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Random Characters Issue on a COMMOODRE PET 2001

Hi
The measurements done without the socket connected
show that there doesn't seem to be a chathode filament
short. That is good.
I wonder about the measurements from CR4. Do you still
have the grounding wire connected? If not, we have
something more than Q3 bad. If so, remove the grounding wire.
I may have not made it clear, the measurements should
be done without the grounding wire connecting CR4.
That was only there to look at the behavior when we
had power on.
Anyway, so far it seems that Q3 has an open base-emitter.
This alone could explain the failure.
Is there a place you can get such transistors their. Is there
anyone in Europe that can suggest a good source for such
parts. The 2N3904 is a common transistor. I have several
sources here in the US but don't know anything about
Europe.
Dwight
 
Dwight Elvey said:
Hi
The measurements done without the socket connected
show that there doesn't seem to be a chathode filament
short. That is good.
I wonder about the measurements from CR4. Do you still
have the grounding wire connected? If not, we have
something more than Q3 bad. If so, remove the grounding wire.
I may have not made it clear, the measurements should
be done without the grounding wire connecting CR4.
That was only there to look at the behavior when we
had power on.
Anyway, so far it seems that Q3 has an open base-emitter.
This alone could explain the failure.
Is there a place you can get such transistors their. Is there
anyone in Europe that can suggest a good source for such
parts. The 2N3904 is a common transistor. I have several
sources here in the US but don't know anything about
Europe.
Dwight
Click to expand...

Hi
I could get access to one 2N3904 transistor (I have some old stuff all around the house ;-)) ; I have replaced mine on the video board.
Unfortunately I still get the green lines ; symptoms are just the same.

Maybe something more than Q3 needs fixing ...

Stéphane.
 
Just to let you know, after checking, I have potential spare for all components on the video board with used components coming from other parts.

Stéphane.
 
Hi
You didn't say anything about the CR4 measurement?
Please don't replace anything until we've finished the
measurements. I was checking things to make sure that
the damagded Q3 was not caused by something else.
If we don't have everything analyzed, you may blow
a good new Q3.
Dwight
 
Dwight Elvey said:
Hi
You didn't say anything about the CR4 measurement?
Please don't replace anything until we've finished the
measurements. I was checking things to make sure that
the damagded Q3 was not caused by something else.
If we don't have everything analyzed, you may blow
a good new Q3.
Dwight
Click to expand...

Hi
OK I see.
I am sorry I did not pay enough attention to your previous message.
I checked CR4 WITHOUT the grounding wire connected.

Stéphane.
 
Hi
Can you make the ohm measurements from
CR4 banded end to ground again. The number
is so far wrong that I just want it double checked.
That is ohms reading power off:
one lead on CR4 banded end
other lead on ground.
test leads as you've done before.
Thanks
Dwight
 
Hi
I have double-checked the ohm measurements :

With CR4 banded end to ground - ohms reading (200k) - power off, I get :
- Red lead on CR4 banded end / Black lead on ground : -0.00 kohms
- Black lead on CR4 banded end / Red lead on ground : -0.00 kohms

I hope this will help.
Stéphane.
 
Hi
This indicated that either Q1 is bad or Q2 is bad.
Can you pull Q2 from the board ( unsolder it ) and
repeat the CR4 measuremnt?
Dwight
 
Dwight Elvey said:
Hi
This indicated that either Q1 is bad or Q2 is bad.
Can you pull Q2 from the board ( unsolder it ) and
repeat the CR4 measuremnt?
Dwight
Click to expand...

Hi
With Q2 unsoldered, I get the same measures like before.
Stéphane.
 
Hi,
My Voltmeter died (battery needs replacement ;-)).
I could see when starting the tests without Q1 that the results were different but did not have the time to write them down.
Stéphane.
 
Great news !

Great news !

Dwight,

I have great news !

Following your last two messages, I have repeated the tests with :
Q3, Q2 and Q1 unsoldered.

In your last message, you identified Q1 and/or Q2 as potential source(s) of the issue.

And you were actually so right

Interpreting the measures drove me to conclude that Q1 was faulty ; I then replaced it, and the screen came back to normal instantly ! It now works like a charm :


pet2001_4.JPG

I am really very grateful to you Dwight for sparing your time and knowledge in debugging my PET screen, and to all of you for taking time and helping me.

I really would like to do something in return, so if there is anything I can do for the community / the forum, I will for sure do it with great pleasure.

Quite frankly, I must admit I really enjoyed this little adventure, and I really do think about doing it again with some other sick PETs around ;) ... (I need to improve my skills in the meantime, I know ! ;)).

However, the repair of my PET is not totally complete.
I have a last (but not least) point to solve : the keyboard mapping is incorrect :confused:.

If for example a press [RETURN], I get the letter 'P' on screen ...

My PET is definitely very malicious ! ;)

I still have the original ROMs set ; if I use it, I have a correct keyboard mapping, but instead of the BASIC welcome screen, I get the TIM screen.

So I have :

- Original ROMs set : TIM + correct keyboard mapping
- Spare ROMs set : BASIC welcome screen but incorrect keyboard mapping

Stéphane.
 
Last edited:
Let's see.. the ROMs on the board I sent you are meant for the "graphical" keyboard, and has a custom Swedish character EPROM. It redefines some characters but doesn't change the mapping. Do you have a business keyboard, identified by the fact you have numbers 1-0 on the top row instead of just the symbols !"#$ etc?

You should have three ROM sets if I recall correctly: the original one from your old PET motherboard, the EPROM set that Dave sent you and the ROM set on the board I sent you. What are the exact numbers on the ROM set from your old board, just to identify that you also didn't have a V4 machine that I downgraded to a V2?

Have you posted a portrait picture of your PET so we can see the keyboard? I can't recall if you did early on in this thread.

A third answer is that either the 6522 or 6520's are partly faulty and decode the keyboard wrongly. You got the 6522 from a 1541 if I recall correctly and bought new 6520's from eBay?

Edit: Oh yes, you have a 2001N a.k.a. 3000 series. You also posted an image early on pointing that your old board have one less chip than what the image displayed, which should mean your old board was V2 just like the one I sent. I don't know if the 2001N came with a business keyboard.
 
Hi Anders,

Yes, I do have a business keyboard (with numbers 1-0 on the top row instead of just the symbols !"#$).
I am not in front of the PET right now, but after contacting the seller today, he told me that the PET was a V4 machine that was downgraded to a V2 !
(he experienced the same issue, but did not tell ;-))

Thank you.

Stéphane.
 
Last edited:
Hi
Looking back at your pictures, I don't see a ground wire from the
CRT mounting screws to ground. The bezel looks to be plastic.
I suspect the damage may have been a discharge someplace.
There should be a ground wire from one of the tubes 4 mounting
screws and the circuit ground.
Also, there is a lot of dust around the second anode ( 10KV wire )
on the tube. This should be cleaned but there is a good
chance of getting a shock from it ( even after turning the
power off ).
I always have a 1 Meg resistor on a grounding wire just to
discharge the tube with. I take a screw driver and attach
the wire to it and to the frame of the tubes mount. I then slip
the screwdriver under the rubber boot of the second anode.
I leave it there while working on things.
I use some rubbing alcohol to remove the dust from about
a 3 inch radius around the second anode. If I'm really
serious, I may completely remove the tube and clean
everything but this is usually enough to keep bad
sparks from happening.
The 1 Meg resistor is just to slow the discharge a little.
I used to just us a wire but was told that too quick
a discharge might induce a charge on parts of the circuit.
I'd never had that as a problem but thought it better
to be safe.
Dwight
 
Aha! In that case you need a different Editor ROM. That is the second last one, counting from the first empty slot and towards the metal shield.

As found out when working with Alex' board, you may need a 2716 or 2516 EPROM instead of the 2532 ones Dave sent you. It could depend on how your programmer works.. well, assuming you have some kind of access to a programmer. In worst case, I could send you a preprogrammed 2716 but we need to determine exactly which ROM version you need to get programmed. It will probably be a good idea to write down the numbers from each chip to get the full picture.
 
sbo said:
I have great news !
Click to expand...

Stéphane,
This is wonderful. You have done a great job with this computer. Your PET have given you some very hard problems which you have overcome with patience and nerves of steel.

Dwight is one of our best troubleshooters. He can fix anything.
-Dave
 
dave_m said:
Stéphane,
This is wonderful. You have done a great job with this computer. Your PET have given you some very hard problems which you have overcome with patience and nerves of steel.

Dwight is one of our best troubleshooters. He can fix anything.
-Dave
Click to expand...
Just add my name to the above. Great work, and fun to watch.

@sbo: did you try your original ROM in D8? I think it should be a 901474-01
 
MikeS said:
@sbo: did you try your original ROM in D8? I think it should be a 901474-01
Click to expand...

If I understand the zimmers database correctly, a 901474-01 editor is for the V2 business keyboard and a 901447-24 editor is for the graphics keyboard. I am confused as to what keyboard Stephane has. I sent him a -24 editor but in the wrong package (2532 instead of a proper 2716 EPROM).
-Dave
 
dave_m said:
If I understand the zimmers database correctly, a 901474-01 editor is for the V2 business keyboard and a 901447-24 editor is for the graphics keyboard. I am confused as to what keyboard Stephane has. I sent him a -24 editor but in the wrong package (2532 instead of a proper 2716 EPROM).
-Dave
Click to expand...
Pay attention! ;-)

Yes, I do have a business keyboard (with numbers 1-0 on the top row instead of just the symbols !"#$).
Click to expand...
 
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