13th Oct 2021, 9:06 pm | #81 | |
Pentode
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Re: Commodore PET 2001 voltage regulators
Quote:
Although the pin/socket is now delivering the voltage, could a safeguard be to have the crocodile clip from the +ve terminal on the large capacitor to CR1/2? Would this cause any additional problems? |
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13th Oct 2021, 9:07 pm | #82 |
Dekatron
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Re: Commodore PET 2001 voltage regulators
It would do no harm to leave a permanent bypass around JP8 (4) but it would be inconvenient whenever you needed to remove the PCB and a highly unoriginal feature to have in a historic machine.
With the connector fitted and the machine running, what DC voltage do you now see -On CR1 tapered end -On top of the JP8 connector (pin 4). The closer these two voltages are to each other, the better the connection between the JP8 pin and JP8 receptacle. |
13th Oct 2021, 9:16 pm | #83 |
Nonode
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Re: Commodore PET 2001 voltage regulators
Bearing in mind the work carried out and looking at the photos I think this should be fine. Your call of course.
Alan |
13th Oct 2021, 9:40 pm | #84 |
Pentode
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Re: Commodore PET 2001 voltage regulators
Hi.
Okay, I took a few measurements over different periods of time and they all seemed to be around the same value. The last two sets I have are as follows: CR1 = 9.04/3v CR2 = 9.04/3v JP8 pin 4 = 9.04v CR1 = 9.08v CR2 = 9.06v JP8 pin 4 = 9.07v These readings were taken when the PET was first switched on, then after running for a few minutes. I did check CR1 a few times, and if this changed, then I checked pin 4 - they were about in sync value wise from what I could see. |
13th Oct 2021, 9:49 pm | #85 |
Dekatron
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Re: Commodore PET 2001 voltage regulators
Those are nice results, little or no voltage drop across the JP8(4) connection now.
I would say you've probably done enough to have confidence in the connection now, especially if the connector has been on and off a few times and still giving good results like that. |
13th Oct 2021, 11:19 pm | #86 |
Octode
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Re: Commodore PET 2001 voltage regulators
Is it worth doing a continuity test from wherever feeds JP8/4 to wherever it needs to go to (sorry I'm away) and wiggling the plug to see what happens?
It might be worth testing as it's a test without power and therefore can't harm any other ICs. Colin. |
13th Oct 2021, 11:26 pm | #87 |
Dekatron
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Re: Commodore PET 2001 voltage regulators
Decent idea, the test points to check continuity between would be CR1 or CR2 tapered end and the +Ve terminal of the offboard capacitor. It would be difficult to hold the probes in place with one hand each and also wiggle the connector, so maybe use croc leads to attach the meter leads to the test points to free both hands for connector, the wiggling of.
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14th Oct 2021, 2:02 pm | #88 |
Nonode
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Re: Commodore PET 2001 voltage regulators
I’ve been giving some more thought to the next steps now there is more confidence in the security of the JP8/P4 connection. In the absence of an oscilloscope, logic probe or EPROM programmer there are still a few options in terms of eliminating possible reasons for the random character display and consequent failure to boot correctly. In no particular order these are a some of the possibilities which might be worth considering: 1. The 555 (UA2) timer circuit should reset the CPU a couple of seconds after switch on. If this doesn’t work correctly the random character screen doesn’t clear as highlighted by Slothie earlier. Operation of the circuit can be verified by monitoring the voltage on the reset pin (40) of the 6502 (UC4) CPU at switch on. The voltage should remain at 0V for a couple of seconds before rising to around 4V (steady). Pin 40 is in the top left corner of the 6502 when viewing the board from the opposite side to the power supply. 2. The PET doesn’t need the two PIAs (UC6 & UC7) in order to boot but faulty chips can cause the random character screen problem. Switching on the machine with the PIAs removed won’t harm the PET although if it does boot properly neither the keyboard nor the external interfaces will function. This is just a simple way of identifying faulty PIAs. 3. During the work so far there has been quite a lot of board manipulation/flexing and elderly PETs are well known for developing poor IC pin to socket contact. Rather than removing socketed ICs completely (fiddly with a risk of pin damage on re-insertion) I’d suggest the lift slightly, squirt with contact cleaner, rock and then push home firmly approach. I’d probably leave the processor and ROM chips alone in the first instance and just concentrate on the 15 socketed DRAMs. 4. In view of the fact(?) that the DRAMs are the only chips which rely on anything other than the +5V supply I still think they are the most likely candidates for damage caused by the JP8/P4 issue. See earlier post. If Sirius is able and willing to provide an EPROM with which to test for individual DRAM failure that would be ideal of course. However there are other ways to at least carry out some checks on the DRAMs. The attached document might help as it illustrates how the dynamic RAM board can be configured to operate with only eight 4116s, albeit at half capacity. With a bit of chip swapping it might be possible to identify groups containing one or more faulty chip(s) and arrive at a set of eight working chips thereby ruling out faults elsewhere on the board. If none of the above suggestions bear fruit it’ll be necessary to start thinking about things like logic ICs, the CPU, ROMs and the like but in view of the previously working nature of the machine I’m hoping that won’t be necessary. I’ve also rather ignored the possibility of supply line failure somewhere along the line for the same reason. Just throwing out some ideas really to help (hopefully) get us started on the next steps. Alan
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14th Oct 2021, 4:30 pm | #89 | |
Pentode
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Re: Commodore PET 2001 voltage regulators
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14th Oct 2021, 4:37 pm | #90 | |
Pentode
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Re: Commodore PET 2001 voltage regulators
Quote:
- I've just checked the voltage of pin 40 on the 6502 (UC4) and it came out steady at 4.88v. I also tested from a startup, and it went from 0v to 4.88v. I checked it a few times, and it did seem steady at 4.88v. - I've also removed both the PIAs (UC6 nad UC7), then started the PET up. The screen didn't seem to change (still garbled). - The 15 RAM ICs have been lifted and contact cleaner squirted around the legs/sockets. They have been seated back in. When switching the PET back on, it still displayed the garbled screen. - I'll have a read through the document to see if we can progress something with this. Tim. |
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14th Oct 2021, 5:38 pm | #91 |
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Re: Commodore PET 2001 voltage regulators
I'm going to program up a 2716 with the 'Daver2' test code and send it off to Tim when it's ready so hopefully that will be available to use as and when we get to the point where we think the system can run some test code.
I may also do a second one with Slothie's code, the only reason that one is not an automatic choice is that I don't have a TMS2532 EPROM to program it into. It's just a lucky fluke that we can plug a 2716 into the UD8 position and it will work, whereas a 2716 plugged into the UD9 socket needs an adaptor to work. I might just 'adapt' a 2716 directly rather than place it in a socket adaptor. With this one, I think we might have to start quite a long way back on the grid. The normal startup sequence is: -Jump to boot code in UD9 -Shortly afterwards, jump to a subroutine in UD8. -In the subroutine, ---Clear the screen RAM ---Initialise the large ICs -Return from subroutine. If the main system RAM is faulty then the machine will get stuck because it will fetch an invalid return address from the faulty RAM. But, it should still have managed to clear the screen by this point. Unfortunately this machine isn't clearing the screen so right now, I'm not sure it will be able to execute test code either, unless we happen to have a dud original UD8 or UD9 PROM - it will still be worth trying one or other of the test EPROMs as a way of ruling out a faulty UD9 or UD8 respectively. So now we come to the usual questions for retromit: What do you have in the way of test gear, in addition to the meter which you obviously do have..? -Logic Probe? -Frequency meter or Multimeter with a 'Frequency' range on it? -Oscilloscope? -Logic analyser or logic 'sniffer' with parallel inputs? -Any other potentially useful test toys such as an Arduino Uno, Arduino Mega or Raspberry Pi? |
14th Oct 2021, 5:40 pm | #92 | |
Nonode
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Re: Commodore PET 2001 voltage regulators
Quote:
Alan Last edited by ajgriff; 14th Oct 2021 at 6:06 pm. |
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14th Oct 2021, 5:48 pm | #93 |
Nonode
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Re: Commodore PET 2001 voltage regulators
No frequency measuring option on the DMM Tim is currently using unfortunately. See manual attached to post #42.
Alan |
14th Oct 2021, 7:08 pm | #94 | |
Nonode
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Re: Commodore PET 2001 voltage regulators
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Alan |
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14th Oct 2021, 7:57 pm | #95 | ||
Pentode
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Re: Commodore PET 2001 voltage regulators
Quote:
I've just tested again, and it was around 2 seconds; it went straight from 0v to 4.88v in virtually one jump; then solid on 4.88v. Tim. |
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14th Oct 2021, 8:02 pm | #96 | |
Pentode
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Re: Commodore PET 2001 voltage regulators
Quote:
Many thanks once again for sorting the EPROM for me. The multimeter I have is a bit basic. It has been planning to upgrade for a while now, so this could be an ideal time. Have you any suggestions on a good upgrade? Unfortunately I don't have an oscilloscope or logic analyser (but, again, open to any suggesstions you may have). I do have a couple of Raspberry Pis, and also a 4116/4164 RAM tester (which is Arduino based) - which I was thinking of using to test the RAM chips from the PET (the 4116). Tim. |
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14th Oct 2021, 8:09 pm | #97 | ||
Octode
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Re: Commodore PET 2001 voltage regulators
As part of my fixing of my dilapidated PET linked to earlier, I had to buy a scope. I bought a generic 6022BL scope off Amazon and it did me fine. About £80-£90 right now. It needs a PC to plug into but I happened to have a spare laptop I could use. It works on Windows 10 and there's a Linux program/drivers for it too.
I learnt a lot about how to use them and still have it at the end of the process. There's plenty of people on here with far more knowledge than I that will have broader recommendations - just thought I'd mention what I used. Colin. Quote:
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14th Oct 2021, 8:19 pm | #98 |
Nonode
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Re: Commodore PET 2001 voltage regulators
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14th Oct 2021, 8:49 pm | #99 |
Dekatron
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Re: Commodore PET 2001 voltage regulators
Agree, since you are lucky enough to have nearly all of the RAM in sockets, by all means test all of the removable RAM in your tester.
If that doesn't score a lucky hit, we may very quickly get to the point where you will need a scope of some sort - a PC-based digital storage scope like the one Colin suggests is ideal and it's fair to say we would not have made any headway with the fault on his PET had he not taken the plunge and bought that. As he said, although the initial outlay is not insignificant, a storage scope is the single most useful bit of test gear you can own and it will continue to be useful for other jobs long after the PET is up and running. I think there is something called a Pi-Scope which is a bit of hardware and software you add to a Raspberry Pi which effectively makes it into a digital scope - I have no experience with that system personally, you'd have to do some research. However - as I'm very fond of saying, always try the things you can try with what you already have and costs nothing, so test those RAM ICs first and report back. |
14th Oct 2021, 9:27 pm | #100 | |||
Pentode
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Re: Commodore PET 2001 voltage regulators
Quote:
Thanks for the recommendation - I'll certainly look into that. Cheers, Tim. |
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