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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old 14th Aug 2023, 12:41 am   #641
SiriusHardware
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Default Re: Commodore PET 4032/8050/3022

New video chip running warmer, well, it is after all a different device but I suppose the main thing is that it is doing some actual work which the old one clearly wasn't.

I'm not sorry that we don't appear to have a RAM fault after all, it's about time this machine gave you a bit of a break.

You're getting along really well with this one, almost on the home straight now. This was IMO a much more difficult one to crack than the 3016 mainly due to the complexity of the video generation circuit, not to mention a wonky monitor thrown in as well. The various test tools you have, especially the NOP tester, the Tynemouth board and the Hantek PC scope have more than earned their keep on this case.

Congratulations on everything you've achieved so far.
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Old 14th Aug 2023, 2:41 am   #642
Mark1960
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Default Re: Commodore PET 4032/8050/3022

Quote:
Originally Posted by ortek_service View Post
(I once found a Z80A CPU IC that only worked for about 30s, but did work for longer if sprayed with freezer spray - Well butane lighter refill fluid, as that was all I had to hand! - to prove it was at fault).
That should probably have a warning “Don’t try this at home”.
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Old 14th Aug 2023, 10:50 am   #643
ScottishColin
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Default Re: Commodore PET 4032/8050/3022

Thank you. Once we got past the CRTC issue then it all got a lot easier. And those white sockets were not so good on this motherboard - I see that mentioned by other people; interestingly I only had to replace one of them on the 3016.

Having said that, the keyboard was a lot easier on this PET, so swings and roundabouts.

I'll let you know how I get on with the IEEE port. Then I need to put the 3016 back together with all the scavenged chips I have used that are still in the 4032. I bet I get a fault there when I put it back together.

Thanks as ever for all your help.

Colin.


Quote:
Originally Posted by SiriusHardware View Post
New video chip running warmer, well, it is after all a different device but I suppose the main thing is that it is doing some actual work which the old one clearly wasn't.

I'm not sorry that we don't appear to have a RAM fault after all, it's about time this machine gave you a bit of a break.

You're getting along really well with this one, almost on the home straight now. This was IMO a much more difficult one to crack than the 3016 mainly due to the complexity of the video generation circuit, not to mention a wonky monitor thrown in as well. The various test tools you have, especially the NOP tester, the Tynemouth board and the Hantek PC scope have more than earned their keep on this case.

Congratulations on everything you've achieved so far.
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Old 14th Aug 2023, 10:53 am   #644
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Default Re: Commodore PET 4032/8050/3022

I'm thinking of leaving the 40-pin chips and the ROM chips in sockets and put all the other logic chips back directly onto the motherboard.

Couple of questions:

1. Do the chips that I'm going to solder directly back into the motherboard need any kind of shim popped under them temporarily as I solder them back in - what I mean is that sometimes I see chips not directly flat on the motherboard but there's a small gap; is this a good idea?

2. It appears that Commodore snipped the legs of chips they soldered directly into the motherboard as they seem shorter to me. Was there a good reason for that?

Thanks.

Colin.
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Old 14th Aug 2023, 11:28 am   #645
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Default Re: Commodore PET 4032/8050/3022

The pins of ICs soldered directly to the PCB are virtually always cropped, I think because if not then there is the potential for them to get folded over and touch each other.

Lifting the chips up a little so the wide parts of the legs aren't resting on the top side pads makes them much easier to desolder again, but it takes some patience to get them sitting exactly level and all at the same height, so the way to do it if you want to is exactly as you say, slip some kind of spacer underneath the IC, solder the pins and pull it out afterwards.

If the machine was mine I would leave all of the ICs already socketed in sockets, that is partly me as a repair engineer always preferring an easy life, but it also gives you the option to quickly check suspect ICs, especially those notorious 74LS244s, in your other PET if the machine ever breaks down again. If you think about it, you have already made frequent use of this facility to 'borrow' ICs from the 3016 for diagnostic purposes during this repair thread, and that was only possible because you left those ICs socketed.

You could theoretically put the 4 x ROMs which weren't socketed directly back onto the PCB, because the Daver2 test code checksums them which is quite a good in-circuit test for them.

If you have to replace the 3446 buffers I would also leave those permanently socketed because you actually do use the IEEE interface, the 3446 buffers are clearly a major weak point and it would be useful to be able to interchange them with the ones in the other PET for diagnostic purposes.
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Old 14th Aug 2023, 11:43 am   #646
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Default Re: Commodore PET 4032/8050/3022

Food for thought. Thank you.

I left it running memory tests this morning for four hours without failure so it currently looks like the RAM chips are all good which is a bonus.

I re-ran the IEEE test program and got the same results (i.e. every bit failing and other errors) so I'll start on socketing the 3446s when the postie delivers them.

Then I need to put in some of those posts for the resistors on the CRT motherboard (they seem to be known as Turret Terminals). Then properly clean the upper case and attach a new sticker.

Then strip the lower black case, sort out the rust and respray.

Then I think that's it for this one - I'll write it all up and put a write-up on my Wordpress page with photos.

Thanks everyone.

I'll start a new thread for my printer when I make a start on that one (CBM 3022 dot-matrix printer).

Colin.

Quote:
Originally Posted by SiriusHardware View Post
The pins of ICs soldered directly to the
PCB are virtually always cropped, I think because if not then there is the potential for them to get folded over and touch each other.

Lifting the chips up a little so the wide parts of the legs aren't resting on the top side pads makes them much easier to desolder again, but it takes some patience to get them sitting exactly level and all at the same height, so the way to do it if you want to is exactly as you say, slip some kind of spacer underneath the IC, solder the pins and pull it out afterwards.

If the machine was mine I would leave all of the ICs already socketed in sockets, that is partly me as a repair engineer always preferring an easy life, but it also gives you the option to quickly check suspect ICs, especially those notorious 74LS244s, in your other PET if the machine ever breaks down again. If you think about it, you have already made frequent use of this facility to 'borrow' ICs from the 3016 for diagnostic purposes during this repair thread, and that was only possible because you left those ICs socketed.

You could theoretically put the 4 x ROMs which weren't socketed directly back onto the PCB, because the Daver2 test code checksums them which is quite a good in-circuit test for them.

If you have to replace the 3446 buffers I would also leave those permanently socketed because you actually do use the IEEE interface, the 3446 buffers are clearly a major weak point and it would be useful to be able to interchange them with the ones in the other PET for diagnostic purposes.
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Old 14th Aug 2023, 11:58 am   #647
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Default Re: Commodore PET 4032/8050/3022

How sure are you of the replacement 6520 which you have in the UB16 position at the moment? The usual failure mode for the 3446 buffers is that two or three bits fail, typically in two different 3446 buffers.

A fault on every single bit of the IEEE port lines seems much more likely to be down to failure of UB16, and if I didn't already know it was one of the first devices you changed I would be telling you to look at that first, but I do know it was one of the first devices you replaced so I haven't really pressed this point so far.

You could try a simple check like poking specific values like 0 and 255 to the output address of the IEEE port and seeing if you see the expected states on the PORTB pins of UB16.
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Old 14th Aug 2023, 12:48 pm   #648
ScottishColin
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Default Re: Commodore PET 4032/8050/3022

Both of the 6520s and the 6522 are currently from the working 3016, so I'm reasonably confident.

But just in case, I swapped UB16 and UB12 - the keyboard and cassette player still worked and more importantly I still get the same errors with the test program.

So I think that's the 6520s ruled out, but it could still be the UB16 socket.

I will socket the 3446s and try again (one at a time using the known working 3446s from the 3016).

Colin.


Quote:
Originally Posted by SiriusHardware View Post
How sure are you of the replacement 6520 which you have in the UB16 position at the moment? The usual failure mode for the 3446 buffers is that two or three bits fail, typically in two different 3446 buffers.

A fault on every single bit of the IEEE port lines seems much more likely to be down to failure of UB16, and if I didn't already know it was one of the first devices you changed I would be telling you to look at that first, but I do know it was one of the first devices you replaced so I haven't really pressed this point so far.

You could try a simple check like poking specific values like 0 and 255 to the output address of the IEEE port and seeing if you see the expected states on the PORTB pins of UB16.
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Old 14th Aug 2023, 1:11 pm   #649
SiriusHardware
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Default Re: Commodore PET 4032/8050/3022

Just to humour me, try these tests:

1)

Code:
POKE 59426,0
With a meter, measure the voltages on pins 10, 11, 12, 13, 14, 15, 16, 17 of UB16.

2)

Code:
POKE 59426,255
Again with a meter, measure the voltages on pins 10, 11, 12, 13, 14, 15, 16, 17 of UB16

I'm not doubting your confidence in the UB16 chip itself, but I am wondering if the system is unable to communicate with it due to a fault on the control or bus signals going to it.
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Old 14th Aug 2023, 3:33 pm   #650
ScottishColin
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Default Re: Commodore PET 4032/8050/3022

After POKE 59426,0, all pins are 0.07V

After POKE 59426,255, all pins are 3.8V.

Colin.


Quote:
Originally Posted by SiriusHardware View Post
Just to humour me, try these tests:

1)

Code:
POKE 59426,0
With a meter, measure the voltages on pins 10, 11, 12, 13, 14, 15, 16, 17 of UB16.

2)

Code:
POKE 59426,255
Again with a meter, measure the voltages on pins 10, 11, 12, 13, 14, 15, 16, 17 of UB16

I'm not doubting your confidence in the UB16 chip itself, but I am wondering if the system is unable to communicate with it due to a fault on the control or bus signals going to it.
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Old 14th Aug 2023, 3:43 pm   #651
SiriusHardware
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Default Re: Commodore PET 4032/8050/3022

Well, that is exactly what should happen, so there's no doubt that the system is able to send data out to the chip.

With the machine running in idle mode at the BASIC prompt, what states or waveforms do you see on the UB16 R/W pin (21) and the RS0 pin (36) and RS1 pin (35)?
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Old 14th Aug 2023, 3:55 pm   #652
ScottishColin
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Default Re: Commodore PET 4032/8050/3022

See attached.

Colin.


Quote:
Originally Posted by SiriusHardware View Post
Well, that is exactly what should happen, so there's no doubt that the system is able to send data out to the chip.

With the machine running in idle mode at the BASIC prompt, what states or waveforms do you see on the UB16 R/W pin (21) and the RS0 pin (36) and RS1 pin (35)?
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Old 14th Aug 2023, 4:00 pm   #653
SiriusHardware
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Default Re: Commodore PET 4032/8050/3022

Activity as expected, so no obvious reason why the system would not be able to read from the UB16 chip. As it stands, we're back to suspecting that all three of the MC3446 buffers are fried. If so, that was quite some feat by whoever managed to do that.
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Old 14th Aug 2023, 4:53 pm   #654
ScottishColin
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Default Re: Commodore PET 4032/8050/3022

One by one. Replaced UC12 and we have an improvement with that replacement.

Onto the next one.

Colin.
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Old 14th Aug 2023, 4:57 pm   #655
ScottishColin
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Default Re: Commodore PET 4032/8050/3022

Two down, one to go.

Colin.
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Old 14th Aug 2023, 5:17 pm   #656
ScottishColin
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Default Re: Commodore PET 4032/8050/3022

Third replaced and all errors have gone and the test runs through clean.

Attached an SD drive to the IEEE port with success so that's done.

Onto the case and the label now.

The only other thing that comes to mind is that there were two ROMs in UD11 and UD12; one marked and pne not. How can I test them?

Colin.
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Old 14th Aug 2023, 6:32 pm   #657
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Default Re: Commodore PET 4032/8050/3022

Four out of four gates in each of three buffer ICs dead. Incredible.

The ROMs which came in the 'spare' sockets, read them in your EPROM programmer and look through the code, you may find some plaintext in there which reveals what it was that they did. Also note down the checksum of the code in each case. Then, go looking through the firmware section of the zimmers site to see if you can find anything which might be the same code, and if you think you find it load the zimmers version and compare the checksum with the checksum of the code in your device.

According to the ROM section of the circuit diagram UD11 and UD12 reside at addresses A000-> and 9000-> respectively.

It is possible, but by no means a given, that the executable code in each EPROM / ROM starts at the first address in the EPROM, if so you might be able to run it by the usual method which I think, but don't quote me, is

Code:
SYS decimalstartaddressofcodeinROM <enter>
Or something like that, you might have to put brackets around the decimal number, basically use whatever syntax you had to use to run that tools utility EPROM which came with your 3016 when it still had BASIC 2 in it.

In this case to attempt to run the code in the A000 ROM you could try

Code:
SYS 40960 <enter>
And for the 9000 ROM you could try

Code:
SYS 36864 <enter>
I would do this with nothing else (no disc drives) attached to the computer because there is no guarantee that the code in either EPROM does start at the first address in the EPROM, so it is as likely to crash as it is to work, maybe more so.
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Old 14th Aug 2023, 6:42 pm   #658
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Default Re: Commodore PET 4032/8050/3022

Do I remember that one EPROM was labelled and the other was not? It is possible that they both contain a single utility or app which is so big that it occupies two EPROMs, so make sure they go back in the original slots you found them in.
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Old 14th Aug 2023, 7:31 pm   #659
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Default Re: Commodore PET 4032/8050/3022

UD11 had a label but the writing was faded off

UD12 was marked Superchip which is referenced here a few times:

http://www.zimmers.net/anonftp/pub/c...her/index.html

I've looked through a copy taken from the STAG but can't see any obvious text either in the file or in a Hex translation.

I'll pop them in and try.

Colin.
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Old 14th Aug 2023, 7:57 pm   #660
ScottishColin
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Default Re: Commodore PET 4032/8050/3022

Ah well. If I put the Superchip EPROM in, the PET won't boot at all - I just get a single left-arrow on the screen.

The PET boots with the unmarked ROM on its own, but I can't get it to do anything.

I'll take a memory dump of each EPROM and pop them on here to see if anyone can make anything of them but having got the PET working, I think I'll put them to one side now.

Colin.
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