Crown XLS 202 Amplifier Repair – Part 4

My hunch is there’s DC output voltage triggering the DC protection circuitry and automatically shutting down the amplifier.

So we need to measure if there’s any DC voltage at the output of the amplifier.   Note, you cannot do this check by sticking your meter at the output binding posts… because those are connected to the relay terminals — and you won’t be able to get any useful reading.

We need to measure where the RED arrow is pointing in the schematic below.  (Of course, the negative or BLACK probe of our meter should be connected to Ground. There is a common ground point on the amplifier, just right below the power transformer location.)

0901outputdc

Channel 1 measures 1.25Volts DC.  This is a good reading, since this is below the 2 Volts DC threshold of the DC protection circuit.  So Channel 1 must be good.

0901channel2DC

Channel 2 measures 54VDC at the same location! Definitely, that’s not normal. NOTE:  I had to put my Fluke meter to MAX mode because I can only measure this DC voltage for a very short instant of time.  The protection relays get triggered and AC power to the whole amplifier is cutoff.

0901npnsuspect

That 54VDC measured at the output voltage is almost the same level of the Vcc+ voltage of the amplifier. So analyzing the schematic above, the only way we could see a high voltage at that location is if one (or more) of the NPN power transistors are shorted!

NOTE: For amplifier model XLS 202, there are only (2) NPN and (2) PNP power transistors. For the higher models XLS 402, 602, etc… the other transistor locations will be filled up.

So we got our suspect!  A shorted NPN transistor in Channel 2!

to be continued…

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