TV worked fine in it's case, now blows fuses since in cab frame...?

[Level42]

I bought a great working Nokia TV (55cm, 20") to use as a replacment in my Galaxian cab.
(Since I live in Europe, a TV is a fine arcade monitor, using the RGB on the SCART connector).

As long as it was in it's case, the TV was working fine. I carefully removed CRT and PCB, as far as I know without damaging anything.

Now I placed it in the original Galaxian wooden frame, and I tested it (without any connections). It immediatly blows out a fuse when powering up. It's not the mains fuse blowing, but one in the higher voltage section it seems.

The PCB is on the original stands, which I moved to fit the new PCB. I diconnected the degaussing coil, but it still blows fuses. Could this have anything to do with the meshwire that's running over the outerside of the CRT ? There's a grey wire going from the neck-board to a clamp. I actualy forgot how this was placed originaly, should this be connected to anything ?

[Level42]

OK, I know a little more now.
The fuse that blows is the fuse in the +300V line in the power circuit. The fuse blows because the  HOT is dead, it's shorted in all ways. Could this been caused by the "mesh"-wire not being in touch with the backside of the CRT ? As far as I can see on my other monitors, this mesh-wire should be contacting (or connected to) the mounting eyes of the CRT and run over the conducting paint on the back of the CRT. It's possible that it wasn't connecting to any part of the CRT at the very first time I started up the monitor, could this have been the cause ?

I need to get a replacment HOT, but electronics part store is closed now for the weekend....

Any help is appreceated ! Thanks

[grantspain]

i don't think it is to do with the earth strap unless its shorted to something-more likely to me is a short on the degauss coil,you can prove this by disconnecting the degauss coil from the chassis
the horizontal/h.o.t/frame transistor will read short in circuit on the outside legs,the best way to read it is to either meter from the chassis ground to the centre pin or remove the transistor and read
the earth strap does need to cover the dag of the tube and link to the crt bolt holes and it should also go to the neck card,but usually without this you only get interference issue's unless of course you have had some sort of flashover

[Level42]

Thanks for your reply grantspain !

I think the strap is contacting both dag and and the bolt holes properly now.

I've desoldered the HOT (or at least I THINK it's the HOT) and it shows shorts all ways between al pins...
so it's dead alright, but wonder what could have caused it to die...

Need to wait until tuesday to get a couple of replacement BUF450A's.

I've attached a snippet of the schematic that shows the bad transistor which I think is the HOT (AD01 is the part number).

The full schematic is here: http://www.eserviceinfo.com/download.php?fileid=4806

[grantspain]

looks like the power transistor,if thats short then that would certainly blow the fuse.it would be wise to check the diodes and caps on that transistor line as well,check the zenor diode at VO14

[Level42]

yeah, so that's not the HOT I guess...the HOT is driving the fly-back right ?
Anyway, I measured the zener V014 and it seems fine (the typical 0,6 V in "through" using the diode test of my Fluke multimeter, something like 1,4V the other way around but I guess that's from the cap that's paralllel to it. Anyway it's not shorted.

The other diode and caps also no shorts. When measuring now on the places where the the power transistor was, there are no shorts as well.

 I also checked the bridge rectifier diodes, all fine.

Could a problem on the secondary side of the transformer cause this transistor to blow ? I would guess not really.....

I guess I just have to get a new power transistor ( a couple really )and see what happens.

Thanks for the help Grantspain...