• Please review our updated Terms and Rules here

Random Characters Issue on a COMMOODRE PET 2001

Dwight Elvey said:
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
Click to expand...

Hi Dwight,

This is what I get :

Banded end of CR7 ( should be about +33V ) : 31.7V
Banded end of CR18 ( should be about +85V ) : 81V

Stéphane
 
Hi
Those are still reasonable values. We'll need
to look a little deeper.
I'll give you some more things to measure
later.
Dwight
 
Hi
Lets start with some more checks:
Measure the voltages on the ends of the resistors R6 and R7.
This are right under the tube. I want to check to see if
it is a chathode to filament short. There isn't an easy fix
for this but we need to eliminate it. The current flow through
these resistors will tell us how it is doing. We can determine
the current by measuring the voltage.
After that, I'd like a few more measurements made with
the shorting wire connected.
First:
Short to ground the banded end of CR3 and measure the both
banded end of CR4 and the non-banded end of CR4.
Next, move the ground short to the banded end of CR4.
Now measure both sides of CR5 and CR6.
That should give us a good picture of the circuit health
in this area.
Dwight
 
Last edited:
Hi,

This is what I get :
(I hope I did it right for R6 & R7 / I have put the probes on the ends of both resistors and checked the voltage; is that correct ?)

- R6 :
0,36V

- R7 :
0,36V

CR4 (with CR3 shorted to ground) :
- BE : 12,36V
- NBE : 12,38V

CR5 (with CR4 shorted to ground) :
- BE : 12,00V
- NBE : 12,11V

CR6 (with CR4 shorted to ground) :
- BE : 11,82V
- NBE : 12,10V
 
Last edited:
sbo said:
Hi,

This is what I get :
(I hope I did it right for R6 & R7 / I have put the probes on the ends of both resistors and checked the voltage; is that correct ?)

- R6 :
0,36V

- R7 :
0,36V

CR4 (with CR3 shorted to ground) :
- BE : 12,36V
- NBE : 12,38V

CR5 (with CR4 shorted to ground) :
- BE : 12,00V
- NBE : 12,11V

CR6 (with CR4 shorted to ground) :
- BE : 11,82V
- NBE : 12,10V
Click to expand...


Hi
Actually, I wanted the voltage to ground as two measurements,
one at each end. My fault for not being to clear. What I needed
was the polarity of the voltage on R6 and R7. The layout
isn't clear in this sections so I should see two measurements
with the same value. That would be the common lead. I'm
looking to see if the tube is sinking or sourcing significant current.
The measurement you've given is about 3.6 milliamperes but
I can't tell the direction without a ground reference.

The other measurements don't sound right, with the power
of and the meter on ohms, measure the resistance between
the location you are using as ground and the connector pin 8.
I suspect you're not measuring correctly but you never know.
Here is what I expect:

Banded end of CR3 shorted to ground:
BE of CR4 about 0.6V
NDE of CR4 about 1.2V

Banded end of CR4 shorted to ground:
BE of CR5 12 to 33V
NBE of CR5 about 0.6V

Banded end of CR4 shorted to ground:
BE of CR6 12 to 33V
NBE of CR6 about 0.6V less than BE

Since the measurements you gave me are quite different
and not what I'd exect to see for a typical failure, I'd like
to have you remeasure them but be careful about the selection
of your ground point. The connector pin 8 is the best point.
Of course, these could be correct and an indication of the actual
failure.
Dwight
 
Hi,

I think I must be misundertanding something :
Can the ground location I use to shortcut CR3 & CR4 be the same as the one I use for the COM probe ?
Because what I am doing right now is I soldered a wire between CR3 (BE) and the connector PIN8, while in the same time I put the Volt meter COM probe in the (same) connector PIN8.

Is that correct ?

Stéphane.
 
I reply to myself :

Using PIN8 as GROUND, I start to get your measures.
The problem was that I used another PIN on the connector and that was tricky, because just by turning on the PET I got a 10V measurement just with the COM probe on that PIN.
(I hope you understand me, sorry for my poor English ;-))

I keep measuring so ...
 
First results :

Banded end of CR3 shorted to ground:
BE of CR4 0.71V
NDE of CR4 1.39V

Banded end of CR4 shorted to ground:
BE of CR5 1V ???
NBE of CR5 about 0.68V

Banded end of CR4 shorted to ground:
BE of CR6 1V ???
NBE of CR6 1V ???

As you can see, I do not get the same measures on CR5 / CR6 ???
I have double checked and I still get the same results.

I use PIN8 as a ground location for both the shortcut (using a wire) and the COM probe.

Important :
- When I shortcut CR4 BE, the screen is much darker going from blank screen to dark green when playing with the brightness.
- The monitor is separated from the PET body during the tests so I can grant access to the video board and the component easier.
(I hope it does not affect the test...)

pet2001.JPG
 
Last edited:
R6 : +1.03V / +1.03V
R7 : +1.05V / +1.05V

One last point I think I did note mention before :
When I play with brightness (no shortcut) :
- Turning to the right : the screen is "stable", green lines, fixed image, no movement on screen.
- Truning to the left : the screen start to shake and to rattle making some noise like sparks ; (i do not try this so often ! ;-))

Stéphane.

My ground (you can see both the black cable going alternatively to CR3/CR4 and the COM probe) :

pet2001_2.JPG


One last detail :
This zone on the video board seems appears to be very hot ? It is dark brown while the rest is more golden.
Could this indicate a component failure ?
(Just close ro CR5/CR6)

pet2001_3.JPG
 
Last edited:
Hi
The 1 V measurement on both sides of CR6 is not
good. With CR4 shorted to ground, we should see Q3 pulling
the NBE of CR6 quite high.
Now for some checks with the voltage off. I'd like you to
measure with the ohm meter of the meter.
First measure with one lead on each side of CR6. Do two
measurements with the red on the BE and the black on the NBE
and then change the leads around.
From the banded end of CR5 to ground ( swap the leads and measure
a second time.
From the NBE of CR6 take the two measurement and then
remove to connector from the back of the tube ( NOT the single
wire that comes from the transformer or the one that goes to
the yoke ( coils on the neck ).
Be careful not to twist or turn the connector while removing it.
It comes strait back. It may be tight and you can rock it just a
little but be careful, not too much.
With the connector off, repeat the ohm meter measurement
from the NBE of CR6.
We are getting close.
Dwight
 
Hi
I forgot to mention the sparking sounds. This may be normal
from dust on the back side of the tube. It may also indicate
a discharge that shouldn't be there, such as something from
inside the gun of the tube.
Dwight
 
Hi,

Do I understand you well if I measure :

CR6 :
RED / BE - BLACK / NBE : xV
RED / NBE - BLACK / BE : xV

CR6 (with CR5 BE to ground) :
RED / BE - BLACK / NBE : xV
RED / NBE - BLACK / BE : xV

CR6 (with CR6 BE to ground) :
RED / BE - BLACK / NBE : xV
RED / NBE - BLACK / BE : xV

CR6 (with CR6 BE to ground & Tube connector removed) :
RED / BE - BLACK / NBE : xV
RED / NBE - BLACK / BE : xV

Stéphane.
 
Hi
With the power off, we will be measuring ohms, not voltage.
Do you have a closeup of the meter. I'll tell you how to set
the meter up for ohms.
Dwight
 
Dwight Elvey said:
Hi
With the power off, we will be measuring ohms, not voltage.
Do you have a closeup of the meter. I'll tell you how to set
the meter up for ohms.
Dwight
Click to expand...

This is what I get with the meter set to OHMS / 200k (PET off) :
CR6 :
RED / BE - BLACK / NBE : 1
RED / NBE - BLACK / BE : 147.3

CR6 (with CR5 BE to ground) :
RED / BE - BLACK / NBE : 1
RED / NBE - BLACK / BE : 147.3

CR6 (with CR6 BE to ground) :
RED / BE - BLACK / NBE : 1
RED / NBE - BLACK / BE : 147.3

CR6 (with CR6 BE to ground & Tube connector removed) :
RED / BE - BLACK / NBE : 1
RED / NBE - BLACK / BE : 147.3

Same measures on all tests ?? Did I miss something ??
 
Hi
Nothing wrong with your measurements, just my instructions
that were lacking.
The measurement across CR6 would seem to indicate that Q3
is not good but I'd still like ou to make a couple more ohms measurements.
Do note that Ohms are always done without the power being on.
With the socket connected to the tube, red lead on NBE of CR6 and black lead on ground.
Next socket removed from the tube, red lead on NBE of CR6 and black on ground.
Another, red lead on CR5 BE black lead on CR5 NBE.
Lastly, Red lead on ground and black on CR4 BE
Repeat the above with the meter leads reversed.
Dwight
 
Hi,

This is what I get :

With the socket connected to the tube :

RED CR6 / NBE - BLACK GROUND : 1

RED CR5 / BE - BLACK CR5 / NBE : 1
BLACK CR5 / BE - RED CR5 / NBE : 113.8

RED GROUND - BLACK CR4 / BE : 0
BLACK GROUND - RED CR4 / BE : 0

Without the socket connected to the tube :

RED CR6 / NBE - BLACK GROUND : 1

RED CR5 / BE - BLACK CR5 / NBE : 1
BLACK CR5 / BE - RED CR5 / NBE : 113.8

RED GROUND - BLACK CR4 / BE : 0
BLACK GROUND - RED CR4 / BE : 0

(same results)
 
You must log in or register to reply here.
Back
Top