11th Feb 2020, 12:21 am | #61 | |||
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Re: Commodore PET 3016
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If I didn't misread it then we may have to search harder for the circuit diagram of your actual board, if available. Quote:
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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11th Feb 2020, 12:38 am | #62 |
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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? |
11th Feb 2020, 12:50 am | #63 |
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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. |
11th Feb 2020, 1:22 am | #64 |
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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? |
11th Feb 2020, 9:25 am | #65 | |
Hexode
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Re: Commodore PET 3016
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11th Feb 2020, 9:28 am | #66 | |
Hexode
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Re: Commodore PET 3016
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Last edited by John Earland; 11th Feb 2020 at 9:29 am. Reason: No photo |
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11th Feb 2020, 9:31 am | #67 | |
Hexode
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Re: Commodore PET 3016
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11th Feb 2020, 10:12 am | #68 |
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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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11th Feb 2020, 10:28 am | #69 |
Hexode
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Re: Commodore PET 3016
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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11th Feb 2020, 10:29 am | #70 |
Hexode
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Re: Commodore PET 3016
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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11th Feb 2020, 11:15 am | #71 |
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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. |
11th Feb 2020, 11:21 am | #72 | |
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Re: Commodore PET 3016
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11th Feb 2020, 11:27 am | #73 | |
Hexode
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Re: Commodore PET 3016
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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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11th Feb 2020, 11:27 am | #74 |
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Re: Commodore PET 3016
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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11th Feb 2020, 11:28 am | #75 |
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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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11th Feb 2020, 11:39 am | #76 |
Hexode
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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! |
11th Feb 2020, 11:45 am | #77 | |
Hexode
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Re: Commodore PET 3016
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Best wishes, John |
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11th Feb 2020, 11:46 am | #78 | |
Hexode
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Re: Commodore PET 3016
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Best wishes, john |
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11th Feb 2020, 11:51 am | #79 | |
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Re: Commodore PET 3016
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So now, can you find out which keyboard connector pins are connected to 74LS145 pin 9 (That one will be keyboard connector pin 10) 6520 pin 10 (That one will be keyboard connector pin A) 6250 pin 17 (That one will be keyboard connector pin J, probably) Once you have those, you should be able to deduce all the ones in between. Draw yourself a little sketch of the keyboard connector pin numbering and add it to your stash of information about the machine so you can refer back to it in future. Edit: I see you are ahead of me. Fast learner. |
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11th Feb 2020, 12:12 pm | #80 | ||
Hexode
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Re: Commodore PET 3016
Quote:
6520 pins 10-17 to connector: 10 - A 11 - B 12 - C 13 - D 14 - E 15 - F 16 - H 17 - J I think!! |
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