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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 10th Feb 2020, 10:50 pm   #61
John Earland
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Default Re: Commodore PET 3016

I've bought a new 74LS145 just in case!!
J
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Old 10th Feb 2020, 11:13 pm   #62
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   #63
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Default Re: Commodore PET 3016

Quote:
Originally Posted by John Earland View Post
Quote:
Originally Posted by SiriusHardware View Post
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.
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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Old 11th Feb 2020, 12:21 am   #64
SiriusHardware
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Default Re: Commodore PET 3016

Quote:
Originally Posted by John Earland View Post
I'm using 2000k - is that right?
As a general rule, when measuring resistance, use the range whose maximum resistance is the next one above the resistance you are expecting to find (so for example if you were expecting to see zero ohms, you would use the lowest range your meter has, maybe 200 Ohms or 2K Ohms in your case). If you were expecting to see 12K resistance, you would initially use the 20K range on your meter, and if you were expecting to see, say, 150K, you would use the 200K range on your meter. If the resistance doesn't turn out to be in the range you expected then of course you can go up or down range to get a meaningful reading if necessary, just as you did when you made the last set of measurements.

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Originally Posted by John Earland View Post
74LS145 Pin 1 to main board keyboard connector pin '1' = 403....
...74LS145 pin 11 to main board keyboard connector pin '10' = 1
Hmm, those results are so far from what I expected that I think the diagram we're working from must not match your actual PCB. Nothing wrong with the way you measured it, either I have misread the diagram or it doesn't match. I'll check again just in case.

If I didn't misread it then we may have to search harder for the circuit diagram of your actual board, if available.

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Originally Posted by John Earland View Post
Hope I've done this right!!
Yes, you did a good job, but possibly on the back of imperfect information.

All of this measurement activity was aimed at trying to get a definite idea of whether the 74LS145 really was faulty without disturbing it and the 40+ year old soldering around it without good reason. The other approach would be to eliminate it from our enquiries by replacing it.
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Old 11th Feb 2020, 12:38 am   #65
SiriusHardware
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Default Re: Commodore PET 3016

Can you take a clearer close-in picture of this area (only) of your main board both on the top side and on the track side? It may be possible to work out the wiring between the 74LS145 and the keyboard connector by tracing the tracks by eye, although of course many are hidden as they pass under the sockets on the top side.

When measuring from various points to keyboard connector pins 1-10 and A-J, how did you determine which pins were which? Are they marked either on the main board connector or on the keyboard connector?
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Old 11th Feb 2020, 12:50 am   #66
SiriusHardware
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Default Re: Commodore PET 3016

By the way, you mentioned you ordered a replacement 74LS145. Did you also order (or do you already have) a suitable IC socket for it to go into?

The other thing you'll need is a small pair of sharp sidecutters with jaws narrow enough to slip betweeen the pins of the original IC.
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Old 11th Feb 2020, 1:22 am   #67
SiriusHardware
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Default Re: Commodore PET 3016

Well, I dug out an alternative version of the PET circuit from which I have cropped the keyboard scanning circuit, see attached. With respect to the connections between the 74LS145 outputs and the connections to the column pins on the main board connector, they are exactly the same and in fact this version of the diagram has the note 'common to all PET computers' in the lower right hand corner.

Try another tack - with your meter on its lowest resistance range, put one probe on pin 1 of the 74LS145, and hold it there while you touch the other probe to each of the main board keyboard connector column pins 1 through to 10. Only one of those pins should be directly connected to the 74LS145 pin 1, that is, only one pin should show zero ohms between the connector pin and pin 1 of the IC. Which one is it?
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Old 11th Feb 2020, 9:25 am   #68
John Earland
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Default Re: Commodore PET 3016

Quote:
Originally Posted by SiriusHardware View Post
Can you take a clearer close-in picture of this area (only) of your main board both on the top side and on the track side? It may be possible to work out the wiring between the 74LS145 and the keyboard connector by tracing the tracks by eye, although of course many are hidden as they pass under the sockets on the top side.

When measuring from various points to keyboard connector pins 1-10 and A-J, how did you determine which pins were which? Are they marked either on the main board connector or on the keyboard connector?
Hi - here are 2 photos:
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Old 11th Feb 2020, 9:28 am   #69
John Earland
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Default Re: Commodore PET 3016

Quote:
Originally Posted by SiriusHardware View Post
Quote:
Originally Posted by John Earland View Post
I'm using 2000k - is that right?
As a general rule, when measuring resistance, use the range whose maximum resistance is the next one above the resistance you are expecting to find (so for example if you were expecting to see zero ohms, you would use the lowest range your meter has, maybe 200 Ohms or 2K Ohms in your case). If you were expecting to see 12K resistance, you would initially use the 20K range on your meter, and if you were expecting to see, say, 150K, you would use the 200K range on your meter. If the resistance doesn't turn out to be in the range you expected then of course you can go up or down range to get a meaningful reading if necessary, just as you did when you made the last set of measurements.

Quote:
Originally Posted by John Earland View Post
74LS145 Pin 1 to main board keyboard connector pin '1' = 403....
...74LS145 pin 11 to main board keyboard connector pin '10' = 1
Hmm, those results are so far from what I expected that I think the diagram we're working from must not match your actual PCB. Nothing wrong with the way you measured it, either I have misread the diagram or it doesn't match. I'll check again just in case.

If I didn't misread it then we may have to search harder for the circuit diagram of your actual board, if available.

Quote:
Originally Posted by John Earland View Post
Hope I've done this right!!
Yes, you did a good job, but possibly on the back of imperfect information.

All of this measurement activity was aimed at trying to get a definite idea of whether the 74LS145 really was faulty without disturbing it and the 40+ year old soldering around it without good reason. The other approach would be to eliminate it from our enquiries by replacing it.
This is the range on my multimeter
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Old 11th Feb 2020, 9:31 am   #70
John Earland
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Default Re: Commodore PET 3016

Quote:
Originally Posted by SiriusHardware View Post
Can you take a clearer close-in picture of this area (only) of your main board both on the top side and on the track side? It may be possible to work out the wiring between the 74LS145 and the keyboard connector by tracing the tracks by eye, although of course many are hidden as they pass under the sockets on the top side.

When measuring from various points to keyboard connector pins 1-10 and A-J, how did you determine which pins were which? Are they marked either on the main board connector or on the keyboard connector?
Looking at the IC there is a notch on the left hand side. No 1 is the bottom left and working anticlockwise round to 16 or whatever the number of pins are - is this correct?
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Old 11th Feb 2020, 10:12 am   #71
SiriusHardware
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Default Re: Commodore PET 3016

Yes, that's correct for IC pin numbering. My question was how did you know which of the keyboard connector pins were 1-10 and A-J? Are they marked in some way?
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Old 11th Feb 2020, 10:28 am   #72
John Earland
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Default Re: Commodore PET 3016

Quote:
Originally Posted by SiriusHardware View Post
Yes, that's correct for IC pin numbering. My question was how did you know which of the keyboard connector pins were 1-10 and A-J? Are they marked in some way?
Ah - mmm - I just assumed that the lefthand one was 1 through to 10 and then from then on J-A??
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Old 11th Feb 2020, 10:29 am   #73
John Earland
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Default Re: Commodore PET 3016

Quote:
Originally Posted by SiriusHardware View Post
By the way, you mentioned you ordered a replacement 74LS145. Did you also order (or do you already have) a suitable IC socket for it to go into?

The other thing you'll need is a small pair of sharp sidecutters with jaws narrow enough to slip betweeen the pins of the original IC.
No I haven't got a socket as such but I do have pin strips which I can use. I could solder those in then add the IC
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Old 11th Feb 2020, 11:15 am   #74
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Default Re: Commodore PET 3016

OK, in that case can you try as suggested earlier, use your meter on its '200' Ohms range to find out which of the main board keyboard connector pins is pin 1?

It should be whichever pin is directly connected, ie, zero Ohms, to pin 1 of the 74LS145.
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Old 11th Feb 2020, 11:21 am   #75
John Earland
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Default Re: Commodore PET 3016

Quote:
Originally Posted by SiriusHardware View Post
Well, I dug out an alternative version of the PET circuit from which I have cropped the keyboard scanning circuit, see attached. With respect to the connections between the 74LS145 outputs and the connections to the column pins on the main board connector, they are exactly the same and in fact this version of the diagram has the note 'common to all PET computers' in the lower right hand corner.

Try another tack - with your meter on its lowest resistance range, put one probe on pin 1 of the 74LS145, and hold it there while you touch the other probe to each of the main board keyboard connector column pins 1 through to 10. Only one of those pins should be directly connected to the 74LS145 pin 1, that is, only one pin should show zero ohms between the connector pin and pin 1 of the IC. Which one is it?
Not sure this is the right layout. The 7145 IC is to the right of the keyboard connector on my board this diagram is showing that it is on the left - or have I misread it??
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Old 11th Feb 2020, 11:27 am   #76
John Earland
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Default Re: Commodore PET 3016

Quote:
Originally Posted by SiriusHardware View Post
OK, in that case can you try as suggested earlier, use your meter on its '200' Ohms range to find out which of the main board keyboard connector pins is pin 1?

It should be whichever pin is directly connected, ie, zero Ohms, to pin 1 of the 74LS145.
Looks like I've got it completely wrong!

Placing the multimeter on 200k and the black probe on IC pin 1 I moved the red probe up the connector starting at the left hand side. They all gave a reading of about 86.something. However, 9th pin up gave 0 ohms! This must be number 1 (there are exactly 10 pins remaining includng this one - and one on its own separated from the the others by a gap) There are 8 before this so these must be the 8 letter ones!
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Old 11th Feb 2020, 11:27 am   #77
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Default Re: Commodore PET 3016

Quote:
Not sure this is the right layout
Don't worry too much about that - there is a big difference between the Layout, which shows how the parts are physically arranged on the PCB, and the circuit diagram, which shows how the parts are electrically connected together. The positions of the parts on circuit diagrams are usually chosen so that the interconnections between the parts can be followed more easily, and often with the signal paths (in the case of amplifiers or radio circuits) going from left to right.
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Old 11th Feb 2020, 11:28 am   #78
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Default Re: Commodore PET 3016

A circuit diagram is not a layout diagram. It shows how the components are electrically connected up, it does not show where they are on the PCB.
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Old 11th Feb 2020, 11:39 am   #79
John Earland
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Default Re: Commodore PET 3016

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' = 0
74LS145 Pin 2 to main board keyboard connector pin '2' = 0
74LS145 Pin 3 to main board keyboard connector pin '3' = 0
74LS145 Pin 4 to main board keyboard connector pin '4' = 0
74LS145 Pin 5 to main board keyboard connector pin '5' = 0
74LS145 Pin 6 to main board keyboard connector pin '6' = 0
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 = 0
74LS145 pin 10 to main board keyboard connector pin '9' = 0
74LS145 pin 11 to main board keyboard connector pin '10' = 0


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.

(I've tried all oms settings I have and all give infinity readings of 1)


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

All gives infinity readings of 1

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.

All have readings of 9.8!
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Old 11th Feb 2020, 11:45 am   #80
John Earland
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Default Re: Commodore PET 3016

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Originally Posted by TonyDuell View Post
A circuit diagram is not a layout diagram. It shows how the components are electrically connected up, it does not show where they are on the PCB.
Thank you - didn't understand that! Told you I wasn't very good!! But I am learning with your help.
Best wishes,
John
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