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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 9th Feb 2020, 2:00 pm   #41
John Earland
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Default Re: Commodore PET 3016

http://www.zimmers.net/anonftp/pub/c...N/320351-6.gif
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Old 9th Feb 2020, 2:06 pm   #42
SiriusHardware
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Default Re: Commodore PET 3016

Looking at your results so far, the measurements for the connections between the 6520 (dot) and the 74LS145 look OK and appear to confirm that in this detail at least, the circuit we have matches the circuit of your keyboard scanning circuit.

When reading the results for the connections between the IC pins and ground and between the IC pins and other IC pins on the same chip, look a bit more carefully at the meter display to see if it is actually indicating Ohms, KiloOhms or MegaOhms. For the IC pin to ground and IC pins to other IC pins on the same chip, I would expect you to be seeing typically KiloOhms or MegaOhms rather than Ohms.
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Old 9th Feb 2020, 2:06 pm   #43
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Default Re: Commodore PET 3016

Quote:
Originally Posted by SiriusHardware View Post
It looks like the 6520 (dot) is the one associated with keyboard scanning.

For 0V, referring to your overall PCB picture, do you see the thick track running down the left hand side of the PCB, along the bottom edge and up the right hand edge? That will be 0V, so you can hold the black probe anywhere on that area when making the second set of measurements. The -ve end of the large capacitor with the cable tie around it is also likely to be a 0V point.
The 6502 (dot) is the original (well at least the one which was there when I got the PET). I have replaced the second one with a new one and no difference was observed - however. When I replaced the one with the dot then there was a difference. The keyboard became even more unresponsive in that the SHIFT key stopped the others returning anything.
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Old 9th Feb 2020, 2:16 pm   #44
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Default Re: Commodore PET 3016

This is the setting I've used on the multimeter
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Old 9th Feb 2020, 2:23 pm   #45
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Default Re: Commodore PET 3016

Here are the Ground to 74145N readings using 20k setting on multimeter:
74LS145N 15 0V 4.30 20k setting
74LS145N 14 0V 4.45 20k setting
74LS145N 13 0V 4.32 20k setting
74LS145N 12 0V 4.40 20k setting
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Old 9th Feb 2020, 3:04 pm   #46
SiriusHardware
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Default Re: Commodore PET 3016

OK, that's fine, and use the same multimeter setting for doing the checks between IC pins on the same IC. The main thing you are looking for is for there not to be a short (zero Ohms) between any of the pins on the same IC.

Can you clarify the situation with this fault? Has this machine had the fault since you obtained it?

Originally you said you had tried replacing the large socketed ICs so we have been proceeding on the assumption that those ICs are ruled out.

If you replace a chip with another chip and the fault does not change at all, there is a fair chance both ICs are identical, and either both are OK, or (if you are very unlucky) both are faulty in exactly the same way.

However, if you replace a chip with another chip of the same type and that changes the fault from one set of symptoms to another, what that tells you is that there is a difference between the two allegedly identical ICs. Usually, this will mean that the second IC is faulty but, crucially, it does not prove that the original IC is NOT faulty. The first IC may have one fault: The second IC may have a different fault. Where there is doubt, the only way to settle the question is to try a third, known working IC from somewhere else.

One thing you could possibly try is to (temporarily) swap the 'other' 6520 which is already in the machine with the 'dot' 6520, to see what happens then.
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Old 9th Feb 2020, 3:49 pm   #47
John Earland
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Default Re: Commodore PET 3016

Just as an exepriment I have replaced all the ICs with the ones that were in there when I got the machine. There is no difference at all. Does this mean that these chips work??
J
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Old 9th Feb 2020, 3:52 pm   #48
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Default Re: Commodore PET 3016

Except that I've just noticed - in BASIC mode rather than BASIC 4, when I press:
QAZ I get the next line along WSX and WSX prodcues WSX - which I wasn't getting before! Everything else though is exactly the same.
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Old 9th Feb 2020, 3:56 pm   #49
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Default Re: Commodore PET 3016

I've just swapped the 6520's around and again - not difference in the main except that when I press the SHIFT key I don't get anything from the QAZ or WSX as I did when it wasn't pressed.
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Old 9th Feb 2020, 4:21 pm   #50
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Default Re: Commodore PET 3016

Using 20k on multimeter This is what I got from both of the 'orginal' 6520's- I paired them below as you can see testing each IC separately. Most pairings return a value even if that was 1 except 37 with 38. This was the same in both ICs.
40 and 39 - 1
37 and 38 - 0.00
37 and 36 - 1
35 and 34 - 1
34 and 33 - 1 all the way down to 26-27 - 1 then
25-26 another value
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Old 9th Feb 2020, 4:40 pm   #51
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Default Re: Commodore PET 3016

I changed the original 6520s and the 6522 with new ones. I am running the machine in BASIC 4 and the same issues arise as with all of the original ICs. I get exactly the same key return patterning as before. When SHIFT is depressed, only 0 . - and = characters on the number pad work and these return graphics with 0 and . returning the same ones and the - and = returning the same ones.
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Old 9th Feb 2020, 4:45 pm   #52
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Default Re: Commodore PET 3016

Running in BASIC 2 returns exactly the same thing except that pressing RUN/STOP button I can get the PET to load a game from the cassette which I can't if it's running BASIC 4.
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Old 9th Feb 2020, 4:46 pm   #53
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Default Re: Commodore PET 3016

Changing the 2 2114s alters nothing - I see no difference
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Old 9th Feb 2020, 4:50 pm   #54
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Default Re: Commodore PET 3016

I am wondering whether I shouldn't just put back all the original ICs
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Old 9th Feb 2020, 4:56 pm   #55
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Default Re: Commodore PET 3016

I'm going to be really cheeky and ask whether anyone close by would be willing to spend an hour or two with me to make sure I am doing these things right? I live near Middlesbrough and work in County Durham so travelling is not a problem for me!!
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Old 9th Feb 2020, 10:00 pm   #56
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Default Re: Commodore PET 3016

I think you need a bit more faith in your own ability.

Going by all the combinations of everything you've done, I am inclined to assume for now that the original socketed chips which were in the machine are probably OK. Which brings us, unfortunately, back to something you have not tried yet, the 74LS145 or its associated connections. I'm also happy with your findings on the input side of the 74LS145, connections from there to the 6520 are OK, the A/B/C/D input lines all show roughly the same resistance to 0V. I'm not sure whether you did the checks for possible shorts between pins 15,14,13,12 of the 74LS145.

There's a bit more checking we can do on the output side of the 74LS145 before going to the next step of replacing it. It's a little bit late for me to get it all down here tonight, I'll try to outline some suggestions for you tomorrow.
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Old 10th Feb 2020, 1:23 am   #57
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Default Re: Commodore PET 3016

Some further checks:

With the keyboard disconnected, power off as before and with your meter on its lowest ohms range check that the resistance between the following pairs of points is very low, at or near to zero ohms.

74LS145 Pin 1 to main board keyboard connector pin '1'
74LS145 Pin 2 to main board keyboard connector pin '2'
74LS145 Pin 3 to main board keyboard connector pin '3'
74LS145 Pin 4 to main board keyboard connector pin '4'
74LS145 Pin 5 to main board keyboard connector pin '5'
74LS145 Pin 6 to main board keyboard connector pin '6'
74LS145 Pin 7 to main board keyboard connector pin '7'
74LS145 Pin 9 to main board keyboard connector pin '8' * Note the IC pin number has jumped from 7 to 9
74LS145 pin 10 to main board keyboard connector pin '9'
74LS145 pin 11 to main board keyboard connector pin '10'

Then with the black probe on 0V and the meter set initially to the 20K ohms range, measure the resistance from main board keyboard connector pins '1' through to '10' down to 0V and check that they all have a similar resistance. The actual resistance is not important, but all ten pins should read roughly the same resistance down to 0V. If you get an 'infinite' reading on them all, go up one ohms range higher and measure again.

With the meter back down to the lowest ohms range, check the resistance between the following pairs of keyboard connector pins to make sure there are no shorts or very low resistances between any of these pairs of main board keyboard connector pins:

1 and 2
2 and 3
3 and 4
4 and 5
5 and 6
6 and 7
7 and 8
8 and 9
9 and 10

Finally, with the meter set to 20K ohms range and the red meter probe held on pin 16 of the 74LS145, measure the resistance from there to each of the main board keyboard connector pins 'A' through to 'J'. For this group of measurements you should see around 10K, perhaps a little less.
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Old 10th Feb 2020, 10:15 pm   #58
John Earland
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Default Re: Commodore PET 3016

Quote:
Originally Posted by SiriusHardware View Post
Going by all the combinations of everything you've done, I am inclined to assume for now that the original socketed chips which were in the machine are probably OK. Which brings us, unfortunately, back to something you have not tried yet, the 74LS145 or its associated connections. I'm also happy with your findings on the input side of the 74LS145, connections from there to the 6520 are OK, the A/B/C/D input lines all show roughly the same resistance to 0V. I'm not sure whether you did the checks for possible shorts between pins 15,14,13,12 of the 74LS145.
I did test the pins as you mention - at 20k setting everyone gave a reading of some description even if that stayed at 1. something

Quote:
Originally Posted by SiriusHardware View Post
Finally, with the meter set to 20K ohms range and the red meter probe held on pin 16 of the 74LS145, measure the resistance from there to each of the main board keyboard connector pins 'A' through to 'J'. For this group of measurements you should see around 10K, perhaps a little less.
On 20k they all gave a reading if 1, on 200k they all gave a reading of about 78.4 (give or take .4)

Hope I've done this right!!

I've bought a new 74LS145 just in case!!
J
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Old 10th Feb 2020, 11:13 pm   #59
John Earland
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Default Re: Commodore PET 3016

I tested the video IC MOS 901447-10 on 2000k with the pairings - 1-2, 2-3 etc. Most gave a reading of 1 or about 780 etc however, pin pairings 21-20 = 003 - Is this significant? Not sure it is.
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Old 10th Feb 2020, 11:19 pm   #60
John Earland
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Default Re: Commodore PET 3016

Quote:
Originally Posted by John Earland View Post
I did test the pins as you mention - at 20k setting everyone gave a reading of some description even if that stayed at 1. something
on 2000k I got
16-15 = 004
15-14 = 008
14-13 = 008
13-12 = 008
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