CRT monitor repair

[Dr.EM]

I have a CRT monitor here. It is broken, it broke several months ago now, but at the time I replaced it with a TFT. Right now though, it's in the way and I think I can sell it on if I repair it.

The fault at the time was it just failed to switch on, and after that I measured a broken circuit across the AC inlet, so i'm assuming its just a blown internal fuse. I probably need to open it to find out what replacement I need. It is model Samtron 78E if that helps at all. Like I say, it has been unplugged for months, are there any electrical hazards with opening it now? Its a good unit really and would be a shame to scrap for parts.

 

[kchriste]

I should be safe to open by now. But be carefull of the 4KV+ anode voltage if it has been powered recently. It's that big, usually red, wire with the suction cup attached to the side of the CRT. The bigger the tube, the higher the voltage and the tube forms a capacitor that holds that charge for a long time. If I plan to disconnect this wire, I always dicharge it by slowly slipping a grounded screw driver (holding the insulated handle of coarse!) under the cup until I hear it discharge. Never do this while the unit is powered up because you can damage the flyback.
Sometimes fuses just blow due to a line surge. Usually just replacing the fuse will only result in you having 2 blown fuses. ;-)

 

[Dr.EM]

Thanks, unfortunately I cant get the damn caseing off though. It seems to clip in at the front and will not come undone. It's probably too battered up to be resold now anyhow in my attempts to remove it, so it'll probably be smashed open with a hammer for parts.

 

[spuffock]

I would like to point out that CRT monitors fail to be GIVEN AWAY at radio rallies these days, before you go and waste time and effort.

 

[Dr.EM]

Yep, that's pretty much what I thought, hoped to mabye get £10 for it being quite a good unit. I did get it open after finding the clips, though there are forced entry marks from previous attempts. I was right, it was the fuse that had broken as I measured oc across it. It oddly suggests either a 15A fuse or a 4A (possibly for fast blow and slow blow respectivly?). Either way, it was fitted in an awkward fuse holder that would need de-soldering to replace and that would just not be worth it. As for parts, there isn't much there. A nice 450v 220uF cap, some power diodes and resistors and some to220 devices, but still not really worth it as once they are desoldered, they will have virtually no lead left for using. So, it looks like it might be scrapped, seems a waste but thats how it goes now I suppose

 

[berniedd]

Unless you can fix it yourself, it probably won't be an economical repair. That's why business companies simply replace a monitor gone bad instead of getting it fixed. For sure, your fuse blew because there's a shorted part in there. If not in the power supply, it's in the horizontal output stage.

Kchriste, you are right about the high voltage dangers in the "red wire with the suction cup." However may I point out that the working voltages there are not 4,000 volts - it's more like 15,000 volts or even higher.

 

[Nigel Goodwin]

Kchriste, you are right about the high voltage dangers in the "red wire with the suction cup." However may I point out that the working voltages there are not 4,000 volts - it's more like 15,000 volts or even higher.

I would suspect even higher?, bear in mind the higher the EHT the sharper the picture, so monitors probably run higher then TV's - and TV's are normally 25KV upwards (except portables, which are probably 20KV or so)

 

[Dr.EM]

I still have it here, not sure whether to take those few useful looking parts. That red wire and cup looked pretty ominous, it was attached to a plastic block on the PCB, would that be the flyback? It was pretty small, but I couldn't see anything else that looked like one, not a whole lot in there. I have taken photos of inside for future reference

 

[Hero999]

I think I can sell it on if I repair it.

For how much though?

You'd be lucky to get £20 for it.

 

[Dr.EM]

Confirmed now that it has no real value, £20 would have been alright though! It's just that I damaged the case opening it and the fuse is fixed in solidly. Also, is the opinion that the fuse went due to a short in the unit, not due to a mild line surge?

 

[Nigel Goodwin]

Also, is the opinion that the fuse went due to a short in the unit, not due to a mild line surge?

It's usually due to a fault, although a good mains spike might have done it - but certainly the odds are for a fault!.

 

[spuffock]

Take a look at the fuse, if it is a glass one you can usually tell what sort of end it met. Shiny black, something gone dead short. Doesnt look blown, maybe it just fell apart, they do sometimes.