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Commodore Pet 3032 no power

@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.

In your post #15 you measured little voltage out of both of the 5V regulators.

Now you are saying that they are both disconnected, and the one in the middle has recovered a normal output voltage on testing (by implication it means that this one is overloaded by a short in the external circuitry). But is its output still normal when reconnected and powering the PCB ? because it was not when you first tested it.

The other regulator on the end has failed, it would appear, and needs to be replaced, but this is the one (from your previous photos) where you were experimenting connecting a power supply to it with incorrect connections that might have damaged it. So when you replace it , it may or may not be operational as it could be overloaded from an issue in the circuit it is powering. So you still have a few issues to sort out here. What I am saying is that while you are voting for a "shooted" regulator at this point, it may have been you who shooted it and the underlying initial faults possibly still remain. So be prepared for that.
 
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@daver2 @Hugo Holden @SiriusHardware
Ok guys! The 7508 voltage regulator has arrived! And I got news for you! The old regulator was indeed shooted. The CPU now gives me 5V. But the reset signal stays high.

I still have a black screen. BUT when i turn of the computer I see a green flash. So the CRT seems to be fine.

The CR11 still gives me no output (0,2mv) voltage. And the regulator tested fine with Daves resistor trick. Hmmmm.
 
Excellent, so one step forwards!

Can you repeat all of the readings identified in post #15 please.

Dave
 
Ok Dave! Here is the readings
Cr10 11v - 5,14v
Cr11 11v - 240mv
Cr12 22,45 - 11,79
Cr13 -5,07 - -11,75

I notice that ud7,8 and 9 get quite warm. But not burning hot.
 
Ok, so there is indeed something wrong with the associated voltage regulator output.

It is not worth doing anything with the logic until we get to the bottom of it.

Tea time here...

Dave
 
Presumably, if the regulator runs in isolation, but has no output, the +5V rail it supplies on the pcb is shorted out and most likely the regulator is in shut down mode. You could verify that the regulator is ok by isolating it and and loading its output with a 6.8 or 8.2 Ohm 5W resistor, to make sure it is able to supply a decent output current @ 5v.

Since you changed some capacitors, it might be worth checking your work, just in case there is a short created by a solder bridge.

Aside from shorted Tant capacitors on that 5V rail (and you don't have any of those), other parts don't short out very often, occasionally the 0.1uF types can.

Since your meter does read 240mV on that 5V rail, you could hunt around on the board where the pcb tracks distribute the +5v and search for the lowest voltage, that would be the location area of the low resistance or short.
 
I was about to post exactly the same thing that Hugo has...

I suspect a short circuit (or low resistance path) between the output of this particular regulator and GND/0V.

It could be C16 (47 uF) or C45, C81 or C84 (10 uF) assuming I have got the correct schematic. Check that these capacitors are in the correct way round (they are polarised). The negative end of the capacitors should go to GND/0V and the positive end of the capacitors should go to the regulator +5V output.

After that, you could have a faulty IC or some conductive 'dirt' bridging the power rail.

Dave
 
Ok guys,
I have double checked the caps. Everything looks good there. There all in the right place and no shorts there. I have also clean the area with isopropanol.

I have no voltage on the 74Ls244 Vcc (pin 20) but I have 4v on pin 1 (1g)!? There is 4 of these chips. The 2 below 7508 has these strange measurements, the other has no voltage.

I will continue to check the card under a microscope tomorrow. Maybe there is a bad joint somewhere?
 
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The 4V is probably sourced from the IC package pin that is driving pin 1 (that device is probably powered).

A bad joint would not (in general) cause a short - but an open circuit.

Dave
 
have been looking at the motherboard under a microscope for a couple of hours now and tested diffrent components for shorts. I found one short on C15 it,s a smal green cap. I think it is a 1000nf cap. Atleast my tester say so. I have not been able to verify this agains the schematic becouse I dont think the schematic on Zimmer match my motherboard.
Anyway I changed the cap to a 1uf regular and the short is gone. But I still have that low voltage on cr11.
Im starting to suspect that the 7805 voltage regulator is bad after all.
 

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C15 is a 1 uF tantalum bead capacitor (aka smurf grenade). This is on the input to voltage regulator VR4. This voltage measured ok.

These devices are polarised. Putting them in the wrong way round is not advisable...

What schematic are you using from Zimmers?

Dave
 
have been looking at the motherboard under a microscope for a couple of hours now and tested diffrent components for shorts. I found one short on C15 it,s a smal green cap. I think it is a 1000nf cap. Atleast my tester say so. I have not been able to verify this agains the schematic becouse I dont think the schematic on Zimmer match my motherboard.
Anyway I changed the cap to a 1uf regular and the short is gone. But I still have that low voltage on cr11.
Im starting to suspect that the 7805 voltage regulator is bad after all.
C15 (a Tant cap) filters the INPUT to the regulator, not the regulator's output. C15 is also in parallel with C17, on the input to the other 5V regulator that is now working, so it is not plausible that C15 could be your problem is it ?

The capacitors on the regulator's input help prevent parasitic high frequency oscillations. The regulator itself is a power output amplifier with a feedback loop to stabilize the output voltage and as such, the risk of oscillations remains.

The short you measured probably just represented a low resistance on the input side of the regulators.

You could just replace the regulator, but at the moment, you have a result where that regulator appears to be working when it is tested in isolation with its output pin disconnected and into a load.

You could do what I suggested in post #67 to check the regulator with its isolated output into a lower load resistor so that the regulator has to supply over 1/2 of an amp. That will tell you if the regulator is ok, or not.

If it is ok, you need to look for a short or defective component elsewhere on the 5V rail that regulator feeds, if it not ok, then you replace the regulator.

As the manufacturers of these regulators pointed out, they are very hardy, seldom fail. Probably the other one was damaged by the test setup you initially had.

When you are looking for shorts, don't use the diode & beep function on the DVM, use it on Ohms range and document the resistance. In this case, as you have just found out, you were led to believe there was a short across C15, when there definitely was not one. That is evident from the schematic.

Also, rather than hunting around on the pcb with tests that could lead you up a garden path, like finding a capacitor that looks shorted, you need to keep consulting the schematic, and targeting the test areas on the pcb. For example you are interested in the circuitry that the output of the specific 7805 regulator feeds, with the low 240mV output, not this regulator's input circuit which is common to both 7805 regulators. You should know that input circuit to the 7805 regulators must be ok, or the other 7805 would not be working properly would it ?

In addition, if logic prevails, your tester would not have been able to measure the capacitance of 1uF on that green tantalum capacitor, if it was indeed shorted out.
 
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@Hugo Holden
Yeah it could be low resistance as you say.

Can u please explain little more in detail how I should do this test. Im not sure exactly how to do this. Becouse it sounds like the test we did aka daves resistor test?
 
@Hugo Holden
If the other regulator was killed by me. How can u then explain why the CPU had no voltage from the begining. Before I did any test at all? And when I changed the regulator I suddenly has 5v.
 
One key difference is that my resistor was a fairly high value - so not loading the regulator up much.

Hugo's test involves a low value (high power) resistor to load the regulator up to about 1 A.

Ah, use the schematics from the 2001N directory instead... I am sure they are what I referenced early on.

Dave
 
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Well,
I unfortunately I dont have any kind of these value resitors. But I'll do the Dave test again.
I report back.

And this test is fine! Right?
Close to 5V.

@daver2 Im getting a little confused. At the 2001N parts list C15 is a 10uf 25v tant cap. not a 1uf. Are u 100% sure this is the right schematic for Pet 3032?
 

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Close to 5V is good yes (for a 5V regulator).

The schematic lists C15 as 1.0 uF...

All you can do is to compare what you have with what is on Zimmers and hope! That is the best we have! Sometimes boards were also replaced, so differ from the labels on the front if the PET...

Dave
 
If you don't have any 5W power resistors about, in the range of 5 to 10 Ohms, probably your best bet at this point is to replace the regulator, if you have one on hand, (since you cannot fully test the original with the light load). Then one of two things will happen, the 5V output will recover or it won't. If it doesn't then you need to look for the fault that is causing the regulator output to be overloaded.

(One other trick if you don't have load resistors is to use a 3 to 5 watt 6V auto lamp).

This is the schematic I have been looking at:


Matches up with C15 being a 1uF.
 
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