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Commodore Pet 3032 no power

@Hugo Holden, Yes, I will buy two at once. Because the other one is probably broken as well. I was thinking of buying from that Ebay seller you recommended. They ship from the US so I should probably get them in a week or so.
But I promise that I will do the same test on these.

Thanks for your help and patience guys :)
 
>>> Black is connected to output.

This is not correct.

>>> Readings are -350 mV.

This is not possible if everything is connected correctly. There is no negative voltage anywhere if the circuit is connected correctly.

Dave
 
@Hugo Holden, Yes, I will buy two at once. Because the other one is probably broken as well. I was thinking of buying from that Ebay seller you recommended. They ship from the US so I should probably get them in a week or so.
But I promise that I will do the same test on these.

Thanks for your help and patience guys :)
Double check post #40, I added a photo.
 
@Hugo Holden
I had a bad grounding. I now tested to PE and my reading on ground is now -0,92V with a load of 8V.
And this is on the 7905 voltage regulator. Not the 7805.
 
Usually a minimum of 2.5V above the output voltage.

-5V - 2.5V = -7.5V (or a more negative voltage) should do the job.

Dave
 
Ok. then I suppose the 7905 is shooted.
I do the same test on the 7805.
I ordered 4 of these. Two each.
I report back
 
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It is fairly difficult to kill one of these regulators to be honest...

You are aware that a 7905 has a different pinout to a 7805 aren't you?

Dave
 
@Hugo Holden
I had a bad grounding. I now tested to PE and my reading on ground is now -0,92V with a load of 8V.
And this is on the 7905 voltage regulator. Not the 7805.

Both of the 5V regulators on the big heat sink are 7805 types. I thought we were checking the two TO-3 cased 7805 regulators here. The modified photo I posted, the base diagrams etc were all for and relating to a uA7805 in a TO-3 case.

Where did the 7905 remark come from ? Maybe when you looked on the TO-3 regulator on the heat sink you saw "7905", but that would be the date code, not the device number ?

As noted in the previous post #28 I made, the Negative Regulators have a different connection scheme to their positive regulator counterparts. In the PET there is a TO-220 cased 7905 regulator on the board near the front edge. When checking that one, note that the center pin and its tab are the input, not gnd as it is in a 7805 TO-220 cased regulator.
 
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@Hugo Holden you are right. I zoomed in it is a 7805. The text is worn off.

@daver2 , if it wasn't dead before,then it is now due to my ignorance, unfortunately.
so I will probably simply replace them to be on the safe side.
 

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@Hugo Holden you are right. I zoomed in it is a 7805. The text is worn off.

@daver2 , if it wasn't dead before,then it is now due to my ignorance, unfortunately.
so I will probably simply replace them to be on the safe side.


If you really want to be on the "safe side" and you are attempting repairs, it pays to look at the manufacturers schematics (commodore PET schematics) and then the individual device data sheets. You would have seen that both these 5V regulators were the uA7805 and that both supply a different array of IC's on the board.

That fact should have alerted you to the notion that it would be improbable, if not extremely unlikely, that both these regulators were simultaneously overloaded and shut down, or that both have failed, when the manufacturer described them as "Blowout Proof".

It would require two independent near identical faults on the two separate +5V supply rails for each regulator to shut them both down. Does that sound likely to you ? More likely there is something wrong with the measurement techniques you are using.

Look up the principle of Occam's razor:

 
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I think maybe Articat should resolder the connections to the pins on the 2 7805 regulators and then try his own suggestion again.

Ok, then the smartest way should be to simply turn the motherboard over and measure the input and output voltage directly on the voltage regulator.

Could you do that please Articat, and let us know what you find. When making the measurements use the rivet on the regulator or the metal regulator case as the GND.

It is not impossible, but it would be highly unusual, for both of the 7805 regulators to fail at the same time.
 
@Hugo Holden Thanks for the input man. But I still belive my measument is fine. Here is why...
I have now tested the other 7805 Voltage regulator and it gives me 5V with @daver2 resistor trick. This is the Voltage regulator locacated in the middle. So I guess this regulator is good!

The regulator I suspect is bad still gives med those -0,9V witch @daver2 says is impossible. I have notice the regulator getting a bit warm and draws 2.5amps. So Im still voting for a shooted regulator.

@SiriusHardware
Thanks for posting man. Yeah l will try that. Iĺl report back.
 
OK here is a update guys...

I have desoldered the voltage regulator (that showed -0,9V ) from the heatsink and motherboeard, and it now test 6V........ So this is about 1V off.
The amps looks like 0,025.

So we have 1 good 7805 drawing 4.96V and one bad 7805 drawing 5,96V.
 
Good work.

Just double checking that these measurements are with the resistor from the output pin to GND as I recommended?

With the faulty regulator out of the board, and on test, what happens to the output voltage if you increase the input voltage slightly. Does it increase? If so, this indicates that the regulator could be internally short circuit and (as a result) may have stressed some of the components downstream.

The only way you should be able to measure a negative voltage on the output of the regulator is if it has shutdown and there is a 'backfeed' from somewhere.

Dave
 
Thanks @daver2
Yes output is connected to ground via a resistor :)

Ok so at 10V the regulator gives a output of 7,06V. And at 7.9V it outputs 5V.

So what do you think Dave. Should we wait for that new 7805 to arrive before we do some more tests?
 
Yes.

If it is not regulating (as you indicate) we need to wait for a replacement before we continue.

It is still strange to me why the two (2) +5V regulators were originally giving us the same reading. I don't like inconsistencies like this...

We need to get the power supplies fully operational before we continue...

Dave
 
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