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.

Thanks.

Colin.

Re: Non-working Commodore PET 3016
 

Hi Colin,

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

https://vintage-radio.net/forum/showthread.php?t=163589

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?

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.

Colin.

Re: Non-working Commodore PET 3016
 

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.

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 don’t have 5v.

I’m 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 I’m only using this as an example, not recommending it.
https://www.amazon.ca/Reinly-Frequen...30258&sr=8-116
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.

Mark

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.

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.

Re: Non-working Commodore PET 3016
 

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.

Colin.

Re: Non-working Commodore PET 3016
 

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.

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.

Re: Non-working Commodore PET 3016
 

Thank you - I will do this tomorrow and let you know.

Colin.

Re: Non-working Commodore PET 3016
 

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.

Re: Non-working Commodore PET 3016
 

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.....

Colin.

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?

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?

Thanks.

Colin.

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?

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:-

http://www.zimmers.net/anonftp/pub/c...N/320349-9.gif

Re: Non-working Commodore PET 3016
 

The voltages seem to take a minute or so to stabilise after powering on - does that make sense?

VR6 pins top to bottom:

+0.02v
-12.40v
-4.97v

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.

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.

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.

Re: Non-working Commodore PET 3016
 

This is what the motherboard looks like now - happy to clean it further if you think it needs it?

Thanks.

Re: Non-working Commodore PET 3016
 

It does not look too bad - I assume the close up shows the missing solder mask only and not bare board on the Ground line? Check anywhere there seems to be any furry marks - especially between pins or between the leads of capacitors/resistors and give it a light brush - be careful not to use nylon if possible - make sure you have a ground strap and do not do it for too long if you have to.

Re: Non-working Commodore PET 3016
 

There's some evidence of green corrosion on the right of the last photo, and a bit of grey fur in other places.

I also note that there ar lots of tantalum bead capacitors all over the board. In other devices they are a 'change on sight' component. Generally though they will be on the power supply rails, and they seem to be at the right voltages so may be something to worry about later.

Re: Non-working Commodore PET 3016
 

My understanding is that tantalum's are usually fine as long as they have plenty of headroom on the voltage for transients they will experience in use - I.e. 10V tants on a 9V line will likely go bad. I think they are 25V ones so hopefully they have a bit.

I agree with Duncan U11, UG2 and EF1 def needs the green dealing with and the leg on UH2 seems rusted as well and if it should not be connected the the rail below it may be shorting to that.

Re: Non-working Commodore PET 3016
 

Back to work tomorrow so unfortunately an early night for me otherwise I won't be able to get up at the right time in the morning, but just to observe that VR6 does seem to be OK now that you are looking for minus 5V on the -bottom- pin as indicated on the diagram attached in #9. ;) So we can now say that the supply voltages are all OK, which is a good start. Good work, Colin.

With no display you are going to start to feel the need for more than just a meter fairly soon, do you remember I asked you if your meter could measure frequency? You said your old one could not, but can the new one?

The monitors on these are quite primitive, they don't have their own onboard free-running line oscillator and instead rely on receiving a steady stream of sync pulses from the main computer PCB in order to activate that part of the monitor circuit.

If you don't have a picture on the monitor there are two main possibilities,

-The mainboard is not generating the sync pulses which the monitor needs, therefore the monitor is not being enabled.

-The mainboard is generating sync pulses, but the monitor has a fault.

To determine which of the above cases is true, the ideal tool at this point would be a scope or, failing that, something which can measure frequency because when the mainboard is working it generates pulse waveforms which have steady frequencies on the horizontal and vertical sync signal outputs to the monitor and if they are there, the frequency of those signals can be measured. We need to know whether those sync waveforms are there or not.

Conventionally, you can detect the sync waveforms with a scope, with a frequency meter, or with a 'logic probe' as suggested by Mark earlier. I'm open to suggestions for any related checks or any other checks which can be done with just a meter to narrow things down further.

One thing which has been said to be a problem on these is bad IC sockets - if you have the confidence to do so, with the power off, try removing each socketed IC one at a time, inspecting them for bent pins and then reinserting them, taking care not to bend or fold any of the pins over as you do so. Try powering the machine on again after reseating each IC. Take plenty of photos before you start and only remove and reinsert one IC at a time so you can't ever get two of them mixed up.

Edit: Just looked at the images and it does seem to have suffered from damp / moisture. If it has caused actual problems, ie, corroded away tracks or VIAs this could be be quite a challenge.

Re: Non-working Commodore PET 3016
 

Hi - thanks for your patience through this. This is the meter I bought:

https://www.amazon.co.uk/gp/product/...?ie=UTF8&psc=1

I attach a photo showing the frequency specs. Let me know if it won't do the job and I'll ask around to borrow a scope.

I'll get the motherboard out today and give it a better clean as per the messages above. I have a chip puller so I'll pull them out as suggested and examine them.

Thanks again.

Re: Non-working Commodore PET 3016
 

I think you've hit lucky (or chose well) as regards your choice of meter as it does indeed have a 'Hz' (frequency) section on the dial. It claims to be able to read up to 10MHz, if that's true it could prove quite useful for this project.

Re: Non-working Commodore PET 3016
 

I don't suppose this will help - leg missing on chip from UD6....

Wasn't me; honest.

Colin.

Re: Non-working Commodore PET 3016
 

Photos.

Re: Non-working Commodore PET 3016
 

I think that is A7 on one of the ROM's so no it wont help - you can usually solder a pin from another scrap tip if there is a stub - you may be lucky.

Re: Non-working Commodore PET 3016
 

Well, it's good that you found that. If this had been any ordinary logic IC we would have said just replace it. Unfortunately that IC is one of the PROMs, it contains factory programmed data which is specific to this model or model range so it is a chip which you can't easily 'just replace'.

Can you see if the other half of that snapped IC lead is still inside the corresponding socket hole? If it is you are going to have to get it out or remove and replace the whole socket. If you do have to replace the whole socket you could replace it with a turned-pin type, plug the chip into it and spot-solder the broken pin to the 'well' on the top of the socket contact.

If the socket hole is vacant what you can do, if you feel up to it, is to first find an unused new component like a resistor or capacitor which has leads which are a nice smooth fit in the hole in the socket, and then solder the end of one lead to the inside of the remainder of the snapped pin. Once you've got it in the right place and at the right angle, trim the lead off to the same length as the other leads on the device. The attached crude sketch illustrates what I mean.

You can also bend the original upper half of the pin inwards a bit and solder the 'new' lead to the outside of it instead. That would be easier to do but it will make the repair more obvious. You may not mind about that as long as it works.

Edit: Top tip from Tim, I've never thought about using another whole leg from a duff chip (rather than a bit of component lead) as a replacement leg. Will have to remember that one.

Re: Non-working Commodore PET 3016
 

Hi - I clipped a leg of a potentiometer I had lying around and soldered that on. It's all back in the socket again, but no joy on the screen.

Attached are photos of the soldering for any criticism, plus a phot of the top of the chip showing the number.

Re: Non-working Commodore PET 3016
 

I'm honestly surprised you managed to get a potentiometer leg to go into an IC socket but clearly you did, so... continue on around the rest of the socketed ICs if you have not already, you may find one or two more like that, or you may find one which has been removed in the past and put back in with a pin folded over. (That is very easy to do, still happens to me sometimes even now).

In the meantime we'll muse over what to look at next. I think probably the simplest go / no go indication for the mainboard is to see whether it is producing sync pulses for the monitor and work forwards or backwards from there. I'll look into where to look for that: Back shortly.

Re: Non-working Commodore PET 3016
 

Looks really good - now we need Sirius magic numbers to measure with your frequency equipped meter - he may correct my view below

(Aside: I was looking at that very one the other day but settled on a set of their test leads for my old meter which is all I really needed... now maybe I will reconsider).

I expect we will be working in this area of the board

http://www.zimmers.net/anonftp/pub/c...N/320349-6.gif

- I think the circuit in the top left is the master clock but, its 16Mhz frequency will be beyond your meter so we may need to look at a later part of the chain - it seems to be generating a lot of phases.

You could usefully check the 1Mhz (Pin 7) and 8Mhz (Pin 3) outputs on the chip just below it G5 74191. As you are using a multimeter you should be safe enough in that area to measure it in reference to the ground pin on that chip (pin 8) https://www.elektropage.com/default.asp?tid=597

Re: Non-working Commodore PET 3016
 

I have checked all of the chips and none have bent or missing pins.

Thanks again all for you patience and help - I really appreciate it.

fyi I have not done anything about the monitor electronics - no cleaning or anything.

You'll need to be patient/specific with me regarding testing frequencies - I've not done that before.

Thanks.

Re: Non-working Commodore PET 3016
 

Its a pleasure the one bit that is good on this forum is people have patience and enjoy helping.

As I have never used a meter to test frequency before I am going to guess you set it to Hz and then put the black lead on Pin 8 (rest it on the base of the pin so it is firm) and then touch the pin with the red probe to measure 7 for 1,000,000 Hz (1Mhz) and pin 3 for 8Mhz - Be very careful not to slip to short leads - that will likely be fatal to one or more of the chips.

Re: Non-working Commodore PET 3016
 

As an aside - in case the monitor electronics are causing a problem (that is an area due to the HT - thousands of volts- that you need to be very careful around) you can always unplug the monitor connections on the corner - this board should still run fine while we are just looking at signals to see if the board is operating.

Re: Non-working Commodore PET 3016
 

I think the relevant diagram for the timing circuit section is this one:-

http://www.zimmers.net/anonftp/pub/c...N/320349-6.gif

Among other things, this part of the circuit generates the 'Horizontal Drive' and 'Vertical Drive' signals. You can find these signals on pins 3 and 5 of connector J7 which looks to be the mainboard's output connection to the monitor.

If those connections are difficult to get a physical connection to you can, alternatively, find the 'Vertical Drive' signal on pin 11 of IC 'UG10' and the 'Horizontal Drive' signal on pin 2 of IC 'UH7'. Try looking at each of those (with the power on of course, and with the meter in 'Hz' mode) and see if you have steady frequencies on both of them. 'Vertical Drive' I would expect to be in the region of 50-70Hz, and 'Horizontal Drive' somewhere in the tens of Khz. If you do find steady frequencies on those points it would also be interesting to know what the 'Duty Cycle' (%) figure is for each of those signals as well - that's another unusual, but potentially useful feature your new meter has.

If you don't find anything which looks like a steady frequency on those two points try looking at pin 3 of IC UG5, where you should see a frequency of 8.00MHz, and pin 7 of IC UG5 where you should see a frequency of 1.00MHz. (The main clock frequency, at 16MHz, is a bit too high for your meter to be able to measure).

All measurements to be taken with the black meter lead on a known 0V / GND point.

Edit: Crossed with Tim, who said much the same in more concise terms.

Re: Non-working Commodore PET 3016
 

Is that the only good bit? ;)

Re: Non-working Commodore PET 3016
 

The other bit is they have a very dry humour

Re: Non-working Commodore PET 3016
 

Not much in the way of readings this time.

UH7
Pin 3 - solid 8Mhz @ 55%
Pin 7 - no reading

No readings on any of the other suggested pins.

Colin.

Re: Non-working Commodore PET 3016
 

Have a look at pins 2 and 6 of the same IC (UG5) - what frequencies (if any) do you see on those pins?

(Incidentally, I assume you meant UG5 in post #41, not UH7)

Re: Non-working Commodore PET 3016
 

I did mean UG5 - apologies

Pin 2 - rock solid 4Mhz @51%
Pin 6 - rock solid 2Mhz @51%

Colin.

Re: Non-working Commodore PET 3016
 

I have work early tomorrow so will have to drop out now but, I wonder if the lack of the 1Meg main Processor clock is because it is dying due to a fault on the CPU or a short on it somewhere, or if this circuit has a wait state system that suppresses that clock - I will need to study the circuits a bit closer as I have never worked on a PET.

It may not seem like it but, we are making good progress - the clock net is both the most complex and most important part of any circuit so proving it is acting as it should is 50% of the problem usually.

Re: Non-working Commodore PET 3016
 

Without that 1Mhz signal I do not think any of the phases will be generated as it feeds U5 the shift register NAND inputs on pins 1 and 2 (It is a 74164) so something is likely suppressing it as the 8,4 and 2Mhz is running it is likely not the Chip UG5.

Re: Non-working Commodore PET 3016
 

Right, that's interesting. As you (Colin) may realise, that IC (UG5) takes in the 16Mhz from the main clock and divides it by two several times. Each successive division is available from the QA, QB, QC and QD outputs. Of these, only the QA (8Mhz) and QD (1Mhz) outputs are used in this circuit but your last measurements demonstrated that the QB (4MHz) and QC (2MHz) outputs are also working - also that the frequency measurement feature on your meter is working very nicely too.

However, the QD (1Mhz) output of that IC either does not seem to be working, or it is being prevented from working by something that it is connected to. As (bad) luck would have it that 'CLK1' signal goes to quite a few different areas on the rest of the mainboard.

The 'Flag' at the end of the CLK1 signal line coming out of UG5 pin 7 on the drawing (with the numbers [1,5,7] inside it) indicates that this 'CLK1' signal goes to other points also marked 'CLK1' on circuit drawings 1, 5, and 7 (this is circuit sheet 6 that we are using at the moment).

It's probably a bit late to go hunting off down those signal paths tonight, so I'll leave you with one final check from me:-

With power off, meter on its lowest ohms / resistance range, what resistance do you measure with

-Black probe on UG5 pin 8, Red probe on UG5 pin 7?
-Black probe on UG7 pin 7, Red probe on UG5 pin 16?

(This check is to see whether the 'CLK1' line is shorted to, or has a very low resistance to, either 0V or +5V).

I'm guessing that UG5 is not in a socket?

Edit: Again, cross posted with Tim.

Re: Non-working Commodore PET 3016
 

One final follow-up for tonight, I've looked at where CLK1 does go to. As Tim observed it goes to the clock input on the 6502 but it also goes to a couple of other places as well.

On sheet 1 - goes only to UC4, (The 6502) which is in a socket
On sheet 5 - goes to 4 * 74LS153 (UE3, UE4, UE5, UE6) - not in sockets?
On sheet 7 - goes to 3 * 74LS157 (UF3, UF5, UF6) - not in sockets?

Re: Non-working Commodore PET 3016
 

Test results

UG5 pin 8->UG5 pin 7 - 0 ohms
UG7 pin 7->UG5 pin 16 - 0.7 M ohms


Socket information:

UF10 - 901447-10

UD9 - 901463-03
UD8 - ?
UD7 - 901465-02
UD6 - 901465-01
UD5 - empty
UD4 - empty
UD3 - empty

UC7 - 6520
UC6 - ?
UC5 - 6522
UC4 - 6502

Colin.

Re: Non-working Commodore PET 3016
 

This one:

...looks very promising. If that reading is really true there is a short between some point on the CLK1 signal line and a nearby 0V. That would certainly kill the 1MHz output from UG5.

Although there are quite a few devices on that CLK1 line it would be very unusual for a semiconductor device to fail absolutely zero ohms short-circuit - several ohms or a few tens of ohms would be more typical, so my thinking at this stage is that you are more likely to have a physical short, like two component leads bent over and touching, or a solder bead or solder splash between the CLK1 signal line and a nearby 0V point.

Your mission (should you choose to accept it) is to follow the CLK1 track which sets out from UG5 pin 7 and follow it to every other place that it goes to on the board, all the while looking for possible shorts to nearby 0V / GND areas.

Afterthought: As the 6502 is in a socket, you may as well try removing it and seeing if that short between UG5 pin 7 and UG5 pin 8 clears when the CPU is out of the board. If it's still there with the chip out, put the 6502 back in and continue looking for a physical short between the CLK1 track and other tracks next to it.

Re: Non-working Commodore PET 3016
 

It is unlikely with 0ohms but, a rusty mark between two very close tracks or pins could create such a short as well so a thorough review of that track inch by inch is worthwhile - note there may be vias (joints between sides of the board) and anywhere it is on an IC pin it could split off both sides as well so like playing a text adventure keep a map marking Vias and Chip/Pin numbers so you explore all parts of the tree as it expands. It is easy to follow a path and forget to go and trace another.