UKC

So, my old (but good) fishfinder on my boat appears to have a loose connection. It's a colour CRT and is losing specific colours. A good whack normally 'fixes' it, but it's got to the stage where whacking no longer works. I can solder, so I was wondering if I'm going to kill it if I try fixing the loose connection / dry joint that's obviously causing the problem? I know there's likely to be capacitors in it that could trouble me, but I guess I can simply discharge these through a resistor to make it safe? Do I need to earth anything before poking it with the iron?
It runs off 12volts if that's of any use.

In reply to Toerag:

Colour CRTs run at a hefty voltage; around 36kV. But very low current (and not present on the end cap, but provided by the encapsulated HT transformer via an obvious HT lead and plug to the body of the tube). So turn it off, and leave it off for some time to let the voltage discharge (it should have a built-in discharge resistor). On a 12V powered display, there should be nothing else in there likely to give you a poke.

Provided you have the unit isolated from any mains supply, it will float to whatever voltage your soldering iron tip is at (hopefully, that's isolated, too), so there should be no need to earth anything.

Connecting a mains earth to the chassis will actually be a bad thing to do, since the unit will no longer be floating, and if your soldering iron tip isn't isolated (e.g is connected to mains earth), any components retaining a voltage bias will be shorted to earth via the iron tip.

You may find that it's not a dry joint, but simply the end cap on the tube, and the contacts have oxidised/corroded (especially in a damp environment like a boat, and even more so if it's a sea fishing boat). Unplug the cap and have a look. Do the usual trick of making and breaking the connection a few times to clear the oxide.

In reply to captain paranoia:

Good-oh, will give it a look tonight

In reply to Toerag:

CRT's have enough lead in the glass that waste ones are normally charcterised as hazardous. But then years of licking TV screens as a kid never did me any harm

In reply to captain paranoia:

that and a careful tweak of any trimmer pots perhaps

In reply to Toerag:

Cracked the case open last night - haven't gone any further, but the problem could well be a small board attached to the pointy end of the tube. The board itself has a big white plastic thing on it which is glued to the back of the tube and the whole board / plastic thingy wobbles a bit relative to the tube. The cables going to the board are glued on as well as soldered and look solid.

In reply to Toerag:


That's probably the 'end cap' I was referring to, if it's basically a squat cylinder. Essentially, it's a thermionic valve socket, since you can think of the tube as a rather special type of valve. The 'pointy end' of the tube is just like a valve base, with a number of metal contact pins mounted in the glass base.

There are three colour 'guns', RGB, and each of these is driven by a signal through those pins, the little board, and from the main board. My suspicion is that if it's not a dry joint on either of the boards, then it is likely to be an oxidised pin on the tube pointy end, or oxidised connector on the mating white socket on the little board. The board may be labeled with colour drive signal names. You will be able to tell which drive is faulty by which colour is missing from the display.

In reply to captain paranoia:

Sounds about right, and I think the colour signal drivers are identifiable. It was just a single colour missing to start with (green or blue, can't remember now), but now the whole display goes off and on intermittently (with shock, like falling off a wave).