"Phil Allison" wrote in message
...
Arfa Daily wrote:
Anyone got the schematics for a Crown XLS5000 ? I'm most interested in
the
board that the mains comes in to. There's a bloody great disc something
inside a big lump of heatshrink, and serious flames shoot out of it
before
every trip and fuse in the workshop goes out ...
If I bring it up on the variac, I can't get beyond about 20 volts before
the
poor thing is jumping off the bench, so there's a good short on there
somewhere. I initially thought that the disc might be a big VDR that was
short, but looking up the part number on the second one that's on there
for
the other half of the amp, it appears to be an inrush thermistor, so I
guess
whatever is short, must be south of that ...
** I found a nice pic:
http://www.musiker-board.de/attachme...005-jpg.84415/
The XLS series of amps use large, 240VAC rated PTCs for inrush limiting
AND fault protection. Room temp resistance is a few ohms rising to over
12kohms when hot. Normally, the relay next door bridges it out soon as the
DC rails come up.
The example I saw late last year was an XLS602 that would simply not
switch on while the PTC was getting very hot. Like yours, you could variac
it up to some extent, then the PTC detected a fault and went high.
Problem turned out to be the toroidal transformer, enamel insulation had
failed in the primary winding creating shorted turns.
BTW:
Very sad to hear of George Cole's passing.
.... Phil
Yeah, I was very sad too, but he did have a good innings lasting until he
was 90. Dennis Waterman was still good friends with him as well. He's
looking a bit rough now, though ...
I saw every episode of Minder that was ever made. They all had very clever
'play on words' titles, and the first two series were actually quite dark
stories. They didn't think it was going to last beyond that, but the comedy
element was deliberately ramped up in the third series by the interaction
between Cole and Waterman, and it became an overnight success. I just loved
the character Arthur Daley for the wicked sense of comic timing and the
malapropism he employed. And of course, it was made by Euston Films, the
absolute kings of drama made in London.
Back to the crown. The transformers in this 5000 model are truly massive
torroidals, and I'm leaning towards suspecting shorted turns on the primary
of the one on the bad channel. The inrush thermistors are the size of penny
toffees - the biggest I've ever seen. From the sheet of flame that shot out
of this one and the number of bench fuses that blew, the short must be right
on top of the thermistor. The fact that you can only get to about 20 v of AC
input before the variac is jumping off the bench, also makes me think
shorted primary.
Unfortunately, it's not an easy task to get to the secondary side of the
PSU, as it's one of those two-board constructions, where one power amp is
stacked on top of the other, and all of the bridges and filter caps for both
amps are, needless-to-say, on the bottom board where you can't get to
anything.
Before going to all of the trouble to get the top board out to see if I can
unplug the tranny secondaries, I'm waiting on the shop getting in touch with
the owner to give him an estimate of what it might cost, depending on what I
find. I'm not sure what the spares position for Crown stuff is like here in
the UK, if it needed a tranny ...
Arfa