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COMMODORE CBM PET 2001-N DESPERATE

Desperado

Desperado

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Joined
Nov 25, 2017
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Good evening everyone and happy holidays to the entire forum!
I kindly ask your opinion on this Pet 2001-N, a gentleman asked me if I can repair it for him...
He told me that when I turned it on it showed the usual screen with random characters and symbols. As soon as I brought it home, I didn't turn it on but when I opened it I immediately saw that there is a cable (number 4 on the connector) that is cut. Also on the connector, on pin 4 you can see a burn mark. What could have happened?
Thanks everyone
 

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Yes, a Merry Christmas (almost) to all.

Pin 4 is the raw DC voltage from the transformer (after it has gone vie the rectifier diodes) to the positive terminal of the large external capacitor. The positive DC voltage 'returns' to pin 4 of the same connector where it is fed to the multiple onboard voltage regulators.

A cut red wire indicates that someone has disconnected the main DC supply from the rectifier diodes to the external smoothing capacitor and the internal voltage regulators.

The burn mark indicates a high temperature, most likely caused by a high current.

So, either one (or both) power supply rectifier diodes have gone (less likely), or )more likely) one of the voltage regulators has gone short circuit (either to ground or from input to output).

This machine will NEVER power on like that (no volts at all), so you are not being told the whole story...

Here is the schematic (most likely): https://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001/320008-2.gif.

Here is the J8 connector pinout:

1734980446150.png

Pins 1 and 5 should be from the transformer. Pin 3 should be GND/0V (black wire). Pin 3 should also have the 'centre tap' of the transformer connected to it.

Pin 4 should be voltage 'out' (a red wire). Pin 2 should be voltage 'in' (also a red wire).

If you remember, there are multiple versions of this PET, so looking at what ROMs, RAMs and video RAMs are present may help...

EDIT1: There are too many wires on that power connector though...

EDIT2: J8 (on a 2001) should be a 5-pin connector. This must be a 2001N machine (as you say in the title) - but my basic argument is exactly the same.

EDIT3: This is the schematic: https://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001N/320349-9.gif, and this is the power connector pinout:

1734981252792.png


Dave
 
Last edited:
I forgot an important detail! When I opened the computer, there was this flying connection!
This Pet has 4116 Ram and the classic roms fortunately!
 

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I have correct my initial post.

Basically, what has been done, is a 'bodge' to bypass pin 4 of the connector. The crocodile clip lead connects the positive output from the bridge rectifier diodes (CR1 and CR2) to the external smoothing capacitor.

You should NOT have a dangling floating lead like this in a metal case that is connected to 0V/GND/EARTH! It is a short circuit waiting to happen!

Dave
 
daver2 said:
I have correct my initial post.

Basically, what has been done, is a 'bodge' to bypass pin 4 of the connector. The crocodile clip lead connects the positive output from the bridge rectifier diodes (CR1 and CR2) to the external smoothing capacitor.

You should NOT have a dangling floating lead like this in a metal case that is connected to 0V/GND/EARTH! It is a short circuit waiting to happen!

Dave
Click to expand...
Yes, I immediately removed that cable in fact.
What do you recommend to do to start?
 
The reason the red wire has been cut is probably because the connector is damaged (hence reconnecting it will probably introduce a poor contact on the power supply - which will be the reason it was cut and bypassed in the first place).

The only thing for it is to put the clip back again (in the same place it was originally) and make sure nothing can short out anywhere.

If the machine originally powered up like this (but with the random screen) there is no reason it should not do it now you have it (unless the owner is not telling you the full truth).

Dave
 
daver2 said:
The reason the red wire has been cut is probably because the connector is damaged (hence reconnecting it will probably introduce a poor contact on the power supply - which will be the reason it was cut and bypassed in the first place).

The only thing for it is to put the clip back again (in the same place it was originally) and make sure nothing can short out anywhere.

If the machine originally powered up like this (but with the random screen) there is no reason it should not do it now you have it (unless the owner is not telling you the full truth).

Dave
Click to expand...
So the first thing to do is to buy a new connector...I hope they can still be found..
 
Er, that's not what I said...

Put the alligator clip wire back on is what I said...

Ultimately, yes, see if you can find a replacement power connector (if you can get the machine working).

Dave
 
With the alligator cable i have this screen and i hear the cassette reader run always... Maybe i have continuos reset state...
 

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Er, how about the power supplies first, followed by the clock signal to the CPU, followed by the SYNC signal on the CPU. Followed by the /RESET signal?

So which actual board have you got (look for the '8' number etched into the PCB)...

Dave
 
Desperado said:
Good evening everyone and happy holidays to the entire forum!
I kindly ask your opinion on this Pet 2001-N, a gentleman asked me if I can repair it for him...
He told me that when I turned it on it showed the usual screen with random characters and symbols. As soon as I brought it home, I didn't turn it on but when I opened it I immediately saw that there is a cable (number 4 on the connector) that is cut. Also on the connector, on pin 4 you can see a burn mark. What could have happened?
Thanks everyone
Click to expand...

The connector simply is not rated to handle the current, or if it was on its spec sheet, it was over-rated or simply it was borderline for longevity.

What happens is a small amount of corrosion occurs on the metal surfaces of the connector where they mate. This has some resistance and some voltage drop develops. In a low current application it doesn't matter much, but a high current one it does, the actual heat generated in a resistance is the current value squared times the resistance. So it is a steep relationship.

Once the connection starts to heat up it "thermally cycles" the connector and the springy nature of it is lost and the spring force weakens, further increasing the resistance and further raising the heat evolved, until it starts to melt and discolor the plastic housing. In this case too, every time the PET is power cycled it results initially at least in substantial additional current for the time it takes to charge the main filter capacitor.

Many a time the connector can be saved by disassembling it, cleaning the surfaces, and re-tensioning the leaflet that wipes the pin. This lowers the resistance and kills the heat production. Some connector types you cannot easily remove the pins in the plug, others are easy. However your one might be too far gone. You could attempt it, the thing is after it is repaired to check the voltage drop across the connector in operation. You can buy other connector assemblies, all they require is the same pin spacing and same number of pins as both parts of the connector should be replaced not just half of it. In this case I would select the highest current rated connector that you can get that will physically fit.
 
Hugo Holden said:
The connector simply is not rated to handle the current, or if it was on its spec sheet, it was over-rated or simply it was borderline for longevity.

What happens is a small amount of corrosion occurs on the metal surfaces of the connector where they mate. This has some resistance and some voltage drop develops. In a low current application it doesn't matter much, but a high current one it does, the actual heat generated in a resistance is the current value squared times the resistance. So it is a steep relationship.

Once the connection starts to heat up it "thermally cycles" the connector and the springy nature of it is lost and the spring force weakens, further increasing the resistance and further raising the heat evolved, until it starts to melt and discolor the plastic housing. In this case too, every time the PET is power cycled it results initially at least in substantial additional current for the time it takes to charge the main filter capacitor.

Many a time the connector can be saved by disassembling it, cleaning the surfaces, and re-tensioning the leaflet that wipes the pin. This lowers the resistance and kills the heat production. Some connector types you cannot easily remove the pins in the plug, others are easy. However your one might be too far gone. You could attempt it, the thing is after it is repaired to check the voltage drop across the connector in operation. You can buy other connector assemblies, all they require is the same pin spacing and same number of pins as both parts of the connector should be replaced not just half of it.
Click to expand...
Ok, thanks so much Hugo! I ll try to clean the connector!
 
That should be 'low' on the priority list - as time spent repairing the connector is (effectively) waste time/money if you have problems with the rest of the machine.

I am not saying don't do it, but if you spend money on replacing the connector - and the PET turns out to be a 'dead duck' - will you recover your losses from the current owner?

If the machine is yours (either to keep or sell) that is one thing; but if you are performing a repair service for someone else, that is another matter.

Dave
 
Desperado said:
Ok, thanks so much Hugo! I ll try to clean the connector!
Click to expand...
As I mentioned if you did attempt it, it involves not just cleaning, it requires disassemby, removal of the oxides from the surfaces and re-tensioning. Spraying cleaner on it would be a waste of time.
In the long run it is better replaced.
 
I have a good reset signal on Cpu and i have clock signals on pin 39 and 37 but i haven't the sync signal on pin 7.
The voltages are good!
 
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