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Commodore Pet 3032 no power


Experienced Member
Nov 23, 2022
I just bought a very nice none working Pet 3032. The tube glows upp but it wont light up. So I have dissconnected the CRT.
The computer motherboard is model 320380. The motherboard is untoched and no previous attempt of repair is visible. It had a very thick layer of dust when I open this baby up. So I guess it never been open.
I cant find this schematic online.

Here are some readings.
big cap 4700uf 22,54v
470uf cap gets -11,87v
voltage regulator VR6 reads -11,87 in 5,08v out
RAM chips reads 11,78V
CPU reads 0,44V???

Can someone please give me a point in the right direction and save another PET :)
As this is your first post - welcome to VCFED.

Don't forget that you will be under moderation for the first 10 posts and to add your location in the world to your profile (if you would like).

First things first. What test equipment do you have and can you read a schematic?

We can point you at the schematics presently.

The PET contains a number of separate voltage regulators, and the IC pins you need to measure (especially on the RAM and CPU) are not necessarily obvious unless you have the schematics and/or the device pinouts from the data sheet.

Thank you Daver2!

I have a multimeter, logic probe , and a ocilloskope. I have done alot of repair on C64 and Amiga but never on a PET. I am no expert with schematics but it has helped me before.

I have now recapped the board. I changed all 10uf and 47uf caps and the 4700uf ,470uf.
I have also pulled the 6502 and test it in a Vic20 and its OK.

Unfortunately the fuse blowed when I started it up again. Its a 32mm 250v slow-blow fuse.
I dont know the amp on it thou. Would a 2A be fine?
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I spent quite a bit of time in nykoping in my dim and distant past. A very nice place.

The fuse rating should be marked on the rear panel of the PET (if it hasn't been removed of course).

The available schematics for your 3032 should be here: http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001N/index.html.

Look at sheet 9 for the voltage regulators. There are 4 of them to measure. Multimeter black probe (negative) to 0V/GND and use your red probe (positive) to take a measurement with. If you have a digital multimeter, the -5V rail should indicate correctly. If you have an analogue meter, you may have to reverse the probes to take a reading.

There are two +5V regulators (VR3 and VR4), one +12V regulator (VR5) and one -5V regulator (VR6). Use the red multimeter probe to measure on each lead of diodes CR10, CR11, CR12 and CR13 respectively in turn (8 voltage measurements in total).

The 'higher' voltage of each lead is the input voltage to the associated regulator with the 'lower' voltage the output.

Am I making sense?

It may be worth checking the layout diagram at http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001 to make sure this looks like your actual board before you start though.

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Nice to hear Daver! I never been there actually. But I have been i Oxelösund its a city a couple of miles to the east. :)

Yeah I think the fuse rating has been removed. I will order a 2A fuse and then I will come back with the multermeter readings.
Thank you for the link to the schematic, and thanks for all the help Daver.

Hmmm I wonder if I have a strange revision of my motherboard 320280, my board has those 8116 ram chips. BTW is it possible to use 4116 ram? and mix them with 8116?


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The 8116 and 4116 DRAM are the same. You can mix and match. You also need to check the suffixes after the number (these identifies the speed amongst other things).

The wisdom I have found is a 1.6A slow blow fuse for 110V and 0.8A slow blow fuse for 220V. But, I will have another look for you when I have a bit more time.

I am familiar with this Dynamic PET board, it is the same as the one in my PET.

Daver2 noted that the 4116 and 8116 DRAMs are the same. Sometimes they also go by other numbers too, Intel for example made a D2117, that is the same part.

Your particular DRAM IC's look very beautiful and the board looks like it will clean up very well. There is also little rust (if any) on the power transformer's lamination stack, so this PET, apart from the dust must have been stored in very dry conditions.

If the CRT heater was going, it means that at that time at least, there was AC power coming from the power transformer to the VDU, and that the VDU's own regulated DC supply was probably working. The VDU circuitry is usually fairly reliable, the usual cause for it not lighting up is a fault on the main board, where the VDU does not receive the H.Drive signal, which it requires for the generation of the high voltages to run the CRT.

The first move in the repair is to make sure the main board's power supplies are working as you are doing. Re-seat the IC's in case there is a bad IC socket connection and after that confirm the CPU is running and look to see if the board is generating the H & V drive pulses for the VDU on the scope. The IC sockets in the PET are not the most wonderful design and rely on a single wipe of one side of the IC pin only, the other side of the IC pin abuts the plastic.

If you tuned out later to have a RAM problem, it would be a shame to remove any of those IC's by mistake. Where any of the RAM outputs are stuck high or low, this prevents the computer from booting and creates a black screen. Though in this case, if you look during the boot up (if all else was normal) there is a flash of garbled characters on the screen as the 555 reset circuit operates.

If you need it there is some information in this article about the DRAM in the dynamic PET, how it operates, and remarks about how to interpret tests to find the faulty IC's. Also 20 or so scope recordings of the DRAM support circuitry that can help in fault finding:

There is a program there at the end of the article, to burn into a TMS2532JL ROM which has memory fill and memory test firmware. If memory below 0400h is working and supporting BASIC you can run those programs without the hardware adapter in the article to check the DRAM from 0400h to 7FFFh.

But there are also various PET-testers too, which are easier & helpful designed by Daver2 and Nivag.

With any luck though, your lovely ceramic DRAM IC's will be ok.
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Thanks guys for all the advices!
I belive we are gonna make this machine work again :)

Im now waiting for fuses, and a 4700uf cap.
The cleaning of the board went well. I'll attach 2 pictures in this post.


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I thought that board would clean up well.

Yes those sockets are the classic white single wipe types.

Generally on these PET dynamic main boards the Electrolytics are fine and don't require replacing. Some boards used more TANT caps than others and they give trouble sometimes because they short out and should be replaced. I think you will find if you subject the electrolytics you have removed from the board to testing for ESR, uF value & leakage, the yellow jacket ones around the DRAM bank and the others you have removed, they will be perfectly ok and they were replaced unnecessarily. It is worth asking for advice about what components should be replaced as a matter of course. On these boards, the Tants yes, but not the electrolytics unless there is obvious trouble with them.

Interesting in the VDU though, it can be worth doing a re-cap, but even in those, often the original capacitors measure ok as many were high quality Nichicon types. Don't replace what looks like a 10uF bipolar electrolytic cap in series with the Yoke's H coils. These are a special part and normally fine and can only be replaced by a film capacitor with a very low ESR, not a bipolar electrolytic.

The main capacitor that I have had trouble with is the large off board 23,000uF one. Others have had problems with it too. They can test ok on the ESR meter but still be faulty and they are over the range for most capacitance meters to verify the uF value. When they are sick, ripple appears in the signals feeding the VDU and because of the +feedback in the vertical scan stage, it makes the raster scan bounce up and down vertically, at a fairly low frequency. If you do need to replace the main capacitor, I would recommend the one in the attached photo from RS components.

The white sockets in my PET were ok. But it pays to check them pin by pin.

If you get a pin from a defunct IC , cut it off close to the IC, and solder a small wire handle to it, you can use it to insert into each IC socket hole and feel the tension and that the socket is working.

Don't put anything other than an IC pin in there as it can damage the socket. If one of these sockets gets any kind of an adapter inserted into them, with pins that are thicker than about 0.3mm, it de-tensions the socket claws and it ruins the socket for use with a real IC later, and in that case it is better to replace the socket. A typical IC pin is about 0.3mm thick.

The first thing I did when I got my PET was to make a duplicate set of ROMs with known good firmware and make Daver2's Pet-Tester ROM. These have come in very handy in fault finding & repairs. The better ROMs to use are the TMS2532JL and the 2716.


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Hugo Holden:
Yes it cleaned up real good.
I did a recap mostly becouse the 4700uf cap was out of range. I dident measure the others, but the computer is from the late 70's so I thought it wouldent hurt :)

I will take your advice and check those white sockets. Commodore had bad sockets on the C64 aswell and maybe these are even worse?

Thanks you for the information on the 22000uf capacitor. I will probably buy one of these right away actually. I have no ERS meter and I dont want any ripple voltage i my machine. So better be safe than sorry.
OK Daver2, I now have some readings.
cr10 11,9v and 349mv
cr11 11,9v and 211mv
cr12 22,97v and 11,77v
cr13 5v and 12v

CR10 and CR11? Maybe those TO-3 regulators are shooted? They dont make these anymore?
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So, assuming you have measured the voltages correctly, you are correct that there is something wrong with the two (2) +5V regulators VR3 and VR4.

VR5 looks OK: 22V in and 11.77V out.

VR6 looks OK: 12V in and 5V out (although I expect these to be -12v in and -5V out).

There are three (3) reasons for the +5V regulators shutting down:

1. They are both dead.
2. There is a short circuit between the +5V rail(s) and 0V/GND.
3. There is a low impedance between the _5V rail(s) and 0V/GND and the regulator is shutting down to protect itself and the PCB/circuitry.

We need to work out which condition applies.

Let me think about the best way to proceed.

EDIT: With the power OFF and your multimeter set to read a low resistance range, measure the resistance across the pins of capacitor C14 (47uF) and C16 (47uF). The first capacitor is on the output of VR3 and the second capacitor is on the output of VR4. Let's see if there is a low resistance there or not first.

Daver2 yeah I mesured it correctly,
You are correct it was -12/-5 on VR6
C16 mesures 350ohms
C14 mesures 54 ohms
54 Ohms would equate to 5 [V] / 54 [Ohms] = 0.092 [A] = 92 [mA].

There isn't a nasty short circuit anywhere (or at least it is not present with the power off).

At this stage I would either be taking one of the +5V regulators out and testing it outside of the PET with a 9V PP9 battery (as input) and a 100 Ohm resistor as a load from the output to 0V/GND and see if it works.

Alternatively, if you have a variac, you could slowly increase the voltage to the PET and monitor the output from the two +5V regulators and see if they start to come up and then 'crowbar' at some point. I would disconnect the power to the monitor for this test though (there should be a power plug/socket in the monitor housing.

Dave, I have taken all measurements with the monitor completely disconnected. I have a manual external PSU but I am not comfortable using this on the PET. Could you please explain in a little more detail how I would go about the first option with the 9v battery. I think this option would suit me
There are three (3) pins on a voltage regulator.

An input pin.
An output pin.
A ground pin.

Download the data sheet for the regulator part to determine which pin is which. Pay particular attention to whether the data sheet identifies the pins from the top of the device or the bottom.

Remove the voltage regulator from the PET.

Connect a 100 Ohm (or similar) resistor between the output pin and the ground pin of the voltage regulator.

Connect the negative end of the 9 Volt battery to the voltage regulator ground pin.

Connect the positive end of the 9 Volt battery to the voltage regulator input pin.

Measure the voltage across the 100 Ohm resistor with the negative lead of the multimeter on the ground pin of the voltage regulator and the positive lead of the multimeter on the output pin of the voltage regulator.

If everything is well, you should measure 5 Volts!