Commodore PET 4032 apparent 6520 failure

[ScottishColin]

I thought there wasn't enough PET action in here, so I decided to break my lovely reliable 4032 and see some help.

What actually happened is I stupidly plugged a power tap into cassette port 1 and since then it's been all downhill.

The symptoms are in the screen attached. It always boots to the monitor as shown. If I exit the monitor, any commands take me straight back into it.

My research so far has led me to believe that the 6520 may be at fault. Indeed, if I remove the 6520 from UD12, then the problem goes away but I cannot then use the keyboard to do anything....

However, if I replace it with a known working 6520, then the problem comes back. I have replaced the 6502, both 6520s and the 6522 and the problem stays with the motherboard.

I reflowed the solder under the socket in UD12 but no difference (all 40 pin sockets have been replaced on this device).

So there's a problem with the motherboard somewhere (And not the 6502, 6520s or 6522 I don't think) caused by me doing a bad thing with cassette port 1.

I've got 5V on pin 2 of port 1 which is a start, but I'm not clear where to go from here.

Help again please and thanks.

Colin.

[Mark1960]

Sounds like something in the cassette interface that might be holding the interupt line low. Do you have a link for the schematic?

[ScottishColin]

Schematic attached - page 28 I believe.

Colin.

[ortek_service]

Originally Posted by Mark1960

Sounds like something in the cassette interface that might be holding the interupt line low. Do you have a link for the schematic?

Originally Posted by ScottishColin

Schematic attached - page 28 I believe.
Colin.

Well looking at that schematic, it seems the cassette lines all go to a 6520 (that suspiciously handles the keyboard) or to the 6522.
I did wonder if the cassette motor supply could somehow get applied to an adjacent pin and damage something connected to that (fortunately, the +5V isn't adjacent, otherwise many devices on that supply could get an over-voltage). So it would seem most-likely that the +5V voltage supply / the motor supply got applied to one of the cassette signals that go to a 6520 / 6522.
However, that would probably only damage those, whereas it sound like there's an issue with the ROM(s) / addressing of these, if BASIC isn't being entered.

But I have had another thought - Do you press a key at power-on-reset to enter the Monitor routine?
If there was a stuck-key, this could result in always entering this. And this might explain why removing the 6520 that controls the keyboard, prevents reading of this key that causes entry to the Monitor.

[Mark1960]

It looks like the most susceptible components would be the 6522 and 6520, surprised there isn’t any protection circuits between those and the external connectors.

You could check all the power supplies are still ok, in case one of the 5v supplies is down. If there is no 5v supply to the 6522 or 6520 they could be pulling the irq output to ground. This would fit in with removing the 6520 removing the problem, even if the 6520 is OK.

[SiriusHardware]

This is far too late to be thinking sensible thoughts but I did find this short snippet regarding entry to the built in machine code monitor.

you can get into the internal monitor by grounding the "diag" pin on the
User port

If the machine keeps reverting to the monitor, maybe it 'thinks' the DIAG pin is being held down at 0V all the time. Maybe it is being held at 0V all the time.

Following the DIAG pin from its connections on J5(5) and J9(20), both of those eventually arrive at UB2 (6520) pin 9, so the 6520 would be a likely suspect if you had not already presumably tried replacing it.

However the DIAG signal also goes to UD5 (2) and it is possible that that UD5 pin 2 is damaged and holding the DIAG line low.

What voltage do you see on the UD5 (pin 2) / UB2 (pin 9) node?

[ScottishColin]

The 6520 is in UB12 - I think that's mis-marked on the schematics.

UB12/9 and UD5/2 both start low at power up and then both go to 1.17V after a few seconds.

By the way, I used the CRTC version of Dave's ROM tester v4 and that runs fine and reports the other ROM checksums OK for BASIC v4 and the keyboard scanning tool works.

I have swapped the 6520 so I think the problem may be elsewhere either on the motherboard or other ICs.

I did notice that Cassette ports 1 and 2 both go through a Q2T2222 in socket UE16; is that easy to test to rule it out?

Colin.

Originally Posted by SiriusHardware

This is far too late to be thinking sensible thoughts but I did find this short snippet regarding entry to the built in machine code monitor.

If the machine keeps reverting to the monitor, maybe it 'thinks' the DIAG pin is being held down at 0V all the time. Maybe it is being held at 0V all the time.

Following the DIAG pin from its connections on J5(5) and J9(20), both of those eventually arrive at UB2 (6520) pin 9, so the 6520 would be a likely suspect if you had not already presumably tried replacing it.

However the DIAG signal also goes to UD5 (2) and it is possible that that UD5 pin 2 is damaged and holding the DIAG line low.

What voltage do you see on the UD5 (pin 2) / UB2 (pin 9) node?

[ScottishColin]

I'm getting 5V on pin 20 of the 6522 and the 6520s.

However, I'm only getting 2.1V on pin 3 of the 6502 - I believe that should be 5v?

Colin.

Originally Posted by Mark1960

It looks like the most susceptible components would be the 6522 and 6520, surprised there isn’t any protection circuits between those and the external connectors.

You could check all the power supplies are still ok, in case one of the 5v supplies is down. If there is no 5v supply to the 6522 or 6520 they could be pulling the irq output to ground. This would fit in with removing the 6520 removing the problem, even if the 6520 is OK.

[ScottishColin]

And now I'm getting varying 5V on the 40 pin devices.

Could we go back to basics and check the +5V line do you think?

Colin.

[SiriusHardware]

Pin 3 is one of the clock-output pins on the 6502 and as such will have a 50-50 high-low output waveform on it. If measuring that with a meter you would expect it to indicate the average voltage. Look at it with a scope and you will see a waveform there.

Go back to basics and check all the supply voltages if you like BUT I think the lowish voltage on UB12(9) and UD5(2) is worth investigating. If your 3016 also has a DIAG line what voltage does that normally sit at?

With the power off and UB12 removed, what resistance do you have between UD5(2) and 0V?

UE16 is essentially a box full of separate transistors. You can do some basic checks on each of the transistors in the package if you can get it out but at the moment I don't see a strong reason to suspect that device.

[ScottishColin]

Thank you - I was rather thrashing around and needed some logic applied.

The DIAG line is 5V on the working 3016.

I cannot get a solid resistance reading on UD5/2 and earth; it varies between 4.4m? and 11M?.

Colin.

Originally Posted by SiriusHardware

Pin 3 is one of the clock-output pins on the 6502 and as such will have a 50-50 high-low output waveform on it. If measuring that with a meter you would expect it to indicate the average voltage. Look at it with a scope and you will see a waveform there.

Go back to basics and check all the supply voltages if you like BUT I think the lowish voltage on UB12(9) and UD5(2) is worth investigating. If your 3016 also has a DIAG line what voltage does that normally sit at?

With the power off and UB12 removed, what resistance do you have between UD5(2) and 0V?

UE16 is essentially a box full of separate transistors. You can do some basic checks on each of the transistors in the package if you can get it out but at the moment I don't see a strong reason to suspect that device.

[SiriusHardware]

With UB12 removed and then power on, does the voltage on UD5 (2) stay the same as when you last measured the voltage, or does it change to some other value?

[ortek_service]

What was the voltage, rather than resistance to groundmon the /DIAG line on the faulty 4032?
- It does seem that this is the most-likely problem, if this cause it to enter the Monitor routine, as Graham had suggested.

The IC's etc. may well affect in-circuit resistance readings, although getting anything less than a few hundred ohms would probably indicate a problem on this line. You could try removing any socketed IC's, that connect to the /DIAg line, to minimise what's connected to it and help narrow down any low-resistance measurements.

[SiriusHardware]

That was mentioned earlier,

UB12/9 and UD5/2 both start low at power up and then both go to 1.17V after a few seconds.

Both pins are connected to the DIAG line in the 4032. I can't see a pullup resistor anywhere on that line in the 4032, but I think if it is not logic 1 / high then it will tend to stick in that 'Enter the monitor' loop.

If it's hovering low even with the 6520 removed then I'm leaning towards a problem with the input on UD5 (2). However the extremely high resistance seen between the UD5 pin and 0V with UB12 removed doesn't make UD5 an obvious culprit.

[SiriusHardware]

...Unless of course there is an internal pullup resistor on the UD5 input which should be holding the DIAG line high?

[ScottishColin]

1.16V - so the same.

COlin.

Originally Posted by SiriusHardware

With UB12 removed and then power on, does the voltage on UD5 (2) stay the same as when you last measured the voltage, or does it change to some other value?

[SiriusHardware]

Still not sure if we are chasing a wild goose here - do you have any 74LS00, and how easily can you get UD5 out?

[ScottishColin]

UD5 is directly soldered, but I'm more than happy to remove/socket it. I'll order a replacement or two anyway.

Colin.

Originally Posted by SiriusHardware

Still not sure if we are chasing a wild goose here - do you have any 74LS00, and how easily can you get UD5 out?

[SiriusHardware]

I've looked at the diagram for the 3016 again and the DIAG line is even simpler there as it only goes to the 6520 pin 9 - there is no equivalent of the UD5 gate in that machine because there is no speaker / buzzer to gate on and off. And yet, somehow, the diag line on that machine has +5V on it.

Assuming three things:

1) In this area, the hardware of the two machines is very similar. It would be reasonable to expect that the normal state of the DIAG line in the 4032 should be high, as it is on the 3016. This is why it's always great to have two similar, if not identical, machines side by side, so you can make comparisons like this and occasionally chip-swap. (There's an idea, does the 3016 have a socketed 74LS00 in it?).

2) The most obvious way to force the machine into 'machine code monitor' mode is to take the DIAG line low, and at 1.16V the level on the line definitely qualifies as a 'low'.

3) Daver2's test code fully works - it ignores the state of the DIAG line and so doesn't care / doesn't notice whether there is anything untoward in that area or not.

For all of the above reasons and given the relatively low cost of a 74LS00 and socket, I'm going to say that it would be worth replacing UD5 to see if that allows it to come out of the MC-Monitor loop. It doesn't seem to have a short or low resistance down to ground, but something may have happened to the internal pullup which would normally keep that input pin held high.

[ScottishColin]

So I've ordered some and just been through my spares box and drawn a blank.

And then I glanced at my MK14.....

I'll give it a try tonight.

Colin.