Random Characters Issue on a COMMOODRE PET 2001

[MikeS]

Hi Mike,

I took that measure by controlling the output tension on the big power supply close to the mainboard. Does it make sense ?

I managed to get for a a few days a working PET at home ; so I can compare tensions on both machines if it can help ...

Stéphane.

It makes sense but doesn't help much. What voltage do you measure across the outside terminals of the brightness control (that should be easy to get at); you should have about 30V.

Can you access the three terminals of the 7812 regulator in the heatsink? Between the middle terminal and the outside terminals you should have 16.5V on one side and 12V on the other.

Be careful that you don't accidentally short anything with the meter probes.

 

[dave_m]

Can you access the three terminals of the 7812 regulator in the heatsink? Between the middle terminal and the outside terminals you should have 16.5V on one side and 12V on the other.

Be careful that you don't accidentally short anything with the meter probes.

Stephane,
Use the black probe (common/COM) on the center terminal of the voltage regulator (VR1) as the reference. We are talking about the regulator on the video board.

 

[MikeS]

Stephane,
Use the black probe (common/COM) on the center terminal of the voltage regulator (VR1) as the reference. We are talking about the regulator on the video board.

You're right; I figured I'd keep it simple since probe polarity doesn't really matter with a DMM (if you really see -12V on the 7805 you've got a real problem) but I guess we should train him properly from the start ;-)

 

[dave_m]

but I guess we should train him properly from the start ;-)

Mike,
I keep forgetting that Stephane is a beginner at troubleshooting electronics. He is learning fast, but it must be difficult for him to understand our advice sometimes.

Maybe we should fly to Paris after all. You could help with the troubleshooting while I attend the Moulin Rouge.
-Dave

 

[carlsson]

You never know. Some of the girls on the shows might be EE students working extra to cover course fees.

As for PET repairs, I can report that the other motherboard I sent to Alex in Italy worked well for a few days. Then suddenly it crashes into the monitor on power up. We have analysed that his old board had Basic V4 while the one I sent to him has Basic V2 of which two chips are in EPROM form. Needless to say swapping the whole ROM set caused the new board to display garbage, and unfortunately the two sets are not interchangable. However Alex has an Willem programmer which he will look up if it can at least read 2532 and thus 2332 ROMs. Hopefully it will work out so he can compare dumps with known good BIN files to determine if either of the two EPROMs I programmed got worn out during the transport or if it is another of the original ROMs that didn't survive the trip. Likewise he might find out which out of his V4 ROMs is bad and could be replaced to get a working PET somehow.

 

[MikeS]

Maybe we should fly to Paris after all. You could help with the troubleshooting while I attend the Moulin Rouge.
-Dave

Aw, you're a much better troubleshooter than I so it would be much more effective if you played with the Commodore PET while I umm, explore the different kinds of pets in Paris; I was there many years ago but didn't make it to the Moulin Rouge, alas.

 

[dave_m]

I can report that the other motherboard I sent to Alex in Italy worked well for a few days. Then suddenly it crashes into the monitor on power up.

Perhaps if the machine language monitor is functional, it could be used to display samples of RAM and ROM to see it there is an obvious problem like a stuck bit?

We have analysed that his old board had Basic V4 while the one I sent to him has Basic V2

Did Alex indicate if the characters looked OK? If the V2 board was for a 9" screen, did it look compressed or stretched out on a 12" screen?

 

[carlsson]

Hm, he hasn't said but we are talking about a CBM 3032 here. I don't think they came as any Fat 40's, only the 4000 series? Or perhaps I'm mixing things up now.

 

[MikeS]

Hey, after Paris we can head south to Italy with a couple of days stopover on the Riviera; waddya say, Dave?

 

[sbo]

Hi!

Day off today in France, so I am going to troubleshoot.
By giving a first look at the videoboard, I do not feel like I will be able to check tensions with the video board in the metal box. I think that I will damage (shortcut) the components with the probes if i do not have a full and easy access to it.
So would it be possible to take it off the monitor to have an easier access to the electronic components for the checks ?

Stéphane.

 

[dave_m]

Stéphane,
One way to check hard-to-reach points on the board is to use test leads (wires) with a very small spring-loaded clip on the end to hook to the lead of a component. This is done with power off and when you are sure it is not touching (shorting) anything it should not, then you turn power on and check the other end of the wire with the meter.

The black probe (common) can just be connected to a bare metal spot on the chassis (metal box) for a good ground point.

Another way is to remove the mounting screws to gain better access to the board, and if you have a soldering iron, you can temporarily "tack solder" a thin wire to the test point and bring the wire out for easy access to the meter.

All this is not easy with high voltage so close by. Be careful and if you are not comfortable with it, it may not be worth doing it.

Perhaps you can find someone to help who has TV repair experience?

 

[sbo]

Hi Dave,

OK no problem ; YES it's a little scary, but I think I can dot it.
I will follow your intructions not to hurt myself nor the components.

Just to summarize, which components should I check ?

For the moment, I have :

- VR1
o Between the middle terminal and the outside terminals I should have 16.5V on one side and...
o 12V on the other
-
o Across the outside terminals of the brightness control I should have about 30V.

Stéphane.

 

[sbo]

This is what I got :
VR1 :
First side : 12V
Other side : 19.1V

BC : 33.7V

(I'm vey proud I did not die ;-))
I used the "tack solder" technic ...


By the way, is it a problem to process the tests with the screen upside down ?

 

[MikeS]

The reason I suggested the outside terminals of the brightness control is that it's probably the easiest place to measure and the voltage there will be a useful clue.

Yes, those monitors are not easy to work on; you pretty well have to remove the CRT to really get at everything, something I don't recommend. Let's see what we can figure out from the test points we can get at (relatively) easily.

 

[Dwight Elvey]

Hi
Some more points to check:
Banded end of CR7 ( should be about +33V )
Banded end of CR18 ( should be about +85V )
The points you've measured so far are great.
They verify that those parts of the board are working as expected.
Each measurement that you make will most likely lead to another
question. This is the method used in trouble shooting. It is much
cleaner than just making a big list and comparing voltages.
Make measurements with the black lead connected to the ground.
You might find a way to have this connection without having to hold it
by hand. This will allow you to use just one single hand for the
red probe. A typical TV repairman will use the "one hand in the
back pocket" method while working with a live circuit. In the digital
world we typically don't do that because of the low voltages we
work with. It is good to follow this method of the TV repairman
for working on monitors.
Dwight

 

[dave_m]

Hey, after Paris we can head south to Italy with a couple of days stopover on the Riviera; waddya say, Dave?

Will it be possible for us to stop in Monaco for a few days? I want to break the bank at Monte Carlo.

 

[dave_m]

Hm, he hasn't said but we are talking about a CBM 3032 here. I don't think they came as any Fat 40's, only the 4000 series? Or perhaps I'm mixing things up now.

Anders,
There are so many variations of PETs. Did they make a 3032 with BASIC4 ROMs?

 

[carlsson]

I think you could upgrade yourself? I have a 4032 with the 9" monitor. That machine is equipped with Basic 4, but otherwise has an motherboard identical to the ones I sent to Stèphane and Alex.

 

[MikeS]

Will it be possible for us to stop in Monaco for a few days? I want to break the bank at Monte Carlo.

Sure! I was actually there many years ago for the F1 race but only walked past the casino. And after Monte Carlo we can spend a day or two on the topless beaches in Nice and places in between; your winnings oughta make you a popular guy...

 

[Dwight Elvey]

This is what I got :
VR1 :
First side : 12V
Other side : 19.1V

BC : 33.7V

(I'm vey proud I did not die ;-))
I used the "tack solder" technic ...


By the way, is it a problem to process the tests with the screen upside down ?

If you can access the bottom of the board, that is fine. Be careful about locating the
measurement points. The picture on the web page shows print through of the circuit
layout, relative to the components identified. It is reversed relative to the component
labels.
As for dangerous voltages. The only electrocution danger is the 85V line. The 10Kv
has very little current and it's primary danger is the from bleeding to death after
you whip your hand across the broken glass of the tubes neck, as you snapped it off,
in the jerk reaction.
How about the measurements???
Dwight