Commodore PET 4032/8050/3022

[ScottishColin]

Voltages

CR6 ringed end. - 5.65V
CR6 non-ringed end. wobbling a bit 11.0->11.3V

Colin.

[SiriusHardware]

how does this schematic show connectivity between UD 14 pin 6 and pin 3 of the 6502?

I should perhaps have stated pin numbers. You are looking for continuity between UD15 pin 6 and the 6502 pin 40 (RESET), as shown on the diagram fragment you posted in #99. Do you have that?

[SiriusHardware]

Originally Posted by ajgriff

For ease of reference I'm attaching the file I intended to post earlier.

Alan

Thanks AJ, great minds etc. (See earlier).

[SiriusHardware]

CR6 ringed end. - 5.65V
CR6 non-ringed end. wobbling a bit 11.0->11.3V

The '5V' output is a bit on the high side but not dangerously so. I remember you had a similarly high 5V on your other PET and it does not seem to mind.

[ScottishColin]

My bad. Why on earth did I think it's pin 3? Eejut.

Yes I have continuity between UD15/6 and 6502 pin 40.

Pin 40 reads 1.44V.

Colin.

Originally Posted by SiriusHardware

I should perhaps have stated pin numbers. You are looking for continuity between UD15 pin 6 and the 6502 pin 40 (RESET), as shown on the diagram fragment you posted in #99. Do you have that?

[SiriusHardware]

OK, so that's a fault, which is nice (we were hoping to find one, after all). So now it comes down to which chip on that circuit node to suspect first - my money is initially on the one which is doing the most work, UD15. Do you think you can get that out and socket it?

[ScottishColin]

Yep.

Normal socket or turned pin?

Colin.

Originally Posted by SiriusHardware

OK, so that's a fault, which is nice (we were hoping to find one, after all). So now it comes down to which chip on that circuit node to suspect first - my money is initially on the one which is doing the most work, UD15. Do you think you can get that out and socket it?

[SiriusHardware]

If we are aiming to put the removed IC back in (in the socket) then you will have to make its pins exceptionally clean (with solder wick) in order that they will fit into a turned pin socket. I'm going to say a conventional socket, if you have both, and even then you should clean up the pins of the IC as much as possible with solder wick so that the pins, thickened by solder, do not force the IC socket contacts apart.

Let's say UD15 does turn out to be faulty, if you wanted to, you could remove the conventional socket and fit the replacement UD15 directly back in the PCB to conserve the original (largely unsocketed) appearance of the mainboard. If you fit a turned pin socket in an antique double sided PCB it will be very, very difficult to remove later without tearing pads off the top side of the PCB.

[SiriusHardware]

When you get the IC out, a simple test you can do on it: Plug it into the centre line of a bit of experimenter's breadboard with the pins on each side straddling the gap in the middle of the breadboard, put your meter on a high ohms range, keep the black probe on pin 7 of the chip and touch the red probe to each of these pins in turn,

2, 4, 6, 8, 10, 12

and

1, 3, 5, 9, 11, 13

All of the pins in the first group should have about the same resistance from those pins to pin 7, and all of the pins in the second group should have about the same resistance from those pins to pin 7.

What we're doing here is taking advantage of the fact that the 7417 is essentially a 'box' containing six identical elements which should all measure the same, so if any of the pins, particularly pin 5 or 6 in this case, reads differently to the same pins on the other gates, that would be a fair indicator that that element of the chip is damaged. If you don't see any difference, that does not absolutely prove that the chip is OK as it is possible for a fault to be so far 'inside' the chip that it causes no detectable difference to the resistances on the external pins.

[ScottishColin]

The only reading I can get is between pins 7 and 14. Nothing else. I'll order a replacement. In the meantime is there any point in me trying it in with pin 6 bent out?

Colin.

[SiriusHardware]

Yes, absolutely. That will disable the power-up reset circuit, The machine may even start up with that pin disconnected. If it doesn't, measure the voltage on the CPU RESET pin and if it is now high, try momentarily shorting the CPU reset pin (40) to 0V/GND to see if it will start.

If it is still low (<2.5V) with UD15 pin 6 bent out then the problem lies with something else on that circuit node, so don't order a replacement 7417 yet, you may not need it.

[SiriusHardware]

The only reading I can get is between pins 7 and 14. Nothing else.

Is that on the very highest (Megaohms) range of your meter? You can also try measuring from pin 14 (red probe kept on pin 14 this time) to the same two groups of pins to see if there are any differences that way.

[ScottishColin]

No sign of anything at powerup.

Voltage on pin 40 - 0.322V

Colin.

[SiriusHardware]

With the power on, can you measure the voltage at each end of R13 (the resistor you checked the resistance of earlier) and say what the voltages are, with respect to 0V?

[ScottishColin]

5.09v / 2.17v

Colin.

[SiriusHardware]

There again we have something very unlikely going on. You say that the CPU reset pin is at 0.322V but the bottom end of R13, which should be connected to it, is at 2.17V. That is impossible if the two are connected together as they should be according to the attached diagram fragment. I wonder if we are coming up against diagram differences again?

Note which end of R13 is the one which has / had 2.17V on it and then power off and measure for continuity between that end of R13 and the CPU reset pin.

[SiriusHardware]

Also measure for continuity between that same end of R13 and the pin 6 receptacle of UD15's socket.

[ScottishColin]

Just done a bit of detective work - that R13 on page 1 is actually R15; it's labelled wrong.

R15 has continuity to the CPU reset pin.

R15 reads 5.1V / 1.7V

Pin 40 6052 reads 1.62V

Colin.

[SiriusHardware]

OK. Good work working that out, now I wonder what other differences lie in wait for us? With the power on, again measure the voltages on both ends of R15.

With the power off, measure the in-circuit resistance of R15. It should be 1K or less, or so we hope. If it reads anything higher than 1K, temporarily unsolder one end and measure the actual resistance of the resistor only.

The UD15 element which was under suspicion appears blameless (so you did a good job taking it out in one piece).

Don't put it directly back in the board just yet even if you plan to do that later because there are five other gates in that chip and we may, in time, come back around to this chip and find that one of its other gates is stopping something from working, in which case we will be glad that it is currently in a socket. (see below!)

If I look at the circuit fragment in #116, I see R13(really R15), UD15 pin 6 and the CPU's pin 40 (RESET) on that circuit node, but there is also a connection heading off-circuit from there to circuit sheets 2 and 3.

On circuit sheet 2, it is connected to the RESET input of a 6520 (UB16), the IEEE port handler, and also to another pin on UD15, pin 13.

On circuit sheet 3, it is connected to the RESET inputs of the 6255 (UB15) and the other 6520 (UB2)

I don't see the RESET line going to anywhere else, but I would be happy to be corrected.

Altogether, there aren't many devices on this circuit node which can be keeping it low, so try the following:

Remove UD15, put pin 6 back into shape and then bend out pin 13 and re-insert UD15. Power, on, what is the standing voltage on the CPU RESET pin then?

Power off, remove UD15, put UD15 back into shape and re-insert it with all pins properly inserted.

Remove the CPU, which is the only other socketed IC on this circuit node, and leave it out. Power on - what voltage do you see on the pin 40 (RESET) receptable on the CPU socket?

[ScottishColin]

R15 resistance is 1.003k when powered off.

Pin 40 voltage is 1.38V (same as R15) with pin 13 bent out.

With no CPU, pin 40 reads 1.69V.

Colin.