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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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20th Sep 2022, 1:43 pm | #21 | |
Tetrode
Join Date: Sep 2022
Location: Newton Abbot, Devon, UK.
Posts: 75
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Re: 1978 Commodore PET project
Quote:
Will check every IC that i removed too |
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20th Sep 2022, 2:17 pm | #22 |
Nonode
Join Date: May 2004
Location: Halifax, West Yorkshire, UK.
Posts: 2,583
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Re: 1978 Commodore PET project
I wouldn’t be in a rush to buy additional test equipment or to de-solder anything. There’s a lot that can be done before considering the need for those options. As Sirius says let’s identify the correct documentation for this board as a basic starting point. Studying these documents will make life easier for all concerned as well as increasing the chances of a successful repair. Alan
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20th Sep 2022, 2:41 pm | #23 |
Nonode
Join Date: May 2004
Location: Halifax, West Yorkshire, UK.
Posts: 2,583
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Re: 1978 Commodore PET project
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20th Sep 2022, 5:58 pm | #24 | |
Tetrode
Join Date: Sep 2022
Location: Newton Abbot, Devon, UK.
Posts: 75
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Re: 1978 Commodore PET project
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The board is 320350 REV E I need to learn what the RAM and ROM chips are so will report back |
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20th Sep 2022, 6:28 pm | #25 |
Tetrode
Join Date: Sep 2022
Location: Newton Abbot, Devon, UK.
Posts: 75
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Re: 1978 Commodore PET project
System RAM - TMS 4108-25JDL-0 (x8)
Video RAM - MPS 2114 45L 1280 (x2) MPS6520 0179 MPS 6522 1380 8237 SY6502A MC6821P 309-408 |
20th Sep 2022, 6:35 pm | #26 | |
Dekatron
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,484
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Re: 1978 Commodore PET project
Hm, that's a little bit confusing. We have diagrams for 320349 and 320351, but not for 320350. This from another forum:
Quote:
I think we'll assume your PCB corresponds to the diagram for 320351 (As AJ suggested) for the time being. Edit - just caught your update with the chip details. |
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20th Sep 2022, 6:46 pm | #27 |
Nonode
Join Date: May 2004
Location: Halifax, West Yorkshire, UK.
Posts: 2,583
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Re: 1978 Commodore PET project
I think the files I posted earlier are as close as you’ll get in terms of diagrams. Alan
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20th Sep 2022, 6:53 pm | #28 |
Dekatron
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,484
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Re: 1978 Commodore PET project
Although there is quite a lot already working on your machine, there is a standard approach to servicing, which starts with
- A visual examination to see whether there is anything obviously physically wrong (burnt or blown up components, damaged tracks, loose wires or connectors, etc). Since you've already gone over the PCB to clean it I would say that you have effectively already carried out this step, and presumably found nothing which leaps out. - Next step is to carry out basic checks on all the supply voltages. Although the behaviour of the machine does suggest that the supply voltages are in the right ball park area, you should never assume so because a supply voltage off or out by more than a certain amount can cause some very odd behaviour so it's always worth taking a short time to verify these voltages. Could you please measure the DC voltages on the component leads indicated on the attached sketch? In case it's not clear from the diagram, we are asking for measurements on the right hand ends of diodes CR10, CR11, CR12 and on the lowest lead of regulator VR6. If you need somewhere to put the black probe while making these measurements, the -Ve (right hand) end of C62, the large electrolytic with the cable tie around it, is connected to 0V. |
20th Sep 2022, 8:39 pm | #29 | |
Nonode
Join Date: May 2004
Location: Halifax, West Yorkshire, UK.
Posts: 2,583
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Re: 1978 Commodore PET project
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By the way, when reseating the ROMs it's important to return them to their original positions. Alan |
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20th Sep 2022, 9:23 pm | #30 |
Tetrode
Join Date: Sep 2022
Location: Newton Abbot, Devon, UK.
Posts: 75
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Re: 1978 Commodore PET project
I have just realised that 2 of the legs on chip 901465-02 are damaged. I'm 99% I was very careful when removing it.
It is different to all the others as is purple with gold legs |
20th Sep 2022, 9:44 pm | #31 |
Nonode
Join Date: May 2004
Location: Halifax, West Yorkshire, UK.
Posts: 2,583
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Re: 1978 Commodore PET project
With reference to the board layout diagram which chip is it?
Alan |
20th Sep 2022, 9:46 pm | #32 |
Dekatron
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
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Re: 1978 Commodore PET project
Regarding the damaged ROM, fixing or replacing that is now the priority because the system won't work at all without a working set of ROMs. One way to try to rescue it is to put it into a conventional 24 pin IC socket and solder what's left of the damaged pins to the contacts on the top side of the socket, then plug the IC+socket into the original mainboard socket. If it is only the narrow part of the IC pin which has fallen off, cut a couple of complete pins off any spare or scrap IC and solder them to what's left of the broken IC pins. In the long term you will probably need to get a Hitachi or Texas 2532 EPROM programmed as a replacement. You won't be able to carry out any of the steps below until you have repaired or replaced that ROM.
Assuming the supply voltages turn out OK, possibly the next thing to do is to measure the voltages on the video RAM data pins. While a meter will only show the average voltage, that may be enough to show whether one bit is stuck high or stuck low because the chips are permanently selected and in read mode for the vast majority of the time. Even better if any bit identified as possibly stuck coincides with the bit Mark flagged up as potentially stuck. After that, in the absence of a scope it may be necessary to remove the video RAMs UF7, UF8, and socket and swap them over to see if that changes the nature of the fault. This is something which can be tried without buying any new parts (with the possible exception of IC sockets). If that exonerates the video RAM ICs (ie, no change when swapped) we can remove the video RAM and force various states on the SD0-SD7 lines to verify the operation of the video latch UF9 and the character generator ROM UF10. The above suggestions assume that BrackenFix feels able to desolder ICs from double sided PCBs without damaging either the IC or the PCB. Ordinarily, we would pursue a more diagnostic approach first but in the absence of a scope we are already almost at the point where the most pragmatic approach is to chip-swap. I'm assuming (BrackenFix) that you don't have a logic probe otherwise you probably would have mentioned it. Last edited by SiriusHardware; 20th Sep 2022 at 9:52 pm. |
20th Sep 2022, 9:48 pm | #33 | |
Dekatron
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
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Re: 1978 Commodore PET project
Quote:
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20th Sep 2022, 9:48 pm | #34 |
Tetrode
Join Date: Sep 2022
Location: Newton Abbot, Devon, UK.
Posts: 75
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Re: 1978 Commodore PET project
I believe it is UD7
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20th Sep 2022, 9:59 pm | #35 |
Tetrode
Join Date: Sep 2022
Location: Newton Abbot, Devon, UK.
Posts: 75
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Re: 1978 Commodore PET project
Ok, I have bought a socket to try and salvage the chip.
Sadly, I don't have a logic probe |
20th Sep 2022, 10:06 pm | #36 |
Dekatron
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
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Re: 1978 Commodore PET project
Don't worry if you can't save it, there are a couple of us here, myself included, who have a few 2532 EPROMs and the means to program them so the machine need not be out of action for long.
We may have to ask you to remove and socket a few other ICs in due course so it might not be a bad idea to stock up with a few common sizes of conventional IC socket, 20 pin for the buffer / latch ICs, 18 pin for the video RAM ICs, 14 and 16 pin for general logic ICs. |
20th Sep 2022, 10:12 pm | #37 |
Dekatron
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
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Re: 1978 Commodore PET project
Incidentally, you may wonder why I specified a conventional socket for the 'carrier' for the injured ROM rather than a turned-pin socket. We have found in the past that inserting the relatively thick, round pins of a turned pin socket into the sockets in Commodore mainboards spreads the contacts in the mainboard socket and makes them unreliable, hence the suggestion to use a conventional socket which has thin, flat pins.
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20th Sep 2022, 10:49 pm | #38 | |
Tetrode
Join Date: Sep 2022
Location: Newton Abbot, Devon, UK.
Posts: 75
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Re: 1978 Commodore PET project
Quote:
I got the traditional carriers and not turned pins. The thin legs broke off, there is the larger part of the leg is still there so I should be able to solder them. |
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20th Sep 2022, 11:14 pm | #39 |
Octode
Join Date: Mar 2020
Location: Kitchener, Ontario, Canada
Posts: 1,265
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Re: 1978 Commodore PET project
With conventional modern IC sockets you might still need to take care before inserting them into conventional retro IC sockets. The modern style tend to contact the outer and inner surface of the IC pins. The retro IC sockets tend to contact on the edges of the IC pins. Some of the modern style have a wider flat pin to solder into the board and these can open up the retro style sockets past the point where they maintain contact force.
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20th Sep 2022, 11:14 pm | #40 | |
Dekatron
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
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Re: 1978 Commodore PET project
Quote:
Of course you can also solder the replacement pin to the outside of what is left of the original pin but then the repair is a bit more more obvious and not quite so strong. |
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