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ScottishColin 1st Jan 2021 5:29 pm

Non-working Commodore PET 3016
Hi - I have posted before (I see it was 2012....) about my old Commodore PET. I finally have got some time in lockdown to work on it.

I had my first PET in 1979 and although that's long gone, my wife bought me a non-working 3016 a few years ago for me to tinker with. Now we're in lockdown (Scotland), I have some time to devote to this, but I'm not sure I really know where to start.

I have a well-used multi-meter, can solder proficiently, am methodical and have spent all my time in IT so I know how to take computers apart and put them back together.

I don't have a scope, nor have I ever used schematics so I'm a novice there.

I have power - the rear of the CRT glows orange so that's a start...

I very much want to resurrect this PET, and recognise I need to learn along the way so if you can be patient and as clear with instructions, I'll be very grateful - If anyone can help me with some basic troubleshooting it would be very helpful.



SiriusHardware 1st Jan 2021 5:49 pm

Re: Non-working Commodore PET 3016
Hi Colin,

There was coincidentally a more recent thread concerning the same model.

Although your problem sounds different (dead?) have a read through that thread as the OP in that case was also at what he considered entry-level when it came to circuit diagrams, but he made great leaps as the thread progressed so maybe it will give you similar encouragement. There are also links to circuit diagrams, etc, in that thread which you will probably need to get to grips with to tackle this.

If you can solder and use a meter, that's you off to a running start. When you say you have power, are you judging that from the presence of a CRT neck glow or have you verified that all of the supply rails are present and at the correct voltage?

If you don't have a scope - which you really do need to consider obtaining for this sort of work - do you have a frequency meter or can your multimeter measure Hz / frequency?

ScottishColin 1st Jan 2021 6:10 pm

Re: Non-working Commodore PET 3016
Hi SiriusHardware

I've been pointed at that thread already by the OP from a Facebook post I made so I have already started reading that thanks and will continue to do so.

I have seen the CRT glowing so assume there is some power there and also have measured 16V DC across the voltage regulator on the motherboard so I think my power supply is good.

When you say "verified that all of the supply rails are present and at the correct voltage?" - that's the kind of level that I'm not at - how do I do that please?

My meter is a simple (cheap) B&Q meter and does not measure Hz/frequency I'm afraid. Do you have any recommendations regarding a scope? I don't think I'll need it for any other projects so one that is good value (ie cheap) would be good.

Thanks very much for the quick response.


SiriusHardware 1st Jan 2021 7:44 pm

Re: Non-working Commodore PET 3016
1 Attachment(s)
No problem, we'll shift down a gear and try to explain everything more explicitly.

Just as a preliminary, can you have a look at this PCB layout diagram (attached below) and tell me whether that matches your main PCB? Ignore the fact that it is for a 3032, just confirm whether the board layout is the same, with the ICs and other parts in the same places as in your machine. Also ignore the red arrows, as they were already there on this image when found online. Identifying the PCB will help us to determine which circuit diagrams to work from.

Also, do you know how the pins on ICs are numbered? It may be necessary to ask you to to take measurements from pin(x) of IC(y), so we'll need you to know how to identify the correct IC pins to do that from.

Mark1960 1st Jan 2021 7:51 pm

Re: Non-working Commodore PET 3016
Hi Colin,
Try measuring 5v power supply at each IC on the board first. On the 14 to 20 pin ICs this should be top left 5v and bottom right 0v as viewed when reading the markings on the top of the IC. That would put a pip or notch in the plastic of the IC at the left end of the IC.

As you already said this was bought non working, check for any empty sockets that might have been robbed in the past.

This is only a general guide to locating power on the ICs so let us know the details of any that donít have 5v.

Iím not sure if the 3016 has multiple supplies at 12v, -5v or -12v, but check the 5v first.

Scopes are a bit pricey for a first start, but you might find a simple logic probe will help a lot. Something like this one, but note Iím only using this as an example, not recommending it.
Mine is from the dark ages made by micronta. You want to find one that can show high, low or undetermined levels and able to detect fast pulses.


Dennis M 1st Jan 2021 7:51 pm

Re: Non-working Commodore PET 3016
I’ll have to look in the loft.
I did a course on repairing these in the 80’s and think the folder is still up there.
Should have the course notes, schematics and pcb layouts in it.

SiriusHardware 1st Jan 2021 8:05 pm

Re: Non-working Commodore PET 3016
As this is a 3016, some of the RAM sockets are likely to be empty even when the machine is fully working. If Colin can confirm his PCB is like the one in the image in #4, we can then explain how and where to measure the various supply rails and where to look for any critical ICs, such as the PROMs, which may be missing.

ScottishColin 1st Jan 2021 8:57 pm

Re: Non-working Commodore PET 3016
2 Attachment(s)
Hi - I attach a photo of my motherboard which looks like the one in the previous attached diagram. There are two banks of TI 4108 RAM chips.

My recollection on PIN numbers on chips is the attached diagram - hopefully that's right?

Thanks very much.


SiriusHardware 1st Jan 2021 9:12 pm

Re: Non-working Commodore PET 3016
2 Attachment(s)
Regarding IC pin numbering, yes that's right. Sorry to have had to ask, just trying to avoid telling you things you already know.

This attached detail from the layout diagram shows where to measure the regulated supply voltages, all with the black lead of the meter on 0V / GND.

I have attached it as a .jpg image and also wrapped up in a .zip file. The version in the .zip file is larger and easier to read.

Note that there are two +5V regulators, one +12V regulator and one -5V (minus five Volts) regulator. All of these voltages should be present when the machine is powered on.

SiriusHardware 1st Jan 2021 9:32 pm

Re: Non-working Commodore PET 3016
I should have said that if there is no obvious 0V / GND point for you to put the black lead of your meter on, the right hand end of the large capacitor with the cable tie around it will be connected to 0V / GND.

ScottishColin 1st Jan 2021 10:23 pm

Re: Non-working Commodore PET 3016
Thank you - I will do this tomorrow and let you know.


SiriusHardware 2nd Jan 2021 12:08 pm

Re: Non-working Commodore PET 3016
1 Attachment(s)
If those voltages are as they should be, could you then let us have a well-lit overhead image of the right hand side of the PCB - especially the area I have marked with a red box on the attached image of the layout.

Try to get the image sharp and in focus as we need to be able to read which ICs are fitted and which ones, if any, are missing. Bear in mind that the forum reduces any attached images to 800 * 600 resolution, it may be necessary to attach the original image as a .zip as well so we can see the detail.

ScottishColin 2nd Jan 2021 5:30 pm

Re: Non-working Commodore PET 3016
1 Attachment(s)
Hi - I got some voltages that wobbled about so I have purchased a new meter to try to eliminate that. However, this is what I have today reading from left to right on your diagram:

VR6 -12V
CR12 moved between 1 and 20V
CR11 +12V
CR10 +5V

Photo attached, along with food for thought judging by what the voltages should be.....


SiriusHardware 2nd Jan 2021 6:58 pm

Re: Non-working Commodore PET 3016
Nice photo. It shows that all the sockets which should have ICs in, especially the PROM sockets, do. The three empty sockets are for 'optional' ICs.

The voltage measurements on first reading do sound worrying, but, my apologies, I should have made clear that the measurements needed to be taken from the nearest of the three legs on VR6 to you, and from the right hand leads of CR10, CR11 and CR12 - not from just anywhere on the components which the red lines point to, but from the exact leg or lead which the red lines on the diagram in post #9 point to.

With that in mind, could you try those measurements again and see what you get this time?

ScottishColin 3rd Jan 2021 4:43 pm

Re: Non-working Commodore PET 3016
Hi - I have a happier set of numbers now:

VR6 -12v
CR12 +11.95v
CR11 +5.07v
CR10 +4.97v

Is the slight difference to the stated numbers anything to worry about?



SiriusHardware 3rd Jan 2021 5:20 pm

Re: Non-working Commodore PET 3016
That looks better although the output from VR6 (the minus 5V regulator) still looks wildly wrong. Could you carefully measure the voltages on all three of the pins of that device and see what voltages you find on the other two pins? Also please read the device number off VR6, it will probably be 79(something).

The other voltages are good, only several hundredths of a volt off what they are supposed to be, that's quite normal. We'll take that.

Can you now give us a bit more detail as to background - you got this as a non-worker and you've therefore never seen it work yourself, we know that - is there anything more you can tell us with regard to its history - did the previous owner have anything to say about it - and what are the current fault symptoms exactly?

On your original wide area photo I thought I saw what possibly might be rust stains or evidence of liquid corrosion, but I wasn't sure. What's your own impression of the general condition of the machine and the PCB?

SiriusHardware 3rd Jan 2021 6:05 pm

Re: Non-working Commodore PET 3016
For information, and for the benefit of anyone who is following or would like to pitch in, the PSU section diagram I am working from is this one:-

ScottishColin 3rd Jan 2021 7:16 pm

Re: Non-working Commodore PET 3016
3 Attachment(s)
The voltages seem to take a minute or so to stabilise after powering on - does that make sense?

VR6 pins top to bottom:


I am using the earth screw as the earth point - I have also tested using the right end of the large capacitor and got the same answers.

The VR6 is marked LM7905CT

With regard to the PET, I bought it as a known non-worker for £50 off ebay in 2012. The case is rusty and will need attention which I'm comfortable with. It came from a shed in a garden in Dumfries.

Current fault symptoms - nothing on the screen so I find it tricky to give any more detail than that. I have three external cassette drives for when I make some progress - some of my original cassettes are also in the attachments fyi.

I attach some photos of its current state and some photos of the 2012 state.

In 2012, I cleaned the motherboard with 99% IPA to get things a little clearer.

Mark1960 3rd Jan 2021 7:24 pm

Re: Non-working Commodore PET 3016
It is odd that the -5v is measuring so accurate as -12v, I’d agree with Sirius to check the markings on the device, but maybe also take a close look to see if it might have been changed at some point. Look for signs of hand soldering or post a close up of the back of the board.

Edit: just saw colin’s reply, so looks like -5v supply is ok after all.

Timbucus 3rd Jan 2021 8:14 pm

Re: Non-working Commodore PET 3016
Looking at the photos of the motherboard is that just dirt or is there corrosion? You will need to clean that off to get anywhere. A If it is only small areas some isopropyl alcohol on a cotton bud will be enough.

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