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Commodore PET 8032-B monitor issue

int 21h

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Hi there
I got a Commodore PET 8032-B today but the monitor doesn't show anything on the screen.
It beeps when powered on, beeps when you hold down the space key after a few seconds
The tube glows on the CRT/
None of the chips seem to get hot after being left on for a period of time and nothing looks blown on the CRT PCB.
Any ideas?
 

dave_m

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The good news is the beeps indicate that a lot of the main board hardware is working. There is 22VAC going to the board from the chassis transformer and a +18VDC regulator on the board. But the tube glow indicates they are probably working to get filament power. What kind of test equipment do you have? Voltmeter, scope?? Do you have the link to the Zimmer schematics for the 8032? Also check/reseat the J7 connector on the main board. This cable takes the video, horizontal drive, and vertical drive TTL signals to the video board.
-Dave

Zimmer PET schematics
 

int 21h

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Thanks Dave
I'll reseat that cable tomorrow! I'v left the machine at work - I have a standard multimeter, no oscilloscope or anything like that (yet) unfortunately. I don't have any schematics (I'm pretty new to all this so I wouldn't really know what I am looking at to be honest, I only understand the basics of 74LS08N, 74LS86AN etc
 

MikeS

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Thanks Dave
I'll reseat that cable tomorrow! I'v left the machine at work - I have a standard multimeter, no oscilloscope or anything like that (yet) unfortunately. I don't have any schematics (I'm pretty new to all this so I wouldn't really know what I am looking at to be honest, I only understand the basics of 74LS08N, 74LS86AN etc
Can you measure the voltages on the six pins of that video connector?

And don't forget to check the connector at the other end of the video cable, inside the monitor.
 

int 21h

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The connector is fine in both places, I reseated it a while ago just to be sure!
The voltages are as follows..
Pin | Voltage
1 | 20mV
2 Blank (no pin)
3 | 3.4V
4 | 20mV
5 | 4.7V
6 | 0.0198V
7 | 0.15V

Will check the AC in a while
 

int 21h

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The AC going into the monitor from the transformer is only 7V... which is well below the above specified 22VAC.. Any ideas what could be causing it?
 

dave_m

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The voltages are as follows..
Pin | Voltage
1 | 20mV
2 Blank (no pin)
3 | 3.4V
4 | 20mV
5 | 4.7V
6 | 0.0198V
7 | 0.15V

Your pinouts are numbered upside down. The key is on pin 6. So that makes: Video = 0.15VDC which does not seem right; I think a mostly blank screen should be > 3.5 VDC, Vertical Drive =4.7 VDC seems a tad high, Horizontal drive = 2.4 VDC is OK. These average DC voltages gives us a good clue on how to proceed. MikeS will know better what the voltages should be.
-Dave
 
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MikeS

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Your pinouts are numbered upside down. The key is on pin 6. So that makes: Video = 0.15VDC which does not seem right; I think a mostly blank screen should be > 3.5 VDC, Vertical Drive =4.7 VDC seems a tad high, Horizontal drive = 2.4 VDC is OK. These average DC voltages gives us a good clue on how to proceed. MikeS will know better what the voltages should be.
-Dave
Actually I think those video signal voltages are in the ball park; anything other than Vcc or Gnd is usually a sign that the board is putting out something.

I'm sure surprised that the CRT is lit with only 7V AC coming out of the power transformer; what are the numbers of the transformer terminals to which the display is connected?

What are the DC voltages on the three pins of the display's voltage regulator?

If it really is only around 7 - 10 V and depending on what you read on the regulator I'd probably suspect the bridge rectifier.
 

dave_m

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Actually I think those video signal voltages are in the ball park; anything other than Vcc or Gnd is usually a sign that the board is putting out something.
Hi Mike,
I was thinking with the screen mostly blank, the video signal would be closer to the +5V side (black rather than white) or do I have that backwards?

I'm sure surprised that the CRT is lit with only 7V AC coming out of the power transformer; what are the numbers of the transformer terminals to which the display is connected?

What are the DC voltages on the three pins of the display's voltage regulator?

If it really is only around 7 - 10 V and depending on what you read on the regulator I'd probably suspect the bridge rectifier.

Good ideas.

Int21h,
Here is a diagram of the transformer. See Table 1.
http://zimmers.net/anonftp/pub/cbm/schematics/computers/pet/8032/8032051-3.gif
 

MikeS

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Hi Mike,
I was thinking with the screen mostly blank, the video signal would be closer to the +5V side (black rather than white) or do I have that backwards?
I think you do; I just checked it and with my DMM the video signal starts around .35 V and goes up as I fill the screen. Another diagnostic test ;-)
 

dave_m

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Mike,
That is a very good test. However, I'm a little worried that Interrupt21's data was 0.15V. That's even below some of his ground readings. Do you think there may be an issue with the video output or should we investigate the 22VAC issue? I guess a shorted diode on the video board can cause this? We should have him remove the J7 connector and retest if it was connected before.
 

crock

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I think you do; I just checked it and with my DMM the video signal starts around .35 V and goes up as I fill the screen. Another diagnostic test ;-)

And also easy to blind type. Press the " key and hold down any of the cursor keys.

I think the voltage on the video signal is believable. Just having the basic prompt will have a very low duty cycle on the video signal.

The AC voltage is worthy of investigaton. If there is a shorted diode I would expect the transformer to buzz loudly and hopefully a fuse would pop. Might be worth disconnecting AC supply to the monitor and measuring AC with no load. Transformers usually work or don't, so any low voltage here would be unusual.

Rob.
 

dave_m

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Might be worth disconnecting AC supply to the monitor and measuring AC with no load. Transformers usually work or don't, so any low voltage here would be unusual.

Rob,
Yes, you are right. My suggestion of removing the J7 will not disconnect the AC load. He will have to unsolder a wire at the transformer.
 

int 21h

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Tried it with my other DMM...... the transformer is actually giving off 20VAC...
Oddly enough... with the connector for the monitor to the motherboard disconnected the screen - sometimes - works (very bright dot in the centre)... then plugged back in very soon after this (I switch it off first of course and then back on after plugging it in) the bright dot shows for a few seconds.. shakes up and down (like a vibration...) and then disappears... not like a standard crt where it fades in the centre.. it just disappears
Edit:
Tried it again and the screen came on and faded.. I guess it's losing/not getting enough power (the motherboard wasn't plugged in)
 
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MikeS

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To me this sounds like the CRT and video board are basically OK. I think your video signal is being held low suggesting something is wrong with the video generator circuit. See upper right corner of this schematic:
http://zimmers.net/anonftp/pub/cbm/schematics/computers/pet/univ2/8032087-08.gif
Now that we know that the display supply is OK I think I agree with you (as I usually do in the end ;-).

Obviously 20mV means he's not using a very good ground for reference or his meter is bad; on the other hand a -5mV video signal doesn't sound right either. What is the voltage between the two outside pins of the video connector and does it change as you type?

Sure looks like a problem in the area Dave's pointing at.
 
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