Sony PVM9044QM CRT monitor - HV discharge from anode cap

[philpem]

I've got a Sony PVM9044QM CRT on the bench -- problem with it is, there's a periodic arcing which sounds like it's coming from the anode cap.

The cable was absolutely pristine and bright red, but I've cleaned the whole anode cap and the cable back to about an inch from the FBT. Still makes a 'crack' sound a few times per second, making the video glitch for a line or two when it does.

I figure I'm dealing with either a busted anode cap (it looks fine...) or some muck underneath it. The muck underneath I'm suspicious of, as the existing anode cap is stuck on with some very sticky grey goop (which I assume is the Shinetsu high-voltage rated RTV silicone mentioned in the service manual).

I put a little bit of pressure on the rubber cap under the cable grip using a plastic trimming tool, and pulling it a little further away from the chassis seemed to stop the noise. Sadly this isn't a viable option as there's nothing to anchor it to and the analog board is already uncomfortably close to the cable.

Couple of questions then...

- How the heck do you remove the anode cap when it's so thoroughly glued down? Is there a trick to this? I've had a quick try with a screwdriver (while I was trying to discharge the HV) and it went straight up and through the RTV and out the other side.

- What's a suitable replacement for the RTV -- I mean I assume I'm going to have to replace it when refitting the anode cap.

- If it turns out the anode cap is shot, where can I get another? Am I looking at a new FBT?

- Does this even need repairing - how much of a risk does this pose to the monitor?

Cheers,
Phil.

[james_s]

I use dielectric grease on problematic ones, you can find it in autoparts stores near the spark plug wires.

Worth checking the HV too, if it's too high it can arc. And clean off any traces of carbon.

[philpem]

Nice - thanks, James.

Just got the anode cap off and found out why it's arcing all over the place. Someone's been in here before. There's a nice layer of carbonised finger grease under the anode cap, and said cap was split almost down the middle.

So now I need to get my hands on a new anode cap...

[james_s]

That'll do it, carbon is conductive, you have to get it all off.

Dunno where you can buy just the anode cap, but it's not too hard to find surplus flybacks. You might also be able to patch it up with some silicone.

[philpem]

Replaced the anode cap with one from a HR Diemen flyback I had lying around. I put a nice coating of silicone grease around the anode port on the CRT and on the rim of the anode cap to seal it.

As it turned out there was another issue too -- the white circular spacer which stops the HV cable getting too near the PCB or the shield. Over time, this has become slightly conductive. So I wrapped two 6in strips of Kapton tape around that part of the wire to add extra insulation and refitted the spacer. Goodbye crackling!

Cheers,
Phil.

[james_s]

That's odd, I've never seen that happen before. Was the spacer carbonized? Did you try cleaning it?

I just looked up that model monitor and realized I have one of them too, beautiful little thing, I fished it out of the e-waste bin at a previous job where it had been unceremoniously dumped.

[philpem]

I dunked it in isopropyl and gave it a good scrub - no change. No trace of carbon, it looked clean.

I brought a grounded screwdriver near the thing and got a nice arc from it, and also managed to draw one from the insulated HV cable. So perhaps the HV cable is the real problem. That's a new-flyback job to replace, by the looks of it. I'd be surprised if the FBTs were still available at a reasonable price.

[james_s]

That's really bizarre, the cable I would expect to see some corona if you hold a grounded screwdriver right up to it but it shouldn't draw an actual arc. The spacer should be completely non-conductive, never had any issue with one of those. You could probably make a replacement out of a plastic lid from something if you have further issues with it.

[Ian.M]

There must be a pin-hole in the cable insulation.  If you want a reliable repair, and the cable is sealed into the flyback transformer (i.e. non-replaceable),  you'll need to locate the pinhole, clean the cable with IPA for several inches either side, and dry thoroughly, then patch the insulation with clear silicone self-amalgamating tape, building it up to double the existing diameter at the pinhole, and tapering it over a few inches either side of the damage to give enough creepage distance along the cable jacket to self-amalgamating tape joint.     As silicone self-amalgamating tape is quite soft, you should probably remove the anode cap from the cable so you can add heatshrink over the whole repair.

Testing for defects by arcing to ground is risky - it can burn out the flyback chopper transistor (line output transistor in TVs), and even burn out the transformer.