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Any good examples on discharging a CRT that is actually charged?

C

cwathen

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Feb 1, 2018
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Bristol, UK
About to do my first ever flyback transformer replacement. The monitor in question (an Amstrad PC-CM) was arcing like hell in 2014, and confirmed due to a dead flyback by an ex-TV engineer I knew. At the time no flyback could be sourced so I just sat on the monitor (complete Amstrad PC system in fact) in case parts were ever needed for my second, working system.

I've just found a NOS transformer and assuming nothing else is wrong hope it will be a simple enough job to swap it out and get a working colour monitor to go on my RM Nimbus PC.

Firstly, I would assume since no attempt has been made to power it up in over 5 years that the monitor would definitely be safe to work on now!

However, just in case I want to try discharging it...the trouble is every video I've ever seen on the subject is on a discharged tube where nothing happens. Part of me would wonder if just having a little wiggle under the suction cup with a grounded screwdriver and getting nothing was because it is discharged or because I didn't do it right.

Does anyone know of any examples of a discharge on a *charged* tube which does go kerpap, mainly so I can see where they've poked to make that happen?
 
the anode lead's connector is a big metal clip in the center of the suction cup, you really can't miss it

if it doesn't pop, it's ok just tap it a couple times with a grounded implement and call it good
 
cwathen said:
About to do my first ever flyback transformer replacement. The monitor in question (an Amstrad PC-CM) was arcing like hell in 2014, and confirmed due to a dead flyback by an ex-TV engineer I knew. At the time no flyback could be sourced so I just sat on the monitor (complete Amstrad PC system in fact) in case parts were ever needed for my second, working system.

I've just found a NOS transformer and assuming nothing else is wrong hope it will be a simple enough job to swap it out and get a working colour monitor to go on my RM Nimbus PC.

Firstly, I would assume since no attempt has been made to power it up in over 5 years that the monitor would definitely be safe to work on now!

However, just in case I want to try discharging it...the trouble is every video I've ever seen on the subject is on a discharged tube where nothing happens. Part of me would wonder if just having a little wiggle under the suction cup with a grounded screwdriver and getting nothing was because it is discharged or because I didn't do it right.

Does anyone know of any examples of a discharge on a *charged* tube which does go kerpap, mainly so I can see where they've poked to make that happen?
Click to expand...

I'm not the best authority on the subject, but I would like to issue the following precautions: First, it matters WHERE you ground the (screwdriver, probe, whatever you shove under the cap). There is a specific path to ground for the tube, usually in the form of a wire surrounding the tube. You want to be discharging to that. Second, although it's highly unlikely that the tube will retain a charge for five years, DON'T ASSUME. Always discharge. And third, with a recently-charged tube there is often a secondary charge that builds up after the first discharge, so DISCHARGE MORE THAN ONCE.

I hope somebody who knows more than I do will chime in on this topic, but in short, make sure you know what you're doing.

-CH-
 
cwathen said:
Didn't pop...job was quite easy. Sadly didn't fix the monitor though :(
Click to expand...

you unfortunately just learned the hard way that "bad flyback" was an incredibly popular catch-all diagnosis for "the horizontal deflection circuit doesn't work and I'm not sure why"
 
My dad taught me to tune the colour and focus on live CRTs when I was a kid, which must, by necessity, be done live. He never taught me to properly discharge the thing before doing any other work though, and I learned that lesson the hard way.

I made myself a probe with a 25W resistor and an alligator clip for discharging stuff. There's about 1kV per diagonal inch of screen on these tubes, so that should give you an idea of what kind of voltage you're dealing with. The good news is that contrary to popular belief, as long as the monitor is turned off, it's not likely to kill you unless you've got heart problems if you accidentally zap yourself, like I have, but it WILL hurt like hell, and you'll may hurt yourself from the reaction if you hit something.

I've currently got a monitor apart with a bad vertical deflection circuit that I need to get back to at some point. In all likelyhood, it's a microfracture in the circuit board or a cold solder joint, because it stopped working after I transported it. Stuff has kept piling up.

I've never encounters any bad flybacks before, but I have had a case of cracked insulation on the anode wire that would produce the occasional zapping noise, like it was producing a minor short with ambient air moisture, but I was able to fix it with some silicone around the base of the wire.
 
maxtherabbit said:
you unfortunately just learned the hard way that "bad flyback" was an incredibly popular catch-all diagnosis for "the horizontal deflection circuit doesn't work and I'm not sure why"
Click to expand...
With the old flyback in the monitor didn't run at all. When powered you got a lot of arcing at the back of the CRT (sounded like a dead short).

With the new flyback, you now get HV, heater glow and the screen does light up.

The problem is that's all it does - the screen is entirely white and doesn't in any way respond to signal being applied or not. The brightness control will affect the brightness of the screen, contrast does nothing. Vertical hold does affect the height of the picture so you can introduce black bars at the top and bottom. The Horizontal hold seems to slightly affect the brightness of the screen and also the amount of CRT whine from it rather than doing anything to the sides of the picture.

CRT whine on this is very loud compared to the identical working monitor I've got.

I would guess from this there is no sync and no horizontal deflection (although I thought lack of horizontal deflection would cause the picture to collapse into a vertical bar rather than light up the entire screen?)

The design is of the type where the flyback is on the main PCB and is then attached to the back of the tube via another small PCB, it isn't hanging off the back of the tube. I'm wondering if components on the tube PCB were damaged from the arcing of the old transformer?

Started another thread in the PC forum on that specific monitor in case it is a common issue but no joy yet. Hopefully someone will know something which might be generically wrong with it based on the symptoms.
 
“Bad flyback” is the diagnosis for when you’ve blown two replacement HOT’s and you don’t know where else to look :)
 
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