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Random Characters Issue on a COMMOODRE PET 2001

Hi
I looked at the layout upside down. The picture shows the traces and
I thought these were from the same side of the board as the part layout.
It was actually a print true of the paper. The traces are flipped right
to left from the parts.
The end you shorted to ground was already ground. It is the end that
is away from the back of R3 that should be shorted to ground.
Dwight
 
Hi,

I tried to short the end that is away from the back of R3 : Same symptom (green lines).

Stéphane.
 
hi
I'm begining to sound really bad but I gave you the wrong
circuit point again. I counted the inversions wrong
from the CRT back.
The spot to ground should have been the band end
of CR4.
There are three diodes next to Q2. CR4 is closest to
Q2.
Here is the way the circuit works. The closer to
ground that the cathode of the CRT get to ground,
the brighter the screen.
Q3 provides an active pullup. If it shorted, it would
make the screen darker. It is possible that the +85V
or that the zener ( CR7 ) is bad. This would cause
the screen to go bright. It is also possible that the
CRT has a short between the filament and the cathode
( hopefully it isn't that ).
Q2 pulls the cathode voltage towards ground when
turned on through CR6. If Q2 shorted, it would make
the screen bright.
The lead of CR4 with the band is connected to the
base lead of Q2. If Q2 is working, grounding this
lead would allow Q3 to pull up the cathode enough
to make it dark.
Going more towards the input. Q1 inverts the input
to Q2.
Q1 is baised on by R1 through CR2 and CR3. This
should cause the screen to go dark if the input
was open. So my prediction of an open video would
have been incorrect for this circuit. It should have
caused the screen to go dark.
For typical failures transistor and diodes usually go short.
For resistors, they tend to go open.
Lets look at the failures for the circuit.
CR7 short brighter
Q3 short darker
Q2 short brighter
Q1 short darker
CR6 short Medium bright
CR5 short brighter
CR4 short little effect
CR3 CR2 short darker
CR1 short brighter
Dwight
 
Hi
Part 2 on failures
The fact that the screen is bright would lead one to
think that most of the flyback circuit is working. This means
that it is unlikely that the 85V line is shorted to ground. If
it were, it would most likely pull the 10KV down as well.
The fact that the brightness knob has some effect would
indicate that the -30V was there as well. This also
says that the flyback circuit is healthy.
Lets look at some resistor failures.
I have yet to see a shorted resistor in my years of fixing
things. Resistor may age and get lower resistance but
never go short. Resistors tend to go open.
R5 is a good candidate. If it went open, there would be no
pullup from Q3. Also R4 would have a similar effect. These
are both high power resistors and likely to fail.
If the resistors R6 or R7 failed open the screen would
go dark. There is the thing called SG4. This is a spark
gap if it shorted, the screen would go bright. Usually
these are a ceramic capacitors with a slot cut in the
edge. Metal fiber or other material may have lodged
in the slot but not too likely.
R2 would cause it to go darker ( no bias for Q2 ).
R1 would make it brighter but it has very little power
and unlikely to open unless mechanically broken.
R3 going open would only make it darker.
Dwight
 
Last edited:
Part 3
What voltages should one expect to see?
Rather than look at every voltage, it is often best to look at a few and
then refine the search when you find something that looks out of
order. Do remember that a fault can cause some voltages that seem
to be more towards the input leads to fail, even though the fault
is farther away. There are limits to this but in general a bad voltage
reading means that the failure is at that location or earlier ( towards the
input ).
Voltages are relative to ground unless noted.
At the junction of CR1and CR2 ( non-banded ends ) the voltage should
be about 1.5 volts, with little or no text on the screen.
The voltage at the banded end of CR4 should be less than 0.4V a voltage greater
would indicate a failure at this net.
The voltage at the banded end of CR7 should be 33V
The voltage across R6 and R7 should be 0.0V. A voltage of 0.2V or
greater would indicate some failure.
Each of these voltages would lead one to another thing to measure,
not necessarily a failure of the component mentioned.
Dwight
 
Hi Stéphane,
it seems to me that there a three ROMs missing. This is a pitty, but there are adapters availible (still?) to fix this problem. It might be possible the RAMs could be defective - but first of all, you need the ROMS. The PET is noct able to do anything useful without them.
Then, you could test the RAM with the Video-RAM on top (left side of your picture). Basically, you need only "page 0" - wimre the right most chips (upper RAM chips on your picture).
Good luck! I keep my fingers crossed!.
Cordialement,
Charly DL4DBY.
The Zimmers page (link by cosam) helped me to fix my PET...
 
No. Basic V2 means three slots are supposed to be empty for future expansion. The board I sent to Stéphane was perfectly well tested by me before I sent it, and I think he tested it with a known working PET after arrival too. So most definitely it is not a motherboard issue.
 
Hi,

I have tried to ground the band end of CR4 :
The screen is now much darker :
- Before : dark green <> bright green
- After : blank screen <> dark screen

But all I have on screen are the horizontal lines like before.

Thank you.

Stéphane.
 
Hi
Good info. I'll have to give it some more thought.
I left the schematics I'd printed out at work
and I have problems down loading here.
So, you say that the screen gets darker but the
heave retrace lines are still there?
Sounds more and more like the +35V is missing.
Wish you had the meter handy.
Without the +35V, it could only blank with +12V
maximum. It wouldn't have the full +35V to work with.
The fact that the retrace are so bright should be
indicating something as well. Can you get a screen
shot with the CR4 banded end shorted?
Dwight
 
Dwight Elvey said:
Hi
Good info. I'll have to give it some more thought.
I left the schematics I'd printed out at work
and I have problems down loading here.
So, you say that the screen gets darker but the
heave retrace lines are still there?
Sounds more and more like the +35V is missing.
Wish you had the meter handy.
Without the +35V, it could only blank with +12V
maximum. It wouldn't have the full +35V to work with.
The fact that the retrace are so bright should be
indicating something as well. Can you get a screen
shot with the CR4 banded end shorted?
Dwight
Click to expand...

Hi Dwight,

This is what I get with the CR4 banded end shorted :
(As you can see, this is much darker than in the previous video ; from black to dark green when playing with the brightness.)


Youtube : http://www.youtube.com/watch?v=WPpiDP1uJQI

Stéphane.
 
Last edited:
Dwight Elvey said:
Hi
It was good to see the dimming but I think that
without the voltmeter, we are still in the dark.
Dwight
Click to expand...

Finally I received tonight my voltmeter.
So we can go on with the tests.

You will have to be patient with me, as I have never used one before.

Stéphane.
 
Stéphane,
Is it a digital voltmeter (DVM) or a VOM with a meter movement? Either will work well. If a DVM, use on "auto-range" if possible.

I would start by measuring the +12V output of the voltage regulator with the meter on "DC". Then check it on "AC" to see if the ripple is bad. It should be close to zero on AC.
 
dave_m said:
Stéphane,
Is it a digital voltmeter (DVM) or a VOM with a meter movement? Either will work well. If a DVM, use on "auto-range" if possible.

I would start by measuring the +12V output of the voltage regulator with the meter on "DC". Then check it on "AC" to see if the ripple is bad. It should be close to zero on AC.
Click to expand...

This is my voltmeter :

41QN2GRgz3L._SL500_AA300_.jpg


@Tez : Thank you. I hope it will be successful ;-) !
@Dave : Should I measure the +12V on the video board or directly on the power supply ?

Thank you.

Stéphane.
 
dave_m said:
Stéphane,
Is it a digital voltmeter (DVM) or a VOM with a meter movement? Either will work well. If a DVM, use on "auto-range" if possible.

I would start by measuring the +12V output of the voltage regulator with the meter on "DC". Then check it on "AC" to see if the ripple is bad. It should be close to zero on AC.
Click to expand...

Theese are the measures I get (on the power supply) :
DC=16.50V
AC=0V
 
sbo said:
Theese are the measures I get (on the power supply) :
DC=16.50V
AC=0V
Click to expand...
Exactly where? On the input to the regulator, that's OK; if on the output that would explain your symptoms with a bad regulator. What do you get on the brightness control?
 
MikeS said:
Exactly where? On the input to the regulator, that's OK; if on the output that would explain your symptoms with a bad regulator. What do you get on the brightness control?
Click to expand...

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.
 
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