Commodore PET 4032/8050/3022

[ScottishColin]

This will keep me busy for a while. Hopefully I've learnt something from last time and will trouble you less.

There's a couple of extra ROMs in there - one labelled SUPERCHIP the other has a label but the writing has worn off. I guess I should remove them before I start to get them out of the way.

There's a couple of interesting patch wires which look like they were done at the factory?

I also suspect I will need help with the printer. I'll keep you informed.

Colin.

[SiriusHardware]

Sounds like you need a 'My other computer is a PET' sticker to put on one of your PETs.

Does the computer actually work?

[ScottishColin]

I've not tried anything yet - I've just driven there and back (Motherwell) to pick it all up. I'll give it all a try tomorrow.

Colin.

[ScottishColin]

So having blown a fuse, I've stripped it down (ish) - I don't think this wire across the on/off switch connectors is a very good idea.....

Now I have to find some 0.8A glass fuses.

Colin.

[SiriusHardware]

Lucky the fuse did blow, to be honest. I can only imagine whoever did that thought they were trying to bridge a faulty power switch.

The machine looks quite nice on the outside, but like your other 'celebrity' PET, seems to have spent some time in a damp atmosphere. Is there a story?

[ScottishColin]

The wire was firmly wrapped around the switch connectors but the ends were out so I suspect they may have shorted to the chassis.

It's gone now.

I have realised that I got too eager so I've started to strip it down, clean it up, test it and then put it back together. I'll keep you in touch.

While I'm waiting for replacement fuses (750mA btw), if there is any testing I can do of the PSU without power I'd appreciate it.

I'll start sperate threads for the disk drive/printer as and when necessary.

Colin.

[SiriusHardware]

Did the original fuse have an extra letter, like 'F' or 'T' as part of the number? 'F' normally means fast-blow, 'T' normally means slow-blow. If you fit an 'F' fuse where previously there was a 'T' fuse of the same value the replacement may blow prematurely.

You could probably start by checking the 'on' and 'off' resistances of the power switch since it appears that someone tried to bypass it. It suggests they thought there was a problem with the switch.

[ScottishColin]

All I can see is D750mA250V.

I'll get on with the switch.

Colin.

[SiriusHardware]

'D' may possibly mean 'Delayed' (slow-blow).

[ScottishColin]

Well the switch checks out ok with my meter. Are there any other tests I can do without power?

Colin.

[SiriusHardware]

If the mains plug is one you can take apart as I expect it will be, open it up and check that the live, earth and neutral wires are where they should be. It should be a given, but the way that lethal wire was left wrapped around the mains switch you can't assume anything.

With the switch in the ON position and a fuse fitted (for this, any old (intact) fuse which fits the fuse holder will do) measure the resistance between the live and neutral pins of the mains plug (which of course should be unplugged from the mains socket!), and compare it with the same measurement made on your working PET.

Check the resistance between the mains plug earth pin and the metal chassis, that should be as near zero ohms as possible.

If the latter is OK measure the resistance between the mains plug live and the chassis of the computer, and the mains plug neutral and the chassis of the computer. The last two measurements should ideally be infinite, too many Megaohms for the meter to be able to read. When making those measurements, make sure you don't accidentally measure yourself.

[ScottishColin]

Ok. Fuses in, switch on resistance between Live and Neutral is 8.5ohms.

On my working PET I get no reading (ie infinity)

The whole PSU is out of the PET so there is no earth connection.

Colin.

[SiriusHardware]

On my working PET I get no reading (ie infinity)

Between the Live and Neutral pins on the mains plug? That can't be right. Which ohms range was your meter set to? Did you remember to turn the power switch ON on the worker? (Otherwise there will be no 'through' connection from Live to Neutral for the meter to measure the resistance of).

If you really see an infinite resistance between Live and Neutral on the mains plug of the working machine, try going up through the resistance ranges until you see a reading that is not infinity.

If you have the PSU out of the 'new' machine, then first, measure the resistance between the earth pin of the mains plug and whatever the earth wire is attached to on the PSU block and make sure that reading is around zero ohms.

If it is, measure the resistance from the mains plug live pin to wherever the earth wire goes to on the power supply block, and from the mains plug neutral pin to wherever the earth wire goes to on the power supply block. Those readings should be infinity.

[SiriusHardware]

8.5 ohms sounds quite low for a mains transformer primary but it might not be, that's why we need a comparative reading from your known working machine.

Can you show us a couple of overall views of the PSU block? Does it include a mains filter unit, typically a cylindrical item with four wires going into one end and a bracket securing its metal casing to the metalwork of the PSU block?

[ScottishColin]

OK - it turns out 40+ years of grot on the 3 pin plug didn't allow me to take a reading (but still did allow the PET to work).

Having taken some wet and dry to the plug, I get 9.2 Ohms on my known working PET and 8.5 on the PSU from the 'new' PET.

Photos attached as ZIP files so you can see the detail. The mains filter was tie wrapped to the chassis and I have taken it out of the tie wraps.

Colin.

[ScottishColin]

Earth pin on the 3-pin plug to earth wire - 0 ohms as below.

Earth pin to Live wire inside the wiring - infinity.

I cannot get to the Neutral wire as it terminates inside some heat-shrink which has another component inside it - I can check by removing the heat shrink - is that necessary?

Colin.

Originally Posted by SiriusHardware

Between the Live and Neutral pins on the mains plug? That can't be right. Which ohms range was your meter set to? Did you remember to turn the power switch ON on the worker? (Otherwise there will be no 'through' connection from Live to Neutral for the meter to measure the resistance of).

If you really see an infinite resistance between Live and Neutral on the mains plug of the working machine, try going up through the resistance ranges until you see a reading that is not infinity.

If you have the PSU out of the 'new' machine, then first, measure the resistance between the earth pin of the mains plug and whatever the earth wire is attached to on the PSU block and make sure that reading is around zero ohms.

If it is, measure the resistance from the mains plug live pin to wherever the earth wire goes to on the power supply block, and from the mains plug neutral pin to wherever the earth wire goes to on the power supply block. Those readings should be infinity.

[SiriusHardware]

No, leave the neutral wire alone at the PSU end.

8 ohms, 9 ohms, hopefully that's about right for the mains transformer primary, then. The big green thing isn't actually a mains filter, it is the usual high value low voltage smoothing capacitor for the +9V supply. It looks as though there isn't a mains filter so that is one less thing to worry about.

We really need to make these measurements with the PSU block assembled and all wiring in place as it was. The aim is:-

A) Make sure that all of the metalwork including the mounting bracket and the frame of the transformer has a direct, essentially zero ohm resistance to the mains plug earth pin.

B) Check to see whether there is any leakage from mains plug live or mains neutral to mains plug earth through any part of the PSU block, its wiring or one of its components.

Throroughly clean up the round eyelets on the ends of the green wire and green / yellow wire respectively and reassemble the power supply block with the transformer mounted back on the mounting posts and those two eyelets firmly screwed back in place. You don't need to cable tie the large green capacitor back into place at this stage.

Measure the resistance from the earth pin on the mains plug to the PSU block metalwork, and to the metal frame of the transformer - should be virtually zero ohms throughout.

Then, with the mains switch on the PSU block turned on, switch to the highest ohms range your meter has and measure the resistance between the mains plug live pin and the mains plug earth pin, and between the mains plug neutral pin and the mains plug earth pin. If those two measurements are anything less than infinity, report back.

[SiriusHardware]

I take it you are still waiting for the correct value fuses to arrive?

[ajgriff]

Just for reference the unplugged neutral to live resistance on my 2001-8 is 12Ω so 8 or 9Ω for later machines doesn't seem unreasonable. Also the fuse in mine is 750mA slow blow.

Good luck with this one Colin. The fact that a tinkerer has been at work potentially makes it even more of a challenge but you're definitely the man for the job!

Alan

[ScottishColin]

All cleaned up with my mini drill/grinder. All back in place and readings are as below.

I currently have a 2A fuse in and I await the new ones.

Colin.

Originally Posted by SiriusHardware

No, leave the neutral wire alone at the PSU end.

8 ohms, 9 ohms, hopefully that's about right for the mains transformer primary, then. The big green thing isn't actually a mains filter, it is the usual high value low voltage smoothing capacitor for the +9V supply. It looks as though there isn't a mains filter so that is one less thing to worry about.

We really need to make these measurements with the PSU block assembled and all wiring in place as it was. The aim is:-

A) Make sure that all of the metalwork including the mounting bracket and the frame of the transformer has a direct, essentially zero ohm resistance to the mains plug earth pin.

B) Check to see whether there is any leakage from mains plug live or mains neutral to mains plug earth through any part of the PSU block, its wiring or one of its components.

Throroughly clean up the round eyelets on the ends of the green wire and green / yellow wire respectively and reassemble the power supply block with the transformer mounted back on the mounting posts and those two eyelets firmly screwed back in place. You don't need to cable tie the large green capacitor back into place at this stage.

Measure the resistance from the earth pin on the mains plug to the PSU block metalwork, and to the metal frame of the transformer - should be virtually zero ohms throughout.

Then, with the mains switch on the PSU block turned on, switch to the highest ohms range your meter has and measure the resistance between the mains plug live pin and the mains plug earth pin, and between the mains plug neutral pin and the mains plug earth pin. If those two measurements are anything less than infinity, report back.