29th Jan 2021, 10:56 am | #321 |
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Re: Non-working Commodore PET 3016
Just had a look at your doc from #311, yes, that's more like it. Glad we didn't give up on the 6502, that check turned out to be worth doing. It leaves us with an interesting new puzzle - what is it on the mainboard which is stopping the CPU or just the address lines from being active? So for now, we are turning our attention to the address line buffers, UB3 / UC3.
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29th Jan 2021, 1:15 pm | #322 | |
Octode
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Re: Non-working Commodore PET 3016
Quote:
Colin. |
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29th Jan 2021, 1:50 pm | #323 |
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Re: Non-working Commodore PET 3016
Just keep doing whatever suits you and I'll work around it. For these resistance measurements a simple statement like
From all pins to 0V = One stated resistance From all pins to 5V = Another stated resistance - will be fine. ....Unless of course you get a significant / unexpected variation in readings, in which case we'll want more detail. |
29th Jan 2021, 1:57 pm | #324 |
Octode
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Re: Non-working Commodore PET 3016
OK. Some curious results.
I first measured with my old meter. With the red probe in pin2, I get a consistent 1.99 reading with the meter set to 2K. I cannot get a reading at all (ie it doesn't change) when I go with the black probe on 0V and moving the Red probe. I then tried my new meter to see what that gave me. It gives a steady 3.7 Mohms on all tested pins using Pin2 as the 5V pin. This is where it got a bit weird. When I placed the black probe on 0v (the earthing screw) I get a 0 reading on pins 9-15 and 22-25 However I started to pick up readings, sometimes intermittent, on pins 16-20. Between 22 M ohms to 61 M ohms. I then found that these readings can be affected by pressing on the motherboard gently between the 6502 and UB3. the readings would reduce when I pressed and increase when I let go. I'll let you puzzle this over...it only occurs with the new meter and not at all with the old, but it is most definitely there on those specific pins. |
29th Jan 2021, 2:05 pm | #325 |
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Re: Non-working Commodore PET 3016
Not quite sure how pin 2 got involved?
Try again but this time use CPU socket pin 8 as your +5V to put the red lead on when checking from the pins to + 5V. Use CPU socket pin 1 as the 0V to put your black lead on when making the resistance checks from the pins to 0V. Try using your old meter on a higher range than 2K, go up-range as high as you need to to get a sensible looking reading. If things are as expected then I would think the resistances will be in the high K or M kind of area. |
29th Jan 2021, 2:10 pm | #326 |
Octode
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Re: Non-working Commodore PET 3016
I used pin2 because it tests as +5V when switched on. I will try again with pins 8 and 1 as suggested.
I also tried going up the range on the switch of the old meter and got nothing. I'll try again with pins 1 and 8. |
29th Jan 2021, 2:29 pm | #327 |
Octode
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Re: Non-working Commodore PET 3016
Very very similar set of results. Intermittent readings with the new meter on pins 17-23 when measuring against Pin1. 3.8M Ohms against 5V with the new meter all the way along
Old meter - no readings at all on any settings against 0V, 0.854 (2K setting) against Pin 8. I know the validity of the new meters resistance readings are in doubt, but it's a bit strange I can consistently get readings against some pins with it. |
29th Jan 2021, 2:52 pm | #328 |
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Re: Non-working Commodore PET 3016
Pin 2 is connected to +5V but via a resistor, so that's why we are using a socket pin directly connected to +5V (pin 8).
You are trying to measure the resistance of IC pins which, as we mentioned earlier, will give different readings on different ranges on the same meter and even different readings in the reverse direction on the same range on the same meter. I would guess your new meter has a higher ohms range maximum than your old meter. Readings in the high M region are not much to worry about, what we were really looking for were very low resistances between address lines and either the +5V supply or 0V. I confess I am surprised that you have a relatively low resistance of 854 ohms from the address lines to pin 8 when using the old meter - is that the case from all 16 address pins to pin 8? And, sorry to reiterate, but this was all with the power off? |
29th Jan 2021, 3:44 pm | #329 | |
Octode
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Re: Non-working Commodore PET 3016
Quote:
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29th Jan 2021, 3:52 pm | #330 | |
Octode
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Re: Non-working Commodore PET 3016
Quote:
Do the address lines from the 6502 connect only to the input of a set of address buffers? Or are they connected to multiple ICs? |
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29th Jan 2021, 5:02 pm | #331 |
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Re: Non-working Commodore PET 3016
They go to the inputs of 2 * 74LS244 buffers ONLY.
Everything else on the bus is on the output end of the buffers. Less than 1K from input to +5V does seem low to me, just waiting for Colin to confirm whether this applies to all of the address lines or some, and if only some, which ones? |
29th Jan 2021, 5:40 pm | #332 |
Octode
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Re: Non-working Commodore PET 3016
OK - in my frustration (and probably yours) about meter readings, I have gone a bought a new one with the same dial as the original (ie 200/2k/20k/200k/2M).
https://www.diy.com/departments/0-60.../541251_BQ.prd All readings so far have been taken with the PET powered off and turned off at the socket switch, but with the plug remaining in the socket. Before I do this again, which switch setting should I be using on the meter? Apologies for yet another stupid question. Colin. |
29th Jan 2021, 5:59 pm | #333 |
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Re: Non-working Commodore PET 3016
There are no stupid questions on this forum.
The readings we are interested in are the ones you are getting between +5V (pin 8) and all of the address pins (pins 9-25, excluding pin 21). Start off on the highest (2M) range and work downwards to lower ranges if any of the readings seem low. |
29th Jan 2021, 6:15 pm | #334 | |
Octode
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Re: Non-working Commodore PET 3016
Quote:
meter set to 2M - 0.138 meter set to 200k - 16.9 meter set to 20k - 2.53 meter set to 2k - 0.870 I had expected to see a relationship between the numbers - am I wrong about that? Regardless, which is the most useful setting I should use please? Thanks. |
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29th Jan 2021, 6:42 pm | #335 |
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Re: Non-working Commodore PET 3016
Those readings make sense to me.
You are measuring one of the silicone junctions inside the chip. The same readings would be shown for a diode that is not connected to any circuit. |
29th Jan 2021, 7:01 pm | #336 |
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Re: Non-working Commodore PET 3016
As was explained earlier in the thread, resistance meters use different amounts of current on different ranges, when applied to a non-linear item like a semiconductor device or junction you will get a different reading from one range to the next.
We'll go with the 2K setting for the first run, what we are mainly interested in is whether this value is approximately the same for all of the pins measured to +5V or only some of them. To consider Refugee's point, after measuring the resistance of all 16 address pins to +5V with the red probe on socket pin 8, could you then make the same measurements again, but keep the black probe on pin 8 and move the red probe from address line to address line. For the second lot with the probes reversed, dial up a range or two if the reading is infinite / overflow. |
29th Jan 2021, 7:20 pm | #337 |
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Re: Non-working Commodore PET 3016
For the benefit of anyone following, here is a close-up on the CPU and its surroundings. From this we can see that the address bus pins of the CPU go only to two buffer ICs UB3 and UC3. The rest of the system gets its address bus feed from the outputs of those buffers.
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29th Jan 2021, 7:44 pm | #338 | |
Octode
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Re: Non-working Commodore PET 3016
Quote:
PET off, Red pin in Pin 8, meter set to 2k: 9 - 0.873 10 - 0.875 11 - 0.874 12 - 0.875 13 - 0.875 14 - 0.875 15 - 0.876 16 - 0.872 17 - 0.876 18 - 0.875 19 - 0.875 20 - 0.874 22 - 0.874 23 - 0.875 24 - 0.875 25 - 0.874 PET off, Black pin in Pin 8, meter set to 2k: All readings were 1. I tried all meter settings and the readings stayed at 1. no matter what. |
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29th Jan 2021, 7:47 pm | #339 |
Octode
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Re: Non-working Commodore PET 3016
I don't know if its helpful, but I've watched a lot of C64 repair videos and they sometimes find faulty 74x244 buffers, so its not unknown for them to go faulty.
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29th Jan 2021, 7:52 pm | #340 |
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Re: Non-working Commodore PET 3016
OK, that's enough to suggest to me that, as all of those buffer inputs read the same, they are probably OK.
Now: You know the contacts you removed from the socket? Do you still have them? If so, replace the address pin contacts (9-25 inclusive - leave 21 in place) only, but not the data pin contacts (pins 26-33). Let's see what happens to the address pins when you have the address pins connected, but the data pins left disconnected. I am beginning to wonder if the 6502 is running into a HALT instruction (or whatever the 6502 equivalent is) when it is connected to the data bus. |