Commodore PET 4032/8050/3022

[ScottishColin]

I've managed to find another motherboard photo with the same patches which makes me think they're meant, along with the fact that the wires sit under the jumpers on the board as well.

https://www.squirrel-crafts.com/main...odore-cbm-4016

I attach a couple of photos as .ZIP files so you can see what I mean.

The patch wires connect the following:

UB1/3 <-> UC3/6

UE6/8 <-> UB13/21

Colin.

Originally Posted by ajgriff

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

[ortek_service]

Colin,

Just to check, did you remove those additional (non-original?) EPROM's in the end? It's not a bad idea to, if they weren't originally fitted from the factory, so it will boot without these - Just in case they become corrupted / damaged.

Chip-swapping can be quite a rapid way to find faults, to save a lot of probing (which can still be a bit inconclusive in the absence of a logic or bus analyser to ensure program is executing the right instructions OK / RAM etc. is all good) - With Spectrums, I often ended-up removing the soldered-in DRAM's, and socketing them all, as it was often only a matter of time before some of these died, and made finding faullty ones / future servicing much easier.

Unfortunately (from what I can tell from the photos), it seems not much else is socketed - other than 6502. So can't swap the other 65xx IC's with your other PET etc, to test those / provide a donor of known-good ones.
(Although if the original 6502 pin25 has only broken-off on the actual pin 'leg', with main 'shoulder' still in place, then it should be possible to graft another leg (or suitable gauge tinned-copper wire) onto it to try it in the other PET - As it's probably nice to keep the original date-coded one, if possible, rather than having a more-modern date-coded replacement).

[SiriusHardware]

Colin: If you can do the checks in #58 and report back?

[ScottishColin]

Hi - just checked continuity on a known working 6502 and everything's good.

I've noticed that I'm only getting 2.3V on pin 3 of the 6502 - is that right? I'm getting 5V elsewhere though.

I'll get on with post 58 tests shortly.

Colin.

[SiriusHardware]

If you are measuring that with a voltage meter rather than with a scope, it may be OK as that is one of the two clock outputs on the 6502 - a halfway voltage measured with a meter suggests that you have what you would expect to be there, a roughly 50/50 square wave waveform.

The pin is spending half of the time high, half of the time low, so the meter, which is not fast enough to keep up with the changes, shows the average voltage rather than the highest or lowest voltage.

You can add pin 3 to the list of pins to scope - the reason I didn't include it is that it doesn't appear to be connected to anything else in the circuit.

[ScottishColin]

Next interesting things:

1) as per your post 58, there is no UF1 on this device

2) pins 10 and 11 have been lifted (or never soldered?) on UD2 - see attached photo.

[SiriusHardware]

It is notable that the UF1 circuitry is drawn in a dotted box, which sometimes means 'this might not be here' or 'not always fitted'.

Look at pin 1 of UD3, which hopefully does exist. What frequency (if any) do you see there? Also on UE3 pin 8?

...And can you still also check the states of the CPU RESET, RDY and SYNC pins please.

[SiriusHardware]

Revised opinion: I think the UF1 circuit is shown in a box because it literally is in a box, inside the square tin can next to the beeper. Is that tin can present on your PCB? If it is, I think UF1 is inside that. If so don't worry about that for now, just look for the 16MHz signal on:-

UD3 pin 1

UE3 pin 8

[ScottishColin]

No box on this motherboard. I have seen that on other photos and wondered what was in there but I don't have it.

UE3 pin 8 gives a nice sine wave and a 16Mhz signal

UD3 pin1 goes to 0.5V but no frequency signal at all.

Colin.

[ScottishColin]

CPU RESET - no reading (ie stays at 0)
CPU SYNC - ditto
CPU RDY - 0.5V reading

Colin.

[SiriusHardware]

CPU RESET - no reading (ie stays at 0)
CPU SYNC - ditto
CPU RDY - 0.5V reading

The above readings are all of interest but let's just look at this one first.

UD3 pin1 goes to 0.5V but no frequency signal at all.

Well now, how strange. Those 'lifted' pins of UD2 (10, 11) - can you get to the pads and make a couple of measurements?

(1) Power on, is there a 16Mhz signal on the PAD for UD2 pin 11?

(2) Power off, is there continuity between the PAD for UD2 pin 10 and UD3 pin 1?

[ScottishColin]

Other frequencies:

UD3 pin 11 - no frequency
UD3 pin 10 - no frequency
UD3 pin 9 - 4Mhz
UD3 pin 8 - 16Mhz
UE6 pin 3 - no frequency
UE6 pin 4 - no frequency
UE6 pin 5 - 500Khz
UE6 pin 6 - 500Khz

Colin.

[ScottishColin]

I have safely removed those two 'extra' EPROMs by the way.

Colin.

[ScottishColin]

UD 2 pin 11 pad - 8Mhz

UD2 pin 10 - no continuity to UD3 pin 1.

Colin.

Originally Posted by SiriusHardware

The above readings are all of interest but let's just look at this one first.

Well now, how strange. Those 'lifted' pins of UD2 (10, 11) - can you get to the pads and make a couple of measurements?

(1) Power on, is there a 16Mhz signal on the PAD for UD2 pin 11?

(2) Power off, is there continuity between the PAD for UD2 pin 10 and UD3 pin 1?

[SiriusHardware]

Just about to head off home - just found this manual which seems to have a lot of mods in it pertaining to particular board versions. At the head of the search result for this there was a sentence 'Cut pins 10 and 11 of UDZ' (Meaning UD2, possibly). This document needs to be searched through carefully for any mention of the mods.

http://cini.classiccmp.org/pdf/Commo...Tech%20Ref.pdf

[ScottishColin]

So that's on P20 of the PDF. I can confirm continuity between pins 10 & 11 of UD2 as per the document.

The other notes (14 & 12) don't work as I extected though - no continuity between the pins identified.

So, not for the first time, I'm a little confused.

Colin.

Originally Posted by SiriusHardware

Just about to head off home - just found this manual which seems to have a lot of mods in it pertaining to particular board versions. At the head of the search result for this there was a sentence 'Cut pins 10 and 11 of UDZ' (Meaning UD2, possibly). This document needs to be searched through carefully for any mention of the mods.

http://cini.classiccmp.org/pdf/Commo...Tech%20Ref.pdf

[SiriusHardware]

You're not the only one. At least with your other PET we were able to assume that the machine matched its circuit diagram.

Couple of options then:

1) You take out the mainboard and see which of the mods in that document have been made to your mainboard, and which ones have not, and we try to reconcile that with the circuit.

2) We assume for the time being that the machine once worked as currently modified, and look at more fundamental stuff instead.

Let's take (2) for the time being.

CPU power pin pin (8), voltage is? If you don't even have a 5V supply on this pin stop there and report back. If you do, continue as below...

CPU pins 37 (Clock in) and pins 3 and 39 (Outgoing clocks) - what do you have there, static signals or waveforms and if the latter, what frequency?

This:-

CPU RESET - no reading (ie stays at 0)

...suggests that the CPU is being held in reset, so let's follow that up. Can you measure and state the voltages on all of the pins of the power-on-reset timer IC UD16, plus the voltages on UE5 pins (1,2) and UD15 (pins 5,6).

I am assuming unless you tell us otherwise that virtually all of the ICs are soldered in.

[ScottishColin]

This is a list of the socketed ICs:

UA3 - 901447-10
UB14 - 6502
UD7 - 901498-01
UD11 - empty
UD12 - empty.

Readings coming next.

Colin.

[ScottishColin]

6502 pin 8 - 5.27V

6502 Pin 37 - 4.45V/1Mhz
6502 pin 3 - 5.3V /1Mhz
6502 pin 39 - 5.1V / 1Mhz

UD16
Pin1 - 314mV
Pin2 - 5.33V
Pin3 - 314mV
Pin4 - 5.65V
Pin5 - 4.08V
Pin6 - 213mV
Pin7 - 314mV
Pin8 - 5.65V

UE5 pin1 - 914mV
UE5 pin2 - 941mV

UD15 pin5 - 4.71V
UD15 pin6 - 1.57V

Colin.

[SiriusHardware]

Regarding the mods, which are described on page 20 of the PDF as you noted, scroll down a little way to page 25 and you find a different layout diagram and interestingly, that one has no tin box next to the beeper. The mod references on this diagram seem to match the ones in the mod list on page 20, so for example on the layout on page 25 UD2 has a line pointing to it with a boxed '7' on the end, and on page 20 the mod describing the cut pins mod is... mod number 7.

From there, let's make a cautious leap and assume that the circuit diagram sheets which follow on pages 26-36 are the specific diagrams for this version - if true then unfortunately we have been using the wrong diagrams up to this point.

One fundamental difference is that on this version it is UE4 (Page 31) which has the honour of being the master clock oscillator. That being the case, you should see 16MHz on UE4 pin 6 and UE5 pin 6. If you can confirm that this is so then we are probably on the right track.