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Commodore PET 8032-B monitor issue

Probably touched the middle pin then!
I was using the ground in the mains as an earth - although I had to move the probe to get contact so that's probably what led to the varying voltages.. I used a proper ground at work to get the 20V. I will double check it again today to be sure - the tube is still lighting so it's probably a contact with earth problem...
But yes, I will bring the machine out to this guy tomorrow and see if he can fix it... I'll report back after that!
I really appreciate the help so far guys :) Thanks
 
These problems usually end up as pretty simple repairs, once you find the cause ;-)

Without a 'scope it's hard to be sure, but it looks like we have reasonable video signals coming from the PET.

Since the filament is lit we presumably have a reasonable DC voltage inside the display, so the AC voltage at the transformer must be OK.

If a fluorescent lights when you hold it against the HOT where the HV cable come out then you have high voltage and probably a correct horizontal drive signal.

A vertical problem would likely give you a horizontal line, so if you have HV then that only leaves the video amplifier. Simple.

If there is no High Voltage then it gets a little more complicated; probably the HO transistor and/or related passive components (as in Tez's case) or the HOT itself.

Also check the two connectors going to the deflection yoke.
 
int 21h said:
But yes, I will bring the machine out to this guy tomorrow and see if he can fix it... I'll report back after that!
Click to expand...

Make sure you bring a hardcopy of the 12" monitor video board schematic and and board layout that I referenced in a previous message. Provide him with this link to the index of the scope waveform traces of various test points referenced on the board schematic to help in the troubleshooting or provide hardcopies of them. Scope Traces

Note that there are two versions of the video board layout and schematic. Make sure you are using the one that matches the board in your machine.
 
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MikeS said:
If a fluorescent lights when you hold it against the HOT
Click to expand...

Forgive my ignorance but I don't know what you mean here - a fluorescent bulb/lamp? :confused:
Edit: Thanks Dave, nearly forgot the print that! I have the right one, seems to match it perfectly
 
Nope we only have standard bulbs in here.. Been meaning to replace a few anyway so I will buy a bunch of them.
Didn't think the voltage was so high it could have an effect on items outside it!
 
int 21h said:
Didn't think the voltage was so high it could have an effect on items outside it!
Click to expand...

I think the CRT anode supply(point A on the video board schematic) is pulsing at over 10000 Volts. Not a lot of current so it may not kill you, but it is sure to put you on the floor. Other places on that board are at 412V, 194V and -155V.

Never be cavalier around a CRT. And this coming from a guy who is not brave enough to have ever taken the case of his PET monitor. :D
 
dave_m said:
I think the CRT anode supply(point A on the video board schematic) is pulsing at over 10000 Volts. Not a lot of current so it may not kill you, but it is sure to put you on the floor. Other places on that board are at 412V, 194V and -155V.

Never be cavalier around a CRT. And this coming from a guy who is not brave enough to have ever taken the case of his PET monitor. :D
Click to expand...
I suspect the CRT anode voltage is considerably lower on a small monochrome tube like this one, but a good thing to stay away from in any case; it's usually the cuts on your hand from hitting sharp objects when you jerk your hand away. Old colour TVs are a different matter...

Depending on where your hands are etc. it's actually those lower 100-500V voltages around the primary of the HOT and the HO transistor that can be dangerous because they can supply enough current to burn and keep you from pulling your hand away, just like the 110 (220 in Ireland) volt mains voltage.
 
Well it lights up.. not as much as your bulb but I guess that's because I haven't put it as near
Image is on its side for some reason...
IMG_0385_zpsb69d9d4d.jpg
 
int 21h said:
Well it lights up.. not as much as your bulb but I guess that's because I haven't put it as near
Image is on its side for some reason...
Click to expand...
Excellent. The horizontal drive and high voltage sections are the trickiest to work on, so since it looks like that's working it should be a fairly simple problem to track down and fix; good luck to you and your friend with the 'scope.

I'm curious to see what it turns out to be.
 
Ok so he doesn't have an oscilloscope.... Don't ask me how he fixes TVs as I honestly don't know.. Looks like I am goosed for now, at least until another board shows up on eBay... which could be a long way off..
 
int 21h said:
Looks like I am goosed for now, at least until another board shows up on eBay... which could be a long way off..
Click to expand...

No way! You have done the hard part in isolating the problem to the video circuit (hopefully). Look at the schematic at the upper left side. There are only 4 NPN transistors in that circuit. If you are lucky, the problem will be with one of them. We will show you a way to check the transistors with power off and using only the ohmmeter function of your multimeter. You may have to remove the module for easier access. Hang in there.
-Dave
 
Sounds good :)
I will be able to attack those tomorrow when I get my solder station.. Left it at work again! I see the ones on the schem, on the board they all seem to be in a line which is handy! By the way.. how bad is the damage when one of those large caps explodes? Smaller ones tend to char the cases etc.. but those look like they could be case dynamite!
 
Didn't even read that properly! (getting tired :p)
No need for the solder station.. however I don't have my multimeter on me either so I will get that tomorrow :p
 
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