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[HELP] Commodore PET CBM 3008 3032 Screen Issue

DrAlis

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Nov 11, 2021
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71
I did it again and get 8.3 vac pin1, 8.8 vac pin2 when connector is plugged in. with connector out i get 6.0 v ac pin1, 9.5 v ac pin2 at connector vs case.
 

DrAlis

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oh and i measure pin 1 and 2 against ground from tp3. if i measure between pin 1 and 2 i get 17.3 vac plugged in.
 

daver2

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Sorry, I am not explaining myself well...

The case has nothing to do with the measurement at all!

You need to set your multimeter to ac volts and put one of the multimeter probes on point 1 and the other multimeter probe on point 2.

The case is the 0V/GND point and is not anything we are interested in for this measurement.

Dave
 

DrAlis

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i obviously dont know much about electronics, but voltages changing when plugged in feel like overload to me with my dc knowledge, right? I am also puzzled how 2 times round 9v can be rectified to round 21v. i'll hang in there and learn
 

DrAlis

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Sorry, I am not explaining myself well...

The case has nothing to do with the measurement at all!

You need to set your multimeter to ac volts and put one of the multimeter probes on point 1 and the other multimeter probe on point 2.

The case is the 0V/GND point and is not anything we are interested in for this measurement.

Dave

Thanks for the clarification. So pin 1 vs 2 is 17.3 vac
 

daver2

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For a 50/60 Hz sinewave, you multiply the AC voltage by the square root of 2 (approximately 1.4) to obtain the rectified and smoothed DC voltage. So 18V * 1.4 = 25.2 Volts. This is only approximate however...

However, if we are measuring the AC voltage on the secondary of a transformer that is only indirectly referenced to ground, there is no such thing as 2 times 9V. See my post #43 for details of measuring the AC voltage correctly.

Dave
 

DrAlis

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This is likely a dumb idea, but i ll ask out of curiosity. I got a 12 v dc source and could clamp it where it is supposed to be on the yoke to get the heater going. i assume this is not a good idea because i would fry more whatever is causing the voltage drop in the first place because i now make more current available, right?
 

daver2

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Correct.

if there is something on the PCB that is shorted out (or causing a large current to flow through the 12V regulator) then the voltage regulator will "shut down" to protect both itself and the circuitry after it. Forcing 12V onto the 12V DC rail could cause the black smoke to escape :-(!

Think of the 12V regulator as a fuse. If the fuse blows - there is a problem. If you replace the fuse with either (a) a bigger fuse or (b) a nail - bad things will happen...

This is why we let the protection devices do what they are designed to.

If I suspect something like a short circuit, I may feed it from a bench power supply BUT I will set the current limit on my power supply to a point where no damage will occur if there is a short circuit (just as your voltage regulator is doing).

No such thing as a dumb idea - you did ask first before doing it and I hope I gave you the reason why not to do it?

Even if we get the CRT heater going - this is not going to help because we need the rest of the circuitry to work anyhow.

Have we got a sensible measurement of the input AC voltage yet?

Dave
 

DrAlis

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Thanks for confirming the idea is not of the good kind. Yes, it is 17.3v ac. i wrote it twice already but i burried it well with all the other stuff including the irrelevant measures agains ground. Thanks for helping Dave! Would be zero fun and progress without you.
 

daver2

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Excellent - so the transformer, bridge rectifier and input to the voltage regulator is OK.

It is getting late in the UK now, so I will have a think about where we go next and get back to you tomorrow.

How good are your soldering and desoldering skills?

Dave
 

DrAlis

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Good morning. Yes I can solder but with basic kit. Things are large enough here. For some reason I find the professionally soldered connections a lot harder to desolder than the self made ones from simple kits. Initially I was happy with the low voltage on 12v rail because I thought that leaves few broken things in creating the voltage, but realized that a broken consumer is an option widens the search space dramatically. I am committed to getting at least one of the two in working order after the invested effort.
 

DrAlis

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I also applied fresh thermal paste on the voltage regulator but that did nothing. I ordered a L7812cv hoping to replace the FS7812 but that will take to arrive.
 

daver2

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>>> I also applied fresh thermal paste on the voltage regulator but that did nothing.

it wouldn't. If the output voltage started off at 12V and then decayed over time as the device heated up - then that could be a solution. However, the thermal paste shouldn't degrade unless it is disturbed.

I would disconnect the OUTPUT pin of the voltage regulator from the monitor circuit. See this website for details of the voltage regulator pinout: https://www.componentsinfo.com/lm781...ut-equivalent/.

You may need to connect a resistor (say 1 kOhm) between the OUTPUT pin of the voltage regulator and 0V/GND/COMMON to act as a dummy load.

If you power the PET up in this configuration, measure the DC voltage on the OUTPUT pin of the voltage regulator relative to 0V/GND. If you observe 12V - then the regulator is working OK (well, at least it is regulating the voltage under no load conditions) and I would suspect a high current consumption fault within the monitor circuitry that we have to track down next. If the voltage is less than 12V, then the voltage regulator is most likely to be faulty.

You only need to desolder the OUTPUT pin of the voltage regulator and gently pull it back through the hole to release it from the PCB. However, you may find it easier to remove the voltage regulator IC completely from the PCB and then temporarily wire the INPUT and 0V/GND pins back in.

Of course, if you are going to remove the voltage regulator completely, you could test it on the bench by feeding a DC supply of between 15V and 22V between the INPUT and 0V/GND/COMMON pin and measuring the OUTPUT voltage.

If the voltage regulator is OK - then a replacement won't help and we can fault find from there.

Do not re-solder the OUTPUT pin of the voltage regulator back into the monitor circuit when you have finished this step if the voltage regulator is OK. I want you to take a current reading next if it is OK...

Dave
 
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DrAlis

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Nov 11, 2021
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Thanks Dave report follows. I just find the handling annoying messing with cathode and yoke everytime taking stuff in and out. i took only the output pin out and did measurements. so with 1k ohm i get around 0.9 v, with no load about 2 v. the current through the 1k resistor is 0.9mA.
 
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DrAlis

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Nov 11, 2021
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Thinking about this it is odd. I would have expected at least the 5.1v i got yesterday...
 

daver2

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Correct - that is strange.

Can you post a photograph of what the IC looks like now - just to make sure we are on the same page?

I have just worked out the next step to see if any high current flow is as a result of the horizontal or vertical monitor circuits - but we need to ensure that we have a working 12V voltage regulator first!

I wonder if 1 kOhm is too high a value?

You could try dropping the resistor value from 1k to 470 or 220 Ohms. I wouldn't think any lower would be necessary.

Dave
 

DrAlis

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Nov 11, 2021
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maybe i baked or bent it to death. i will desolder it and test it out of circuit...
 
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daver2

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I assume the pin connected to the red probe is not attached to the PCB at this stage? Just want to confirm...

Also, a word of caution, the metal tab of the voltage regulator is internally connected to 0V/GND. If the heatsink doesn't have a mica washer and an insulated "top hat" then the heatsink will also be at 0V/GND potential. Please make sure that your red clip isn't inadvertently contacting the heatsink - otherwise the OUTPUT of the voltage regulator will be shorting out!

Dave
 

DrAlis

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Nov 11, 2021
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71
Thanks I was careful not to create unwanted bridges. I desoldered it and it acts strange. Connected it to regulated power supply on input: as i crank up starting at 3v output voltage drags along at around 80% of input. when i hit 10v it collapses to around 2v. connecting a resistor does not seem to change anything. def no 12v here or does it need a special load or overheat instantly?
 
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