Commodore PET 4032/8050/3022

[SiriusHardware]

Maybe a bit like Vectrex / arcade machines vector rather than raster-scan systems, but hopefully not requiring the same higher speed horizontal frequencies on the vertical deflection.

The Hdrive and Vdrive signals going out from the PET mainboard are just short digital pulses not unlike sync pulses, so as such, the TTL-Level vdrive, hdrive and video signals can all be combined by a relatively simple circuit into a composite video signal. There are several PET output to composite video converter circuits online.

It could be handy to have one of these as a diagnostic aid, as it would be a way of immediately finding whether a 'nothing on the monitor' fault was due to a fault on the mainboard or a fault in the PET monitor.

Once inside the monitor the digital vdrive and hdrive pulses are stretched and shaped further.

[ScottishColin]

Right - back a bit early for reasons.

This is now turning into a project to take my mind off things, so here's my plan.

I'm going to put progress as it happens on my wordpress site as a way of summarising what has happened along the way.

I'll start separate threads for each device once I've got past this power issue.

Right now, I have the slow-blow 750Ma fuses and have given the board a good clean with IPA, lint free cloths and cotton wool buds. Nothing looks awful, although there is a bit more rust that I would like on the chips on the right hand side of the board; let's see what happens when we get there. Also one of the Voltage Regulators (VR4) may have been replaced in the past as it is now bolted onto the board, unlike the others.

Also there's a couple of patch cables that look old and 'meant' so I'm assuming they were done at the factory.

So right now, I can restart the power issues we were going down. The power supply is bolted back into the chassis and I'm happy with the earth. Am I OK to insert a fuse, power on without the motherboard and then start to measure voltages?

Colin.

[SiriusHardware]

Yes, leave all of the low voltage output connectors disconnected from the mainboard, then:

Move the mains power switch on the PET to the ON position,

Plug the mains plug into a mains socket, ideally a switched one which you can turn off quickly if need be, and turn it on there. This is just to put some physical distance between you and the PSU block just in case it takes off, although I don't expect it will. It sounds like you don't need anything else bad going on in your life at the moment.

If nothing untoward happens, make AC voltage measurements between the following pairs of points on the low voltage side of the mains transformer while the power is on:

The two blue wires going to transformer terminals 4 and 6
The two brown wires going to transformer terminals 7 and 8

You can make these measurements either on the transformer terminals or at the connectors on the other ends of the wires, whichever is physically easiest. Make sure you keep away from the mains side terminals of the transformer / mains wiring area.

[ScottishColin]

OK - no bang or release of smoke which is good.

Voltages below:

The two blue wires going to transformer terminals 4 and 6 - 0.28V
The two brown wires going to transformer terminals 7 and 8 - 15.6V

Colin.

Originally Posted by SiriusHardware

Yes, leave all of the low voltage output connectors disconnected from the mainboard, then:

Move the mains power switch on the PET to the ON position,

Plug the mains plug into a mains socket, ideally a switched one which you can turn off quickly if need be, and turn it on there. This is just to put some physical distance between you and the PSU block just in case it takes off, although I don't expect it will. It sounds like you don't need anything else bad going on in your life at the moment.

If nothing untoward happens, make AC voltage measurements between the following pairs of points on the low voltage side of the mains transformer while the power is on:

The two blue wires going to transformer terminals 4 and 6
The two brown wires going to transformer terminals 7 and 8

You can make these measurements either on the transformer terminals or at the connectors on the other ends of the wires, whichever is physically easiest. Make sure you keep away from the mains side terminals of the transformer / mains wiring area.

[SiriusHardware]

Sorry Colin - problem with being at work is that people keep trying to give me stuff to do. Very annoying.

Brown wire voltage sounds spot on, blue wire voltage, not so much.

With the mains power off and the connectors still disconnected from the mainboard can you try measuring the resistance between one blue wire, the one attached to transformer pin 7, and the other blue wire, the one attached to transformer pin 8? (Note I got off to a good start in #43 by mixing up the transformer pins associated with each colour, but no matter).

[ScottishColin]

OK - remeasures after cleaning.

4/6 - 18.3V
7/8 - 15.6V

Resistance between 7/8 is 0.3 Ohms

Also note that the blue wires are on 7/8 and the brown wires on 4/6 (see photo).

Colin.

[SiriusHardware]

Yep, my error in #43, as belatedly noted in #45.

The voltages now sound good, so reconnect the low voltage side connectors to the mainboard and try turning it on and see what happens.

[ScottishColin]

Everything is apart right now so it'll take a little while to get it back together.

In the meantime, please could you let me know where you see those voltages on the schematics as I can't see them?

Colin.

[SiriusHardware]

Whether it works (which would be nice) or not, measure the DC voltage with respect to 0V on the following points:

CR7 (Ringed end).
CR7 (Non-ringed end).

CR5 (Ringed end).
CR5 (Non-ringed end).

CR1 (Non-ringed end).
CR1 (Ringed end).

The first PDF page of the circuit diagram AJ linked to in #24 shows the overall layout with the positions and orientation of these three components on it.

PDF page 12 shows the circuit diagram of the power supply, from which you may see that we are measuring the voltages going into and out from the +5V, +12V and -5V regulators.

[SiriusHardware]

In the meantime, please could you let me know where you see those voltages on the schematics as I can't see them?

The voltages are not written on the diagrams, which is unfortunate, so my judgement of whether they are roughly right or badly wrong is based on measurements which were made from the same points in earlier PET threads. The PSUs / PSU circuits in them all are all very similar.

[ScottishColin]

Nothing on the screen and no speaker chirp.

On further investigation, on the upside, there's an orange glow in the neck of the tube.

On the downside, one of the 6502 pins has gone and it's pretty stuck inside the white socket so it looks like I'll have to get the cutter out and cut the socket and replace it.

Any views on normal sockets vs turned pin before I start soldering?

CR7 (Ringed end) - 10.2V
CR7 (Non-ringed end) - 5.1V

CR5 (Ringed end) - 19.6V
CR5 (Non-ringed end) - 12.3V

CR1 (Non-ringed end) - minus 11.9V
CR1 (Ringed end) - minus 4.9V

Colin.

[ScottishColin]

Got it - thanks.

Colin.

Originally Posted by SiriusHardware

The voltages are not written on the diagrams, which is unfortunate, so my judgement of whether they are roughly right or badly wrong is based on measurements which were made from the same points in earlier PET threads. The PSUs / PSU circuits in them all are all very similar.

[ScottishColin]

Quick update - I got lucky and fished it out. It's pin 25 which still has continuity from the socket to UD14/pin 17 so I think we're good.

Colin.

[SiriusHardware]

Quick summary: Pin 25 has come off the CPU and for a time you thought it was lost inside the corresponding socket, but you have managed to recover it?

If the CPU has only lost the thin lower half of its pin 25 you could re-pin it by fitting it into a conventional 40 pin socket and (if necessary) by soldering across the gap between the half pin and the corresponding contact in the socket, or you could solder a pin harvested from a scrap / surplus IC to what is left of the original 6502 pin, much like the way you repaired one of the original ROMs in what I am now calling PET 1. (This is PET II).

Obviously the ideal way would be to have a replacement 6502 with good pins.

The 'Chirp' which you should be hearing on start up is generated by a firmware routine which should be executed quite early on in the initialisation process, but since that is not happening we can surmise that the CPU is not presently able to execute code, although the CPU having a leg missing may have some bearing on that.

Repair or replace the CPU however you mean to and then let us know when you are ready to proceed. If you do go to replace the CPU socket then you might want to approach it from the angle of 'which type of socket does my NOP generator / my RAM / ROM board / my diagnostic clip fit into best', because we may need to use those at some point.

[Mark1960]

Pin 25 is A15. My guess is that the input to address buffers from the 6502 would float high, so the 6502 would not be able to access the low 32k of ram, no stack and no zero page.

[ScottishColin]

OK - known working 6502 installed and still nothing on the screen.

In terms of diagnostics tools, I have the Tynemouth board with a simple tester in it, Dave's PETTester v4, Slothies tester, a NOP generator and my scope.

I tried the Tynemouth board and still nothing there.

Out for a while - I'll test continuity from all 6502 legs when I get back in to make sure that's all good.

Colin.

[SiriusHardware]

We'll let you do that first and if you are happy that you have good continuity from there to everywhere else then the next thing will be to look at the clocks and the operation of the reset circuit, it won't be long before you are needing to use your scope again so brush up on your scope-fu.

I think I saw that only one of the standard ROMs is in a socket and that will be the 'edit' ROM which can be replaced with Daver2's test EPROM but not with Slothie's which can only replace the 'Kernel/Kernal' ROM.

[SiriusHardware]

I may be otherwise engaged later, so, some things to check with the power on, when you are ready.

The state of the RESET pin of the 6502 (Pin 40)?

Should be a 16MHz signal on UF1 pin 6, UD2 pin 10, UE3 pin 8 and UD3 pin 1.

8Mhz on UD3 pin 3,
4MHz signal on UD3 pin 4,
2MHz signal on UD3 pin 5,
1MHz signal on UD3 pin 6, UE4 pin 10, UE4 pin 8 and UD3 pin 13.

Measure the frequencies on
UD3 pin 11
UD3 pin 10
UD3 pin 9
UD3 pin 8
UE6 pin 3
UE6 pin 4
UE6 pin 5
UE6 pin 6

Look for a 1MHz signal on CPU pin 37 (Clock-in)

CPU Pin 7 (SYNC) - is it in a steady state or active?

CPU pin 2 (RDY) - is that in a steady state or active?

[ajgriff]

Originally Posted by ScottishColin

There's a couple of interesting patch wires which look like they were done at the factory?

I'd like to know a bit more about these wires as, judging from the photos attached to post #1, they don't look to me like factory add-ons. With reference to the schematic which points are connected together by each of these wires? We already know that this PET has been worked on in the past and these connections might provide some fault finding clues.

Alan

[ajgriff]

Please ignore the previous post as the wires are clearly shown on the layout diagram. Sorry for red herring!

Alan