with it. I think Gavin also produces replacement rom adapter boards. You can find replacement 6550 ram chips but they are expensive and as I found out still fail days after testing good (May have a few in the draw but I sold my Chiclet PETs to thin down my collection).

 

[daver2]

I wondered what had happened to you recently Andy. Yes, I am in the same boat sometimes!

Don’t forget you can purchase adapter boards to be able use modern(ish) devices.

It all depends on what you want to achieve of course. If you want to ‘get it going’ for the interest and learning, then look at adapter boards. Once you have got it working, you can then decide to what to do with it in the future (either leave it with the adapters, or put the original parts back and use the RAM/ROM replacement card).

Dave

 

[Nivag Swerdna]

So .. swap the ROMs into sockets that appear to currently have good ROMs and re-run the test
You need to put the F ROMs into non-F sockets as these are currently replaced by the ROMulan RAMulator.

Also... Whilst we are at it... Disable RAM replacement

PS
When swapping ROMs try and keep x000 ROMs in x000 sockets and x800 ROMs in x800 sockets as the CS lines are significant. I am currently eating cake and drinking coffee in a Garden Centre (recommended) so cannot easily check the schematics. Theoretically we should be able to test all your ROMs by cycling them through H1 or H2 (if I remember that correctly), H6 and H7 are currently replaced. It would be good to verify the H7 ROM as we will need that for PETTESTER later.

 

[daver2]

I am drinking coffee too. But my wife is not permitting me cake!

Enjoy yours!

Dave

 

[AndyG]

Enjoy …. Might try something Stronger soon

Andy

 

[AndyG]

On the subject of this Commodore PET, …. After sorting out the rom/ram issue I would consider replacing the belt on the datasette … these can be found on eBay… the old belts just deteriorate with age and are simple to replace.

 

[Nivag Swerdna]

On the subject of this Commodore PET, …. After sorting out the rom/ram issue I would consider replacing the belt on the datasette … these can be found on eBay… the old belts just deteriorate with age and are simple to replace.

I haven't actually got around to the cassette drive on my PETunia yet... please could you post the sizes and I will order one for when I get around to it! Thx

 

[AndyG]

Strap for Commodore PET 2001 Series Professional Computer Datassette Drive Belt | eBay

Find many great new & used options and get the best deals for Strap for Commodore PET 2001 Series Professional Computer Datassette Drive Belt at the best online prices at eBay! Free delivery for many products.

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Hi Gavin try this link on eBay. Just one belt is needed.

I think I have also ordered from Futurewas8bit

�

SKU	QTY
COMMODORE C2N 1530/1531

 

[Nivag Swerdna]

OK so my socket numbering from memory is wrong...

F000-F7FF is H4 and F800-FFFF is H7... in your screenshot you see H4 as $12 016 aka 6540-016 aka 901439-04... this is from the ROMulan RAMulator and is as expected. Assuming the other ROMs substitutions are OFF then we see...

...and ignoring the transient ones...

$ED 020 aka 6540-20 aka 901439-13 normally lives in H1
$9C 022 aka 6540-22 aka 901439-15 normally lives in H2
$25 023 aka 6540-23 aka 901439-16 normally lives in H6

So looks like the set you are heading towards is...

ROM 3.0 - Basic Level lII - 28 pin ROM type 6540 - Series 2001
Location ROM # Part Number
H1 6540-020 901439-13 OK
H2 6540-022 901439-15 OK
H3 6540-024 901439-17
H4 6540-025 901439-18
H5 6540-021 901439-14
H6 6540-023 901439-16 OK
H7 6540-026 901439-19
A2 6540-010 901439-08 CharGen assumed correct




There are also some words here.. https://github.com/GavinAndrews/hardware/blob/master/memory/documentation/Using_VOSSI.pdf

Keep swapping things around until you are sure you know what you have that is working or not. Many of these devices are active low so a broken trace or socket tends to turn things on and they can end up contending, so if doubt remove as much as possible and test one at a time.

PS
I do have some replacement ROMs (https://www.tindie.com/products/nivagswedna/replacement-pet-rom-6540-pre-programmed/) , normally they get rejected by the purists who only want originals! (But I like them). And I also have a replacement for 6550s (https://www.tindie.com/products/nivagswedna/pet-8k-ram-replacement-for-6550-pet-2001s/) if it comes to that..,

 

[ajgriff]

And I also have a replacement for 6550s (https://www.tindie.com/products/nivagswedna/pet-8k-ram-replacement-for-6550-pet-2001s/) if it comes to that..

When I repaired my 2001-8 I ended up with six faulty 6550 RAMs and as AndyG highlighted earlier NOS replacements are expensive as well as having a tendency to die on the shelf. For me Nivag's 8k board made good financial sense as well as ensuring longer term reliability. I've stored all 16 of the original chips inside the PET's case.

Alan

 

[Nivag Swerdna]

I just checked and although it is not immediately obvious... Set 6 on your ROMulan RAMulator is compatible with your ROMs...

So when you are done swapping ROMs you can switch to Set 6... i.e. Bank OFF ON ON OFF and then selectively Enable or Disable ROM ranges to use a mix of on- and off-board ROMs.

 

[jucole]

Many thanks for the really helpful advice!

The cassette belt had perished so i bought a belt from an ebay seller which had a similar thickness and I best guessed the length - it seems to work ok.

Using Vossi I tested the RAM; now i thought I had a decent set (I did the video ram swap method) turns out quite a few where dead! I stole some RAM from a PET donor board (which will be the next repair) and the ones on there were pretty much are all dead! (except 4, i think this board was struck by lightening!! lol); Interestingly if you get a real bad RAM Vossi and drop it in it shows all the RAM as "bad"! but it's just a single RAM IC. So potentially if you have a couple of real bad one it will give you a real headache trying to find the really bad ones! I think the best way is to rip out all the RAMs and place it in 2 at a time until you have an "OK" set, then swap out one side and if it shows good / bad you can set it asside and move on to the next one; once you have a good set of 2 you can add those in, and just repeat the process and go through the remaining chips.

I'm going to keep swapping out the ROM as described by Nivag so i know where I am with those IC's - I will report back.

I've not really decided what to do with replacements - it's just a shame this particular PETs parts are so obsoleted now.

I've seen the heatsinks they add to these chips and in one instance a case fan - although i guess the lower operating temp will extend the life a bit, I think the most sensible solution would be to store the chips and use a ROM/RAM board.


I'm going to move on now to the donor PET PCB board and look to fix that; I can test the ROM set in the decent board with VOSSI; this #2 board is the same type of board but I think it has the BASIC 1 ROM set. This board will be fun as someone has put a new CPU socket in and it has 2 bodge wires on the back and i think some of the socket pins are loose or badly soldered, so perhaps this board might prove and interesting repair!

 

[Nivag Swerdna]

When you get to RAM testing I recommend using PETTESTER as it has an extensive MARCH RAM test which should give you confidence in the chips you settle on. The 6550s fail in interesting ways... including being enabled when they should be quiet causing much confusion!

PS
http://www.zimmers.net/anonftp/pub/cbm/firmware/README.txt gives the ROM numbers for the various sets in their two forms e.g. 6540-nn and 9014nn-mm which might reduce confusion

 

[Nivag Swerdna]

If you have deep pockets you can still buy 6550s....

MPS6550 Commodore PET 2001 RAM - CASE: DIP22 - MPS -6550 | eBay

Find many great new & used options and get the best deals for MPS6550 Commodore PET 2001 RAM - CASE: DIP22 - MPS -6550 at the best online prices at eBay! Free delivery for many products.

www.ebay.co.uk

 

[jucole]

I marked all the good / bad ROMs. Yeah those new old stock 6550s are cool but too expensive for my pocket! I'll have a think about the other RAM options.

"When you get to RAM testing I recommend using PETTESTER as it has an extensive MARCH RAM test which should give you confidence in the chips you settle on"
OK. i will do that next.