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Random Characters Issue on a COMMOODRE PET 2001

dave_m

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I really feel unlucky because the monitor seemed to work just fine... I suppose this happens often anyway...

Stéphane,
In technical work, when things go wrong, we mention something called "Murphy's Law":

"Murphy's law, sometimes called in jest, the fourth law of thermodynamics" which states: "If anything can go wrong, it will."

So do not despair. We shall overcome. :)
-Dave
 

MikeS

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Mike, Can one get to TP1 without zapping oneself?
-Dave
Don't know; I suspect that he has the later model monitor from the one in my 2001.
But looking at the parts layout diagram it looks pretty accessible; behind the 7805 heatsink, beside the two filter caps. You could also check at the regulator or one of the resistors.

We shall overco-ome... reminds me of the 60s ;-)
 

Dwight Elvey

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Hi
I just started looking at this thread. I've not the time to read everything in it.
I did look at the screen video. This is what it looks like when there is no
video signal and no sync. The types of things that can go wrong are things
like the connector being on wrong or a wire broken.
I guess someone else has asked but like I said I don't have the time
to filter so much.
What test equipment does the OP have? Is there a schematic someplace
that I can reference?
Dwight
 

MikeS

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Hi
I just started looking at this thread. I've not the time to read everything in it.
I did look at the screen video. This is what it looks like when there is no
video signal and no sync. The types of things that can go wrong are things
like the connector being on wrong or a wire broken.
I guess someone else has asked but like I said I don't have the time
to filter so much.
What test equipment does the OP have? Is there a schematic someplace
that I can reference?
Dwight
Hi Dwight; fancy meeting you here in the PET department ;-)

That was my first thought, the cable or the connectors, but the OP says they're OK. Symptoms are the same with a known good MB so it looks like it's in the monitor.

Those scan and retrace lines look kinda far apart, and the brightness also seem kinda high (won't go down to black) so I'm kinda suspecting a higher-than-normal voltage somewhere (but what do I know).

There are several versions of the monitor, but I suspect this is the one:

http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001N/321445.gif

Thanks for taking an interest.
 

Dwight Elvey

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The brightness is a symptom of no video input, The peaks of the video
create the darkness, not the other way around.
If he has an ohm meter, he might test from the circuit board
of the video to the video out. It looks just like an open wire to
me. The bright lines are just the veritcal retrace. Normal
for missing video drive.
Since he has been taking it apart and putting it back together,
a broken wire is the most likely. Often with crimped wires, they
look good to the eye but are broken inside. The only way to
check these is to measure them in the system from end to end.
The video circuit is simple enough that if there is a failure there,
it can be easily fixed, once tracked down. No black magic anywhere.
Swapping stuff blindly is always asking for more troubles.
Dwight
 

sbo

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Hi,

Back to the operations.

By reading the last posts, I thought it might be interesting to replace the video cable completely.

So I have made a brand new one which I have tested first on a working PET with success.
Unfortunately, when I tried it on my PET, it made no change.

So now I have :

- Power Supply : Tested OK
- Video cable : Tested OK
- Motherboard : Tested OK
- Monitor : Unknown State ???

Thanks for supporting,

Stéphane.
 

alker

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Hi, once again as posted some pages before: in my opinion the CRT board which is below the monitor has issues.. I once changed the big capacitor on this CRT board and it was alive again; my guess: it is the CRT board!
 

MikeS

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Hi, once again as posted some pages before: in my opinion the CRT board which is below the monitor has issues.. I once changed the big capacitor on this CRT board and it was alive again; my guess: it is the CRT board!
Yes, we did read your previous posts ;-) and yes, you're almost certainly right that there's a problem on the monitor board.

Dwight and I have disagreed a few times before ;-) Sometimes he's right and occasionally I am, and this time I think that missing video wouldn't quite explain what we see on the screen. Didn't hurt to double-check the cable though.

Let's see what some of those voltages look like, if we can.
 

Dwight Elvey

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Hi
There could also be something open on the boards.
That is why I said to measure from the video out
driver on the mother board to the diode on the
input of the video. Something as simple as a broken
solder joint on the board could open the signal.
Is there also a schematic for the mother board?
If the video board is as it shows in the pdf Mike sent,
There are only two transistors in the video driver.
A quick measure of voltages should show the problem.
It is also possible that the tube has a short between
the filament and the chathode. If it is to the ground
end, it would do the same.
Some voltage readings might help find the problem.
Dwight
 

Dwight Elvey

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Hi
I'm not able to down load these images for some reason.
All I get are the red X.
I'll try on another system tomorrow.
Dwight
 

dave_m

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Hi
I'm not able to down load these images for some reason.
All I get are the red X.
I'll try on another system tomorrow.
Dwight
Yeah, I get that for some reason with internet explorer. You can either right click and save the file locally and then open the gif file or use firefox.
 
Last edited:

carlsson

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I wonder if those GIFs on the FTP server are saved as interlaced GIF or something. I have witnessed it for years that many of them are impossible to open with MSIE, but on the other hand perhaps it is another reason to switch browser.
 

sbo

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Hi, once again as posted some pages before: in my opinion the CRT board which is below the monitor has issues.. I once changed the big capacitor on this CRT board and it was alive again; my guess: it is the CRT board!

Hi,

Following Alker's recommendation, I was thinking about replacing this big capacitor.
- Do you think this could make the debugging go a step further ? In other word is it worth trying it ?
- What should I take care of ? I have been awarned of high voltage on the Video Board, even after the computer has been powered off.

Or is there anything else I should do before to try this ?
I must confess I have been a little confused by the last posts a bit too technical for me ... ;-)

Stéphane.
 

MikeS

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Hi,
...
I must confess I have been a little confused by the last posts a bit too technical for me ... ;-)

Stéphane.
I'm not surprised; that's just us self-styled "experts" trying to show each other how smart we are and just confusing everybody (including ourselves) ;-)

One question: do you have (or can you borrow) a meter to measure some voltages? I'm assuming that an oscilloscope is not an option?
 

Dwight Elvey

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Hi,

Following Alker's recommendation, I was thinking about replacing this big capacitor.
- Do you think this could make the debugging go a step further ? In other word is it worth trying it ?
- What should I take care of ? I have been awarned of high voltage on the Video Board, even after the computer has been powered off.

Or is there anything else I should do before to try this ?
I must confess I have been a little confused by the last posts a bit too technical for me ... ;-)

Stéphane.

Hi
Lets do some real trouble shooting before doing any more easter egg hunting.
Can you locate the transistor Q202 on the video board?
Most of these boards have printed lables.
Also, as Mike asked, can you get your hands on a voltmeter?
Dwight
 

Dwight Elvey

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I'm not surprised; that's just us self-styled "experts" trying to show each other how smart we are and just confusing everybody (including ourselves) ;-)

One question: do you have (or can you borrow) a meter to measure some voltages? I'm assuming that an oscilloscope is not an option?

Hi Mike
I'm tending to think it is something in the video board as well but at this
point, it is just a guess. I think there are still some more experiments
that we can run to narrow it down a little more.
The symptoms still point to missing video. That doesn't say where the failure
is. I just mentioned the cable because it was the most likely place, considering
that it had seen the most handling. I was expecting some ohm meter measurements
rather than replacing it.
There are a few experiments that we can do with just a piece of wire. It does
require locating the video drive transistor though ( Q202 ).
Dwight
 

sbo

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I'm not surprised; that's just us self-styled "experts" trying to show each other how smart we are and just confusing everybody (including ourselves) ;-)

One question: do you have (or can you borrow) a meter to measure some voltages? I'm assuming that an oscilloscope is not an option?

Ahah !! ;-) that makes me feel better !! ;-)

OK I have finally decided myself to get a meter ; I will have it in 3/4 days.
Oscilloscope is not an option... At least for the moment...

A friend of mine offered to come with a manual (digital) oscilloscope ; coudl it help ?

Stéphane.
 

sbo

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Hi
Lets do some real trouble shooting before doing any more easter egg hunting.
Can you locate the transistor Q202 on the video board?
Most of these boards have printed lables.
Also, as Mike asked, can you get your hands on a voltmeter?
Dwight

Thanks Dwight.
I managed to locate tonight the tansistor Q202 on the video board.

As I just said, I'll get the voltmeter in 3/4 days.
Stéphane.
 

Dwight Elvey

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Thanks Dwight.
I managed to locate tonight the tansistor Q202 on the video board.

As I just said, I'll get the voltmeter in 3/4 days.
Stéphane.

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
 
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