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PET 2001-N with non-functioning transformer

Dr. Killemov

Member
Joined
Oct 1, 2022
Messages
10
Location
Holts Summit, MO 65043
Hello, everyone! First time poster here, but I've been collecting for years. I have this 2001-N that I've finally decided to take and have it checked out, but alas, she is absolutely dead. It's obvious that the transformer isn't feeding the machine any voltage at all and I'd like to start there. Are there common issues with this transformer that I should look out for? Please assume I'm an absolute noob at any diagnostics (it wouldn't be too far from the truth, honestly) but that I do have a love for anything vintage and this would be my very first restoration project. It's a daunting task, from my POV but fascinating!
 
I have not heard many issues with the transformer itself. Most of the issues I hear with the power supplies on the PETs is the big blue capacitor being dead or leaking.
 
Hello Dr. Killemov, welcome to the forum. In order for us to be able to help we first need to get an idea of what you really feel your current level of knowledge is, and what sort of test / measurement equipment you have available - for example, whether you have a multimeter and know how to use it?

In your introduction you say that it's obvious that the transformer isn't feeding voltage, but how is it obvious? Are you making that deduction from an apparent lack of life / activity, or have you made some voltage measurements already?
 
And another PET surfaces for repair.

Welcome to VCFED. I would suggest that you get at least 10 posts under your belt before we start debugging in earnest, because your posts will be under moderation until then and could be significantly delayed until the moderators review them.

I second Sirius's post.

There are a couple of recent PET repair threads where we have already tackled measurements of the transformer voltages if you would like to do a bit of browsing. I'll hunt a link out for you later.

Dave
 
And another PET surfaces for repair.

Welcome to VCFED. I would suggest that you get at least 10 posts under your belt before we start debugging in earnest, because your posts will be under moderation until then and could be significantly delayed until the moderators review them.

I second Sirius's post.

There are a couple of recent PET repair threads where we have already tackled measurements of the transformer voltages if you would like to do a bit of browsing. I'll hunt a link out for you later.

Dave
First thing to do is check the fuse with a voltmeter. PET power supplies are very robust
 
Remove the fuse and check it with your multimeter set to a low value resistance range... This is the safest way! No mains voltages...

Disconnect the transformer connector from the PET main logic board and then we can probe the transformer SECONDARY connections with a multimeter set to 20V (or so) AC range. Disconnecting the PET prevents any damage to the logic board until we know that the transformer is behaving itself.

It would help if you could post a photograph of your particular transformer connections, including the wire colours. That way we can tell you what to measure.

One thing I would do at this point in time is to check that the mains plug earth pin is solidly connected to the metal case of the PET. Choose the lowest resistance range on your multimeter and check for a reading less than 1 Ohm (if you can). I would also use a residual current device to power the PET from the mains. That way, if you make contact with something you shouldn't, you live to post the saga! Safety first.

Here is the thread I was talking about https://forum.vcfed.org/index.php?threads/cbm-8032-capacitor.1238793/post-1269513. Start from post #34.

Dave
 
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The
Hello Dr. Killemov, welcome to the forum. In order for us to be able to help we first need to get an idea of what you really feel your current level of knowledge is, and what sort of test / measurement equipment you have available - for example, whether you have a multimeter and know how to use it?

In your introduction you say that it's obvious that the transformer isn't feeding voltage, but how is it obvious? Are you making that deduction from an apparent lack of life / activity, or have you made some voltage measurements already?
I say it's obvious to me, but really, it was obvious to the computer shop I had taken it to to get checked out. The gentleman there said that it was plain dead...absolutely no voltage coming from it, which I thought was a little odd, to be honest. I'm fairly proficient at using a voltage meter, but as for working on this particular machine, I have very little knowledge.
 
I have not heard many issues with the transformer itself. Most of the issues I hear with the power supplies on the PETs is the big blue capacitor being dead or leaking.
That's the thing, really. In my reading up on it, I came to the same conclusion. As noted in my response below, I had taken it to a shop to have it looked at. It was obvious to the gentleman that looked at it and he's been in business for 30-some-odd years so I just sort-of defaulted to his judgment because of that experience. I will, however, take things into advisement and do some poking around in there with my own voltmeter and start my own bug hunt. It's as good a place to start as any.
 
Hello Dr. Killemov, welcome to the forum. In order for us to be able to help we first need to get an idea of what you really feel your current level of knowledge is, and what sort of test / measurement equipment you have available - for example, whether you have a multimeter and know how to use it?

In your introduction you say that it's obvious that the transformer isn't feeding voltage, but how is it obvious? Are you making that deduction from an apparent lack of life / activity, or have you made some voltage measurements already?
Remove the fuse and check it with your multimeter set to a low value resistance range... This is the safest way! No mains voltages...

Disconnect the transformer connector from the PET main logic board and then we can probe the transformer SECONDARY connections with a multimeter set to 20V (or so) AC range. Disconnecting the PET prevents any damage to the logic board until we know that the transformer is behaving itself.

It would help if you could post a photograph of your particular transformer connections, including the wire colours. That way we can tell you what to measure.

One thing I would do at this point in time is to check that the mains plug earth pin is solidly connected to the metal case of the PET. Choose the lowest resistance range on your multimeter and check for a reading less than 1 Ohm (if you can). I would also use a residual current device to power the PET from the mains. That way, if you make contact with something you shouldn't, you live to post the saga! Safety first.

Here is the thread I was talking about https://forum.vcfed.org/index.php?threads/cbm-8032-capacitor.1238793/post-1269513. Start from post #34.

Dave
I will absolutely give this a shot, Daver2, once I get it back to the house, come Monday morning, and I'll delve into that thread ASAP.
 
Excellent.

Is it possible for you to add your location to your profile. We know where you are located then (there maybe VCFED members nearby) and what timezone you are operating in.

Dave
 
Checkout the mains power supply at: http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/8032/8032051-3.gif. This is pretty much applicable to all PETs.

The tests below are with the PET *** DISCONNECTED *** from the mains supply.

The things in the PRIMARY side of the transformer (mains = lethal side) is the plugtop, the mains cable, the power connector in the PET, the fuse and fuseholder, the switch and the transformer primary winding.

With an intact fuse - and the PET's switch ON - you can measure the resistance (using a multimeter) between the LIVE and NEUTRAL pins of the plugtop. This should be a fairly low resistance. When you turn the PETs switch OFF, this resistance should increase to infinity (an open circuit). This is a good test for the mains side BEFORE plugging it into the mains.

*** WARNING *** Do not hold onto the probes of the multimeter (or the pins of the plugtop) whilst performing this test. The back emf generated when you disconnect the multimeter probes as a result of the collapsing magneting field within the operational transformer will 'nip you'... I usually use crocodile clips (or similar).

If you don't get a low resistance, you can chase the circuit around with your multimeter until you find which component is broken. However, just watch any back emf jolts. They tingle a bit!

Dave
 
Checkout the mains power supply at: http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/8032/8032051-3.gif. This is pretty much applicable to all PETs.

The tests below are with the PET *** DISCONNECTED *** from the mains supply.

The things in the PRIMARY side of the transformer (mains = lethal side) is the plugtop, the mains cable, the power connector in the PET, the fuse and fuseholder, the switch and the transformer primary winding.

With an intact fuse - and the PET's switch ON - you can measure the resistance (using a multimeter) between the LIVE and NEUTRAL pins of the plugtop. This should be a fairly low resistance. When you turn the PETs switch OFF, this resistance should increase to infinity (an open circuit). This is a good test for the mains side BEFORE plugging it into the mains.

*** WARNING *** Do not hold onto the probes of the multimeter (or the pins of the plugtop) whilst performing this test. The back emf generated when you disconnect the multimeter probes as a result of the collapsing magneting field within the operational transformer will 'nip you'... I usually use crocodile clips (or similar).

If you don't get a low resistance, you can chase the circuit around with your multimeter until you find which component is broken. However, just watch any back emf jolts. They tingle a bit!

Dave
If you have a good fuse, the next item to check is the line filter. This is a small silver barrel unit the size of your thumb hidden behind the transformer. It has 2 blue wire and 2 black wires coming from it. 40 years ago this filter was needed to stop the pet from interfering with TV reception. Now that we have digital TVs it is not needed and can be removed.
However the wires into and out of this filter must be correctly rerouted. A faulty line filter can keep blowing fuses.
A faulty transformer is the least likely problem with Dr. Killemov's computer.
Commodore Pets, 8050 drives and printers all use fuses. It is a good idea to have spare fuses for all these units.
 
If you have a good fuse, the next item to check is the line filter. This is a small silver barrel unit the size of your thumb hidden behind the transformer. It has 2 blue wire and 2 black wires coming from it. 40 years ago this filter was needed to stop the pet from interfering with TV reception. Now that we have digital TVs it is not needed and can be removed.
However the wires into and out of this filter must be correctly rerouted. A faulty line filter can keep blowing fuses.
A faulty transformer is the least likely problem with Dr. Killemov's computer.
Commodore Pets, 8050 drives and printers all use fuses. It is a good idea to have spare fuses for all these units.
I will take all this under advisement and take lots of pics in the process. I really appreciate the help. I'd like to see her at least powering up in some way before Halloween and this goes a long way towards that goal.
 
Hello,everyone! I'm sorry to have been gone so long, but I've been stuck up in the hospital for almost two weeks with viral pneumonia and my kidneys decided they weren't gonna work that hard and darned near just shut down on me. I'm back though, and not feeling too terribly bad, so I'll be checking in with y'all with some progress on my Pet, probably sometime Saturday. See you all then!
 
Sorry to hear you have been in the wars Dr. K. As much as we love our old computers here, I think you were right to prioritise the repair of your own body first. We'll pick up again when you are ready.
 
Okay...finally over pneumonia and all the aftereffects of it all and so I finally dug into the PET. Here are a few pictures of what's going on inside it. The wiring had been messed with, that's for sure. The black wires on the mains side were complete and hadn't been cut and rerouted, it looks like. However, the blue wire coming directly in from the power cord was wired to the white wire. The green ground was correctly wired as well. The red wire on the mains side is tied up and hasn't been used at all, so that's as far as I've gotten. I made an executive decision to remove the transformer completely(taking care to note what wire goes where) and see if I can probe it, wire it back in the way it was and check it again. That may seem to be a needless step, but I really wanted to get a good look at the underside of the transformer anyway. Attached are pictures of what I've discovered.
 

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This has been seriously butcherd!

We need to tread carefully, as the mains side has been disturbed, possibly by someone not knowing what they are doing.

Have you found the schematic of the transformer wiring online?

Dave
 
Ok, back from Church. I was streaming both services today - so pressure on!

The schematic for the transformer section is here http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/8032/8032051-3.gif.

Make doubly sure that the mains wiring earth is connected to the PET metal case.

The mains blue wire should be connected to the transformer white wire.

The transformer red wire is unused.

The mains brown wire should be connected to the fuse, that should be connected to the switch, that should be connected to the transformer black wire.

I, personally, would have connected the brown live mains wire to the switch, with the fuse after that...

Do NOT plug it into the mains until we have performed some tests with a multimeter though.

Does this make sense to you?

Dave
 
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