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Commodore PET 8032-B monitor issue

int 21h

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I tested those voltages again with the other multimeter but they remain unchanged, exactly the same as before; I will have a look at that schem now!
Right now the voltage on the 2 outside pins on the motherboard are pin 1: 0.15v and pin 6: 0.02v
I'm using the mains as a ground
 
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dave_m

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Right now the voltage on the 2 outside pins on the motherboard are pin 1: 0.15v and pin 6: 0.02v

Pin 1 = 0.15V indicates that the TTL video signal is at logic 0 instead of pulsing. I wish you had a logic probe. As it can detect pulses, you could probably find the bad IC or shorted node in a few minutes of testing. A cheap one will cost about $20 USD. Or can you borrow a scope?

info on Logic Probe
 

int 21h

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MikeS

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Right now the voltage on the 2 outside pins on the motherboard are pin 1: 0.15v and pin 6: 0.02v
It's pin 7; there is no pin 6; let me ask again:
What is the voltage between the two outside pins of the video connector and does it change as you type?

I'm using the mains as a ground
If you're going to measure voltages you should find a better ground; pins 2, 4, and 7 are all ground and should read 0V.
 
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dave_m

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int 21h;259069 Edit: Are these any good? [URL said:
http://www.ebay.co.uk/itm/NEW-AND-BOXED-Handheld-Logic-Probe-Pulser-/181053618573?pt=UK_BOI_Electrical_Test_Measurement_Equipment_ET&hash=item2a27a2f98d[/URL]

http://www.ebay.co.uk/itm/Laser-526...t=UK_Hand_Tools_Equipment&hash=item53f2a50d61

The second one does not mention if it detects pulses. It probably does but you should ask the vendor before buying that one. All these cheapos are made in China and the quality varies a lot.
 

int 21h

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I'll buy one of those probes if they are decent, not sure if they are or not!
Anyways here is a voltage output from those 2 ICs
UD4
1- 0.22v
2- 3.047v
12 - 0.94v
13 - 4.94v


UC2
8- 5.03 v
9- 0.2v
10- 0.18v

Edit:
Just saw the above posts, give me a minute or two to get those details
 

dave_m

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Let me ask again:
(It's pin 7; there is no pin 6)

If you're going to measure voltages you should find a better ground; pins 2, 4, and 7 are all ground and should read 0V.

Mike,
His original measurements were numbered backwards. Correcting that, his ground measurements were all 0.020V which is pretty good.
 

int 21h

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Just double checked those numbers... That's what I am getting (5.03v on pin 8 of UC2).
Running the probes on the video connector on pins 1 and 7 gives 0.13v
And connecting pin 7 to the mains earth gives 0.02v
 

dave_m

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Edit:
Just saw the above posts, give me a minute or two to get those details

What Mike is asking is a good test of the video signal. Using the black lead of the voltmeter on J7 pin 7 (pin 6 is blank), continuously measure pin 1 while someone enters a bunch of input data including return keys on the keyboard. If the voltage start to rise above the 0.15V, we have an indication of good video data.
 

MikeS

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Just double checked those numbers... That's what I am getting (5.03v on pin 8 of UC2).
Running the probes on the video connector on pins 1 and 7 gives 0.13v
And connecting pin 7 to the mains earth gives 0.02v
Sounds like you're counting the pins the wrong way.

The IC notches are on the left, so starting at the notch and going counterclockwise puts pin 8 at the right rear corner; sounds like you may be starting at pin 7 and going clockwise...
 

dave_m

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Something wrong there; UC2/8 (5.03V) is (or should be) connected to pin 1 of the video connector, which you say is .15 or .13V

Mike,
Yes, something funny there...
Also he has different voltages on UD4 pin 12 and UC2 pin 9 which are connected. I wonder if he is locating pin 1 on the ICs correctly or ???
 

int 21h

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Yeah I put a good few chars in, then I reset the machine to be sure and I held down the spacebaruntil it beeped about 13/14 times
Edit
I went by what I learned in CS a few years ago... for the pinouts
Untitled_zpsf5d6279e.png
 

int 21h

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Sorry I didn't see the bit about Repeat and M... it went up to 0.4v this time when I held them down!
Nor did I see the (few) above posts about IC pins
 
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