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Another sick 2001 PET - no video display whatsoever

C

circuit

Experienced Member
Joined
Feb 16, 2015
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175
I've pulled out my other 2001 PET to fix the "garbage screen" it had, but I'm disappointed to find that the monitor is not powering up at all.

It's been stored dry in my house at room temperature and previously the monitor powered up fine.

No whine (no sounds at all, and no faint CRT power-on "thud" sound), no glow in the back, no raster lines, nothing when I power it off. I've had PET monitors fail but still appear to receive power - never like this.

I checked the connections on the transformer and it's still disconnected, it's not been moved from it's position in a long time and overall the machine is in excellent cosmetic condition with no rust or corrosion visible.

Power is still getting to the logic board because the cassette drive whirrs momentarily, the ROMs and RAM heat up, and there is 5v getting to the internal cassette port etc. It's as though the CRT section is not connected at all.

I looked in the back and could not see any type of fuse on the CRT board, I reseated all connectors - still nothing.

Unfortunately I don't have an oscilloscope, just a multi meter, so my own fault finding may be limited. However, I was wondering if this is a known issue?
 
circuit said:
I've pulled out my other 2001 PET to fix the "garbage screen" it had, but I'm disappointed to find that the monitor is not powering up at all.

It's been stored dry in my house at room temperature and previously the monitor powered up fine.

No whine (no sounds at all, and no faint CRT power-on "thud" sound), no glow in the back, no raster lines, nothing when I power it off. I've had PET monitors fail but still appear to receive power - never like this.

I checked the connections on the transformer and it's still disconnected, it's not been moved from it's position in a long time and overall the machine is in excellent cosmetic condition with no rust or corrosion visible.

Power is still getting to the logic board because the cassette drive whirrs momentarily, the ROMs and RAM heat up, and there is 5v getting to the internal cassette port etc. It's as though the CRT section is not connected at all.

I looked in the back and could not see any type of fuse on the CRT board, I reseated all connectors - still nothing.

Unfortunately I don't have an oscilloscope, just a multi meter, so my own fault finding may be limited. However, I was wondering if this is a known issue?
Click to expand...


Is the CRT powered by the transformer? Check the voltage...
There are two wires going from the transformer to the CRT:

https://upload.wikimedia.org/wikipedia/commons/b/b3/Commodore_PET_2001_with_lifted_top.jpg

I'm not sure about the PET 2001 ('till now mine always worked fine so I've never opened the CRT assy), but the 8032 CRT board has some (two?) resistors that are mounted diagonally: one side hitting the board, the other side of the resistor raised few mm. from the pcb. IMO they act as fuse, too.
One day my 8032 CRT stopped to work suddenly, and somebody here suggested me to check them. I found one of them was burnt. Since I hadn't another like that, I replaced it with a normal resistor. No more problem, 'till now!

If you can't find that kind of resistor, won't hurt to check all of them... a multimeter will be ok for that.

if you also have a 30xx, you can swap the monitors for testing purpose, they are exchangeable.
 
Check the voltage on the brown wires off the transformer. Should be ~19.5v AC for a 9" screen, or 21v for a 12" screen.

Next look at the board in the back of the monitor. Don't touch! You're seeking a resistor that has popped. It's on legs, sort of, lifted up off the board. Cannot remember which one it is, but if it is blown it should be obvious - black scorch marks, cracked, odd smell, etc. This is (apparently) an intentional "fuse" resistor that goes to protect the video circuit. I read it on a thread here when searching old PET diagnostic threads.

Edit: giobbi beat me to it!

Be careful in there. Even when turned off the screen can maintain a charge. Stay away from the HT lead (is plugged into the side of the tube, looks like a rubber sucker cup). Especially if it is cracked in any way!
 
JonB said:
Check the voltage on the brown wires off the transformer. Should be ~19.5v AC for a 9" screen, or 21v for a 12" screen.

Next look at the board in the back of the monitor. Don't touch! You're seeking a resistor that has popped. It's on legs, sort of, lifted up off the board. Cannot remember which one it is, but if it is blown it should be obvious - black scorch marks, cracked, odd smell, etc. This is (apparently) an intentional "fuse" resistor that goes to protect the video circuit. I read it on a thread here when searching old PET diagnostic threads.
Click to expand...

ah ah ah same advise at the same time. But mine wasn't so obvious to find (no crack, no marks, no smell). I had to check it with my multimeter.
 
circuit said:
No whine (no sounds at all, and no faint CRT power-on "thud" sound), no glow in the back, no raster lines, nothing when I power it off. I've had PET monitors fail but still appear to receive power - never like this.
Click to expand...

The video board needs a good 15 KHz Horizontal Drive (TTL) signal from the main board to use as the switching signal to generate the high voltage. Check that signal on the main board with a voltmeter on DC and see if you get about 2 Volts.
 
Thank you, I will check the horizontal drive signal. I will also do a board swap (I thought my PETs had different motherboards but they're actually identical)

I did check the power from transformer is getting all the way up to the monitor board, so that would seem to support the suggestion that it's not being switched on by the video signal.

I will report back once I've checked it all, thanks for the helpful comments so far :)
 
circuit said:
I did check the power from transformer is getting all the way up to the monitor board, so that would seem to support the suggestion that it's not being switched on by the video signal.
Click to expand...

There are several components in the video board that can cause a lack of high voltage including the +12V regulator that converts the low voltage AC from the PET transformer into DC, but yes, check the easy things first on the main board.
 
dave_m said:
The video board needs a good 15 KHz Horizontal Drive (TTL) signal from the main board to use as the switching signal to generate the high voltage. Check that signal on the main board with a voltmeter on DC and see if you get about 2 Volts.
Click to expand...

It's just under 1.5v with nothing connected, dropping to approx 1.4 with the video cable connected.

Just before I checked this, I had the PET up on it's stand and it accidentally fell shut.

When this happened, the screen started displaying with a perfect picture!

I swapped the RAM around and was able to boot into BASIC

I ran a test program counting incrementally.

the joy was short lived! :mad: After a while of running the test program, I switched it off and saw a bright dot slightly off centre, which faded out slowly.

After this, it's back to exactly the same behaviour as before: booting up but with no apparent power to monitor
 
Further update:

(for ease, my faulty PET I will call PET#1, the fully working machine is PET#2)

I removed the board from PET #1, and swapped in the known good board from PET #2. It doesn't display anything.

Conversely, the board from PET#1 displays fine in PET #2.

So I guess the issue is with PET#1s PSU or monitor section?

Just to re-iterate the previous comment, I did briefly have it working, and the picture was completely fine (perfect, stable, sharp) apart from the stray bright dot on power down.
 
circuit said:
It's just under 1.5v with nothing connected, dropping to approx 1.4 with the video cable connected.

Just before I checked this, I had the PET up on it's stand and it accidentally fell shut.

When this happened, the screen started displaying with a perfect picture!

I swapped the RAM around and was able to boot into BASIC

I ran a test program counting incrementally.

the joy was short lived! :mad: After a while of running the test program, I switched it off and saw a bright dot slightly off centre, which faded out slowly.

After this, it's back to exactly the same behaviour as before: booting up but with no apparent power to monitor
Click to expand...

Sounds like it's time to start checking solder joints.
 
I agree about cold solder joint. If you can get to the monitor board while powered on, flex board with the handle of a screwdriver then check for solder cracks on power transistors mounted on heat sinks with a magnifier. If hard to locate but does power on and off as you flex it, can also darken the room and look for a tiny flash on the PCB while flexing. If it's a power connection or horizontal output drive can see a tiny spark. Larry G
 
Thanks everyone, sounds pretty unanimous, I will investigate this over the next few days and let you know what happens!
 
I gently flexed the monitor board and sure enough, with a little pressure the back of the CRT glows and I get a perfect clear display! Thanks everyone!

I've not worked on CRTs before, but since this now appears to be a reasonably straightforward issue, I feel more confident tackling it myself. So I'll discharge the board and look for suspect solder joints - is there any point in particular is prone to failure?
 
Yes but it doesn't matter because that's never the one it ends up being. :)

Get yourself a loupe, and prepare to spend a lot of time looking at different angles. Sometimes it's easier just to reflow every solder joint.
 
circuit said:
I gently flexed the monitor board and sure enough, with a little pressure the back of the CRT glows and I get a perfect clear display! Thanks everyone!

I've not worked on CRTs before, but since this now appears to be a reasonably straightforward issue, I feel more confident tackling it myself. So I'll discharge the board and look for suspect solder joints - is there any point in particular is prone to failure?
Click to expand...

Don't forget to check the cable connector that brings the video data and timing signals to the video board for good connections. As well as the low voltage AC wires to the video board.

The bright dot on turn-off is due to a bad capacitor, but I don't remember which one. Does anyone know?
 
Chase down the simplest symptom. If the CRT filament glows on/off. There is AC from the transformer to the monitor board on pins 1 and 2 then to a bridge rectifier feeding DC to VR1, a 12V regulator then to the filaments if I'm looking at a close
enough schematic. Don't keep flexing just look with a magnifier and follow the circuit path. Cracks around VR1 the 12V regulator would be likely or the wires into pins 1 and 2 of the connector. Larry G
 
It can also be helpful to poke the wire/lead at the solder joint with a screwdriver when looking with magnification. Hairline cracks in solder can virtually disappear when there's no movement.
 
I usually use some non-conductive piece of plastic or wood and poke away, starting at connectors and any other places of the board that look like they've developed hot spots, etc.
 
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