Crown K2 power amplifier repair

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This Amplifier would sometimes work , sometimes would not and other times it would just shut off for no reason. These faults can be very hard to track down but these amps when the go wrong and its not the power transistors then it normally down to the caps on the main board. Capacitors tend to drift out of spec as they age and also increase in ESR. A lot of caps when they go bad have visual signs. These signs can be bulging tops or bodies from pressure that as built up inside from getting hot or electrolyte leaking from them onto the PCB they are attached to. Not all caps will show these signs but will still be bad and this is a classic case I had right here.

First things first if you intend to play about in side a crown K series amp you need to trip the crow bar to discharge the main capacitors or risk a very unpleasant shock. ( you have been warned)

To do this you need to short out TP1 on the main PCB to the nearest resistor leg. Looking at the picture its pretty hard to see the actual test point as it can be obstructed by the resistors.

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While doing this be very careful not to touch the heat sinc as this will be at a different potential to the test point and the resistor legs until it as been discharged.

Now That I had the amp discharged and safe to touch I set about removing the PCB. This is a picture of the rear panel that has the input and main board attached to it.

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The back panel unscrews and will slide out of the amplifier with the PCB’s attached after all the wires from the harness have been disconnected (Be careful with the screws the small ones go on the side of the main board).

Once the input board as been removed I could get to the main board and see all of the caps that I was interested in checking.

Most of the people I’ve ever spoke to regards Crown K series amps as always told me that if its acting up to check all the 100uf caps and replace any that are suspicious. Well there are a lot of caps on this board and I wasn’t in the mood for doing them all so normally I would grab a ESR meter but that’s broke. So another way to check ESR was with my scope and a function generator. its not as fool proof as with a meter but it gives a good indication of a bad capacitor from a good one. What you do is pass a 100khz square wave into the capacitor while observing the same signal on a oscilloscope. For this to work in circuit you need to make sure that your signal in lower than 500mv so as not to turn on any semiconductor junction’s. What happens when you see a good capacitor is that the resistance at 100kHz is almost a flat line with slight waves in it as a capacitor is almost a short circuit at 100kHz AC. A bad capacitor with higher ESR will have resistance at this frequency and will just reduce the amplitude of the waveform on the scope.

Picture of good capacitor

( you can see how the waveform as collapsed as the capacitor acts as a short)

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Picture of bad capacitor

(The wave form is still present due to the bad capacitor having a resistance and not shorting the signal)

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With this method I found four truly bad capacitors and two suspect capacitor on the main board. These were all 100uf that seems to back up the advise Id been given.

The two capacitors that I believe cause this issue are c23 and c24. These are the two capacitors on the right of this picture.

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Along with the capacitors there are a few other things that I was advised to check just in case. These were some resistors on the main board that age badly and drift out of spec. Although these resistors were fine on this amplifier I’m going to list them for my reference and anyone else that may have issues with a K series.

Resistors r136 and r236 should both be 33r next to the big white transformer.

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Resistor r5 that is a 1/8 watt at 47k

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Well The amps been used for 3 gigs since the repair and no complaints yet. Ill probably come back and do some editing on this page but for now ill just stick it out there.

Cheers Kris

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