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Random Characters Issue on a COMMOODRE PET 2001

Dwight Elvey

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Jun 21, 2003
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Santa Cruz
Hi
We can do more with a voltmeter but this is a simple test that we can
run with a piece of wire.
You need to connect one end of a piece of wire to ground. Touch the
other end to the base lead of the Q202. If you don't know which is
the base lead, try all three one at a time. Just be careful not to touch
anything else and that it is a circuit ground you are connected to.
If the circuit is working correctly, touching ground to the base
lead should cause the CRT to go dark.
If this doesn't cause the CRT to go dark, we'll know that it is something
towards the CRT that has failed and if it does go dark, it means it
is something towards the input.

Dwight

Hi
I should note that this is not a general test for all the transistors.
It is just that this particular transistor can be tested this way because
of its circuit.
Dwight
 

sbo

Experienced Member
Joined
Oct 8, 2010
Messages
98
(Sorry for the stupid questions...)

Can I use the metal box of the computer as a reliable ground or is there a specific connexion I should use ?
Also, just to be sure and to let you know before to proceed, I have two transistors labeled :
TIP29 A7907
TIP30 A7907

Stéphane.
 

MikeS

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Messages
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Toronto ON Canada
(Sorry for the stupid questions...)

Can I use the metal box of the computer as a reliable ground or is there a specific connexion I should use ?
Also, just to be sure and to let you know before to proceed, I have two transistors labeled :
TIP29 A7907
TIP30 A7907

Stéphane.
Any of the three high even-numbered pins (4,6,8 ) of the 6-conductor video cable will be a good ground. It will be interesting, but since I love arguing with Dwight I'll go on record that I doubt very much that it's a problem with the video signal; I'm pretty certain that total lack of video will result in a dark screen, and the video signal alone could not account for what looks like widely-spaced scan and retrace lines.

Just be careful that you don't inadvertently damage something else; there are some relatively high voltages in there that probably won't hurt you but could easily take out a transistor or two.
 
Last edited:

Dwight Elvey

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Joined
Jun 21, 2003
Messages
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Location
Santa Cruz
(Sorry for the stupid questions...)

Can I use the metal box of the computer as a reliable ground or is there a specific connexion I should use ?
Also, just to be sure and to let you know before to proceed, I have two transistors labeled :
TIP29 A7907
TIP30 A7907

Stéphane.

Hi
Those don't sound like the ones I'm thinking of. These should be
small TO92 type transistors. I think the TIP's are all power transistors.
I'm looking the layout:
http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001N/321446.gif
I see Q202 left of the top center. There is a lead from the CRT that should be
yellow attached to the board at a point labeled 'K'.
It connects to Resistor R251 ( 100 ohm ) to the collector of Q202.
The transistors we are looking at are small black parts with three leads.
Dwight
 

Dwight Elvey

Veteran Member
Joined
Jun 21, 2003
Messages
4,844
Location
Santa Cruz
(Sorry for the stupid questions...)

Can I use the metal box of the computer as a reliable ground or is there a specific connexion I should use ?
Also, just to be sure and to let you know before to proceed, I have two transistors labeled :
TIP29 A7907
TIP30 A7907

Stéphane.

Hi
Definitely do not ground those!!!
Those are the vertical output drivers.
Q202 is one specific transistor. From the top it should
look like a semi-circular black thing, about 1/4 inch in diameter.
Dwight
 

dave_m

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Feb 2, 2009
Messages
3,608
Location
Southern California, USA
(Sorry for the stupid questions...)

Can I use the metal box of the computer as a reliable ground or is there a specific connexion I should use ?
Also, just to be sure and to let you know before to proceed, I have two transistors labeled :
TIP29 A7907
TIP30 A7907

Stéphane,
Double check to see if your board looks like this one:
http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001N/321446.gif

There may possibly be several variations of video boards.
 

MikeS

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Messages
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Location
Toronto ON Canada
Stephane: do you have a copy of the parts layout diagram (321446)? I would recommend that you print a copy if you don't already have one, make certain that it corresponds to your particular PC board, and double-check every part number and location before you start shorting things to ground.

Edit (Dave): great (??) minds think alike ;-)
I'd wait for that meter, myself...
 

Dwight Elvey

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Joined
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Messages
4,844
Location
Santa Cruz
Hi
That board layout matches the schematic at:
http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001/video-1.gif
At least close enough to find the transistors.
Look at layout:
http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001/video-layout.gif
You'll find Q1 and Q2 at the front of the board, way under the tube. There is a resistor there,
R3. Shorting the end closest to the back of the board should give us the same indication.
I recommend attaching a wire to it with the power turned off. If you can get at the bottom of
the board, it would be easier. There is an island with R3, Q1B and the banded end of CR3.
Dwight
 

sbo

Experienced Member
Joined
Oct 8, 2010
Messages
98
Hi Dwight,

I hope I understood well.
I tried to connect R3 resistor (the end closest to the back of the board) to the ground.

It had no effect on the screen.
Still the green lines ...

Stéphane.
 
Last edited:

Dwight Elvey

Veteran Member
Joined
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Messages
4,844
Location
Santa Cruz
Hi
If the grounding didn't blank the screen the problem is in between there
and the screen. when you have the volt meter, we can look some more.
Dwight
 

sbo

Experienced Member
Joined
Oct 8, 2010
Messages
98
OK!
I will let you know when I will receive the volt meter.
Thank you.
Stéphane.
 
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