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Commodore PET restoration help needed

S

SkyCaptain

Experienced Member
Joined
Apr 18, 2020
Messages
85
Location
Basel-Stadt, Switzerland
Hello good people of the Vintage Computer Forums,

I bought my first non working Commodore PET and am a bit at a loss regarding the lack of resources i can find online.

I have a pretty clear issue but no amount of googling has given me any useful results.

When I turn on my Machine, all i see is a "#" symbol on the top left corner. What could be the cause for this.

One real problem with this machine is that i see at least 3 custom modifications (reset button for example), so its going to be even harder for me to actually trouble shoot.

If anyone has an idea of where to start the trouble shooting, i would highly appreciate it!

I will provide pictures later but i need a camera that actually works for these kind of pictures. my shitty smart phone camera just doesnt work.

Thanks a lot!!
 
What model of PET do you have?

You can find schematics and other resources on Zimmers here http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/index.html

A single # in the corner is unusual. The rest of the screen is blank?
One of the first things the startup process does is clear the screen of the random values in video ram, so it seems the startup process is getting that far at least.
If the video ram was bad (which is common) you would normally see all of the same character repeated over the whole screen. If the character rom was bad, the # character wouldn't look correct or the whole screen would be white or green as if a reverse space was covering the whole screen.
If the system ram was bad, I wouldn't expect the startup process to get as far as clearing the screen and you'd likely be seeing random characters over the whole screen.

If you have a cassette drive for your PET (or if one is built in), listen when you first power on for the sound of the cassette motor turning for about a second and then it should stop.
The motor is not controlled directly by the buttons on the tape drive. When you press play on the cassette drive, the computer senses that you have pressed a key and turns on the motor.
If the motor turns on and off like it should, that means the CPU is running the kernel routine that polls the sense line and turns the motor on and off.

My first guess is that you may have a bad Basic ROM. The edit ROM seems to be clearing the screen and it wouldn't get that far without the Kernel.
The # character is the first thing that is normally printed in the top-left corner for Basic v2.0 ### COMMODORE BASIC ###
So possibly the basic rom is degraded and crashing after the startup process jumps to basic.

If you can, post (or link to) some photos of the PET and the "custom modifications" you have questions about.
 
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I see the photos now. What you have there is a 2001-N.
ROM upgrade kits and option rom socket often leave one or more ROM sockets empty.
I'd lookup and double check the ROMs you have there to make sure you don't have one missing or an extra one in an option rom socket.

Yeah, that's a reset button. Looks like it's connected to the 555 power-on reset chip.
Try pressing the button and you should get a repeat of the power on sequence but with the video ram already cleared.

The CPU is definitely running the startup routines. Something is crashing it before it starts basic. Most likely a bad ROM.
 
So how many ROM slots are taken up by default? Does an upgrade mean they were all replaced or that some were added? My question more or less is if i can reverse the upgrade by taking out the added ROM chip? Like, i assume the one with the yellow sticker on it is after market. taking it out would leave 3 slots empty, and i do see pictures online of this mainboard with 3 empty slots.

Sorry, i want to be very careful here and not just do things that i dont know the effect of.
 
So how many ROM slots are taken up by default? Does an upgrade mean they were all replaced or that some were added? My question more or less is if i can reverse the upgrade by taking out the added ROM chip? Like, i assume the one with the yellow sticker on it is after market. taking it out would leave 3 slots empty, and i do see pictures online of this mainboard with 3 empty slots.

Sorry, i want to be very careful here and not just do things that i dont know the effect of.
 
SkyCaptain said:
I finally have pictures to show!

But yeah, first order of business is cleaning the machine

https://imgur.com/a/dqFy3in
Click to expand...

That's a 2001N motherboard, the second revision of the CBM PETs after the 2001. It uses dynamic RAM.
Your issue is likely depending on dirty ROM socket(s) or bad ROM(s). The CPU can go as far as clearing all the video RAM, that's done by the uppermost ROM. Then it gives control to the Editor and Basic ROMs and that's where things go bad.
I would also remove those three diodes close to the 74154, since they were clearly part of a bigger mod. They can't do much in the way they're left now, but also there's no reason to leave them connected.
Do you have some test equipment and a way to read ROM/EPROMs?

Frank
 
iz8dwf said:
Do you have some test equipment and a way to read ROM/EPROMs?

Frank
Click to expand...

Hi Frank

I do not as right now but its on the list of items to buy. I was looking at one of these: https://www.ebay.ch/itm/USB-TL866II-Plus-Programmer-EPROM-EEPROM-Flash-MiniPro-BIOS-AVR-AL-PIC-SP-MCU/123937160356?_trkparms=ispr%3D1&hash=item1cdb3afca4:g:I-cAAOSwiv5doZtc&enc=AQAEAAACcIQvEcHUrT7nmUC3yY5qbPyaBN1nJEDYW8MyypsJPgXKsfKNAdshQDYpIhXQjoOgo8Zlb%2BTkru%2FwX%2Fv6xrSNurJXVi8L1duuxaV0EKzhKgVZFDjb5KaXJgCEi2iDI9cEzP2nFAm9XptB9Yslrc6uD916y4G9pwa9ag6QSe0%2BsICP0QHdfZK15W66FKqHAZQHh%2FrEO5hRl96aFr%2FiKR4GMjGR1vMVYEgvjPb3bI7Uz3V0KTRsEMF%2FI4xX6t524AlOQqwLM20xmtaZzKZm0KbL72FZd9PRLdUZzwPtsSXSNsxUdp2ujzmnwXHPzWLxMigKVivLumur9MLEo1IjUvR57IGaNHE7jhjBk2iRE7%2BZNOHcQ%2B1lvUKZSK5Y3o8v86OUKY5t%2BMrSOmal1G4MNZxQBk2chiTmVBFCfl49I7kdKE1vupPoNWQPbaPKNGZYCrGDiHFoXwD0HYjOLMa6UFf3X9siJgnYR5ROdIzBCdLuLV3ov6E3faT3wsbPmEuGFST7ObHpyhvmL1R7u8de%2F8%2BipjqtYQectMYa20DWQK3%2FRPzXKtxcY%2FvVn0McGEhlz0CCv8sw9gkJ2X6tYjNPV%2BhM1tj80fZ%2BmnDe%2FyRv5UZDeJYN0qlBL%2FabhkGTl%2FnIANb6jgyBzz4k%2FmCgMUUejQc%2Blfn4vIUxVdNsnEYxrojYh1%2FbgH6aN5A0Di1Id2kyEv1mh1OF1GveZdkJkcqqDz3nmFady0fBzQazZN0beBu4aViWWkR0Cbo7QHZV4aLjIeV3wO1zhtyt7T20VpO5DYVh8D8Q1xrkYqhObvYGc7Gyq7UFSFr2J0MEqvZIK2nkTlS1UQ%3D%3D&checksum=123937160356a01e966445f2454ab7704214ef0bdfa6 but im not sure yet if it can read an eprom too so further reading is necessary on this end too. Im gonna need a multimeter too. Im not sure about removing the mods just yet even though that was my first idea too as other people on the 6502 forum said that this was a bad idea because some of these could actually be from the factory.

Emmanuel
 
Welcome to VCFED.

You will need a multimeter and a logic probe as a minimum to perform fault finding.

To repair any faults you find will generally require soldering and desoldering skills.

The fact that the screen clears is very good news.

Gut feeling tells me that you may have a faulty ROM, or it may be something to do with the butchery around the 74151.

Yes, a list of the ROM numbers would be good.

Two further things we may need is a NOP generator (this can be made yourself from two 40 pin IC sockets and some resistors) and a copy of my PETTESTER code to burn into a 2716 EPROM to replace the EDIT ROM. This provides a series of tests to checkout the basic machine operation. Either you can burn your own EPROM or ask someone nicely to send you an EPROM and recover their costs.

Where are you in the world?!

Dave
 
Before your test equipment arrives you can try reseating the socketed ROMs by carefully lifting them from their socket and pressing them back in.

It looks like you have BASIC 2.0 with a couple add-on roms. The rom with the yellow sticker and the one with clear tape on it are obviously add-ons, I can't read what they are from the pics.
Maybe take a closer pic of the back third of the motherboard.
 
SkyCaptain said:
So how many ROM slots are taken up by default? Does an upgrade mean they were all replaced or that some were added? My question more or less is if i can reverse the upgrade by taking out the added ROM chip? Like, i assume the one with the yellow sticker on it is after market. taking it out would leave 3 slots empty, and i do see pictures online of this mainboard with 3 empty slots.

Sorry, i want to be very careful here and not just do things that i dont know the effect of.
Click to expand...
For Basic v1.0 all of the ROM sockets would be filled. The Basic 2.0 upgrade used larger capacity ROM chips and would leave three sockets empty.
Sockets D3, D4 and D5 would normally be empty from the factory.

I would remove the two extra ROMs. As Buzbard said, the rom with the yellow sticker and the one with clear tape on it are obviously add-ons. It's a good idea to remove those so you know that's not what's causing the system to crash.
Carefully lift one end of the chip just slightly with a small flat screwdriver, then the other end, back and forth until the chip comes out of the socket.
Old sockets like this can really hold on to the chip if it's been in there a long time.

901465-01 in D6 is Basic (part 1)
901465-02 in D7 is Basic (part 2)
901465-03 in D9 is the Kernel
901447-24 in D8 is the Edit ROM

The Kernel is required for CPU startup and the Edit rom is responsible for clearing the screen so it would seem those ROMs are OK. Or at least working well enough to get as far as clearing the screen.
 
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Reading ROM chips with a EPROM programmer.
Most of these mask ROMs do part of the address decoding with the pins. Some of the EPROM programmers also leave the programming voltage on all the time. Knowing this, it is possible to make reading adapters for the specific ROMs. One can look at the schematic to determine which pins will be pulled high and which will be pulled low to enable reading the ROM. I usually use a stack of three machine pin sockets to make reading adapters. I like using these as it is easy to pop out the pins you don't want to cross connect ( like the VPP especially ). In the case were you'd want to read several of these, one could make a universal reader with some resistors and dip switches. For those pins that enable being pulled high, you use a 2K to 5K pullup resistor to the +5V. For those that you wish to pull low to select, you can connect to ground by shorting that pin the ground. It is fine to short the pin side of the pullup resistor to ground as it will be only about 1 or 2 milliamperes of extra current draw.
Make up a list of switch setting for each ROM used.
Dwight
 
SkyCaptain said:
I do not as right now but its on the list of items to buy. I was looking at one of these: https://www.ebay.ch/itm/USB-TL866II-Plus-Programmer-
Click to expand...

Emmanuel,

For fixing very old computers, do not buy an EPROM Programmer unless its states that it can program the 2716 and 2732 chips and hopefully the 2532 chips. The 2532 is a pin for pin replacement, but the more common 2732 will do with a simple adapter.

For some reason I cannot see the photos you posted. Can you check? Are all 24 pin ROMs on sockets? How about the big 40 pin chips?
-dave_m
 
Hi Dave,

A multimeter i have, the logic probe is ordered, and an eprom programmer will come around next months payday. Honestly i thought to myself that if im doing this im going the whole 9 yards and will program my own eprom, including buying what ever part necessary. Dont really know how to describe it... basically, im a man on a mission.

So taking out the two ROMs that are definitely upgrades (if its BASIC 2.0(from what im understanding so far)), im left with the following 4 ROM chips:

Left to right (by how the board is in the machine) i have:
901465-03
901447
901465-02
901465-01

I have resocketed them but im afraid ive only made things worse. Now, the hash does not show up, but when i turn it off and on again, it does show a full screen of random characters, before going black

I will add more pictures to the Original Post. There is some stuff i have previously not even noticed. There are also pictures of the the Add-on ROM with the yellow sticker, it also has some rather weird features.

Also I am from Basel, Switzerland
 
Hi Dave, Yeah my posts still need to be approved so it takes a while for them to show up here. I keep posting new images to the imgur link ive posted earlier.

and thanks for the info on the EPROM programmer, ill be looking more carefully keeping this in mind.

Emmanuel
 
Hi Hutch,

I have removed the other two Chips and i have resocketed the 4 remaining chips. The ROM chip with the yellow sticker has been removed, now starting the machine doesnt show the hash symbol anymore. Turning it off and on again does flash a full screen of random characters before turning black again.

Emmanuel
 
dave_m said:
Emmanuel,

For fixing very old computers, do not buy an EPROM Programmer unless its states that it can program the 2716 and 2732 chips and hopefully the 2532 chips. The 2532 is a pin for pin replacement, but the more common 2732 will do with a simple adapter.

For some reason I cannot see the photos you posted. Can you check? Are all 24 pin ROMs on sockets? How about the big 40 pin chips?
-dave_m
Click to expand...

I can confirm that the TL866 programmer is struggling with our old PET EPROMs. It cannot provide some required voltages. I did a deep internet research when looking for a proper programmer and bought this one:

http://www.mcumall.com/comersus/store/comersus_viewItem.asp?idProduct=4282

I am able to write 2532, 2732 and 2716 EPROMs with this but not all types and brands. Following the link there is a button which links to the supported device list. Once you get a good programmer I highly recommend to write a 2716 with Daves sophisticated Petester for the Edit ROM Slot. This may lead you the way.

Best regards
Denis
 
SkyCaptain said:
Hi Hutch,

I have removed the other two Chips and i have resocketed the 4 remaining chips. The ROM chip with the yellow sticker has been removed, now starting the machine doesnt show the hash symbol anymore. Turning it off and on again does flash a full screen of random characters before turning black again.

Emmanuel
Click to expand...

It's not unheard of for things to change like this after re-seating the chips. Since things did change, then it's logical to assume that something physical may have changed when you pulled the chips out or put them back in the socket.
The old sockets are notoriously unreliable. The legs on the chips and the contacts in the socket can oxidize over time and create an unreliable connection.
Did you notice if the legs on the chips were blackened, or had a powdery white coating? Sometimes Deoxit D5 can help clean the oxidation from the sockets and the chips.

That said, it's likely that one or more of the ROM chips is simply dead. These old MOS mask ROMs can and often do just die.
I've had ROMs that were just completely dead and some that were still readable with good data in them but with random bits failed throughout which caused the CPU to crash when it eventually hits an invalid instruction.

The 6502 requires the first 512 bytes of RAM to be working or it has no temporary storage for variables and many CPU instructions won't work.
Since the CPU is getting as far as executing the instructions to clear the screen, at least the first 1K of RAM is probably fine.
The Kernel ROM holds the startup vector so it's safe to assume that at least the last two bytes in the Kernel ROM are OK but other parts of the ROM could have faults.
The edit ROM holds the screen editor and things that are different across different PET versions. Things like the keyboard matrix (graphic vs business), the screen size (40 or 80 columns), region (50/60Hz)

Signs still point to one or both Basic ROMs. Do you have a cassette drive to test the things I mentioned in my earlier post?

I would recommend considering a RAM/ROM replacement board to help with troubleshooting. It can selectively replace individual ROMs and banks of RAM to test which ones are bad.
If everything is working it can also upgrade your ROMs to 4.0 and expand your RAM to a full 32K
Link: https://www.tindie.com/products/tynemouthsw/romram-replacement-board-for-commodore-pet/
The smaller board just replaces all the ROMs but the larger board lets you select which ROMs to replace so you can isolate the fault.
 
Last edited:
Hi all,

So in regards to a EPROM programmer, ive been seeing some "good" experience with programming 2732 chips at 21 Volts instead of 25 Volts as required by running.
Thank you for the link miata, this seems like a very good deal. On my own research i came across a new product by the same chinese company who made the TL866, the XGecu T56 Universal Programmer and it states it can go up to 25 Volts so im kinda interested in buying it to find out.

So on my shopping list is now:
An Eprom programmer
A Logic Probe
ROM/RAM replacement board for Commodore PET
one or two 2716 (for the pet tester program)
a few 2732 (for replacing the ROM)

I do not have a cassette drive, although i have a external floppy drive that i know worked with a PET 3032 when i last used it 15 years ago or so. It has been in a closet since then.
 
So i have been able to measure voltages on the board and its looking good. I have the 5V, the 12V and the -5V working fine. I also gathered that the machine was a 16KB machine upgraded to 32KB.
The chips in the first memory bank are of a different make then the ones in the second bank. Can i simply remove the the second bank and run the machine on 16KB again? or is there more changes needed to the motherboard?

Thanks
 
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