Repair tips for dead Crown XLS602 for newby

[timbert]

Grabbed this for $50 and thought I'd have a go at fixing it. I can understand about 20% of the schematic below and have a Fluke digital multimeter. I've fixed a half-dozen NAD amps with help from people here (anatech) and it's been a beautiful experience.

Upon powerup there are no lights or sound or fan, only lots of heat within a few seconds from the big 17W resistor near the 110V AC point. External fuse and power switch are ok, and there's 120v AC going into the thing that looks like a small spare tire. Heh. Anyway, the schematic has that resistor valued at 23W, but either way it's really hot. That tells me there's a short somewhere, right?

It does not appear anyone else has been in there, and there are no burns or anything obvious, so before I really start tearing it down to get to the bottom of the board and measuring (and ordering) parts I thought I'd ask here to see if I'm wasting too much of my time. I did a quick continuity test on the top surfaces of all 16 TO-3 cans and they all seem to be connected (low resistance).

My next step was to check voltage at what you guys call the "rails" and see if they are +/- 95v DC per the schematic, but I'm not sure exactly where to do that and whether it will tell me anything.

There is a Crown service center in town, but what fun is that?

Thanks in advance.

[llwhtt]

Sounds like the AC input relay is not closing, do you hear a click shortly after turn on? That big resistor should be out of circuit shortly after turn on. The "spare tire" is the power transformer. Check the voltages in the relay drive circuitry to start.

Craig

[nigelwright7557]

Originally posted by timbert

going into the thing that looks like a small spare tire. Thanks in advance. [/B]

If your that ignorant of electronics you seriously need to be taking it to a qualified engineer. There are lethal voltages inside these amplifiers.
Is risking your life worth it ?

[jaycee]

The softstart circuit isn't working by the sound of it. The relay for this might be faulty or the circuit controlling the relay might have a fault.

Make yourself a lead with a bulb in series with the Live/Line/Active wire of the mains. This will save a lot of possible damage and also alert you that things are not right.

You need to be really, really careful as the fault sounds like it's in the primary side of the power supply. One wrong move and you can easily end up causing a fire, or getting a nasty potentially fatal shock.

[timbert]

Thanks all, and you are right as the relay makes no click at all so I will start there.

And please pardon my attempt at humor about calling the transformer a "small spare tire" as I know the purpose and the danger of transformers.

I have brand new leads on my Fluke to prove it. Heh. (There I go again).

[llwhtt]

The green power LED is powered by the low(er) voltage power supply and should run relay or no relay. Though it could be bad you should still have a lit power ind. You said there were no lights at all, correct? I'd check there first, should be + & - 24 VDC.

Craig

[wg_ski]

The light bulb test should be the very first thing that's attempted with this or any other amplifier. Bypass the soft start and plug 'er in through the light bulb - you'll get a lot of information in a few seconds. If it stays on, then the soft start isn't starting for a reason - perhaps a good one. If the bulb goes dim after a few secs, you should know one or two quick voltage measurements later what's wrong with the soft start.

[megajocke]

Probably shorted output devices/filter caps/rectifier. Looks like the softstart is powered from the same transformer as it's soft starting. If the output is shorted it won't turn on.

Good foresight by Crown adding those PTC resistors protecting the soft start resistor. Many amps burn up/explode their soft start resistor if something like this happens.

[wg_ski]

Originally posted by megajocke

Good foresight by Crown adding those PTC resistors protecting the soft start resistor. Many amps burn up/explode their soft start resistor if something like this happens.

Or blow a fuse. But wait - some DJ is gonna blow the fuse, wrap it in aluminum foil, and keep right on playing.

[megajocke]

Maybe that's why many pro amps use circuit breakers instead of fuses - making the equipment DJ-protected besides the usual protection from over-temperature, short-circuit and so on