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homertooties 20th November 2005 09:27 AM

Cambridge audio A3 i
Hello all wonderng whether anyone out there has got any ideas...

Got a Cambridge audio A3 that is acting up,first put it on plays music lovely ,after about 10 mins there is a fault on - v supply ,there are 4 leds in board one will go dim then speaker relay comes on ,because power rails are sent to speaker output.

Switch off leave for a minute switch on okay for a while then it happens again ,i have taken the ouput stage out( 2sa1216 and 2SC2922 out loaded the B to E connections with 100 ohm resistors ,all voltages on channels same as each other,tested the output as far as concerned they are okay swapped them over and put them back in,did it again after about 10 mins same place on left channel,changed drivers bd139 and bd 140 still same the predriving circuit seems to be working fine same voltages on both channels,no shorts,diodes okay.took the output stage out and put in some a and c top3 trannies out of buggered marantz.

The amp played music for a while then blew fuses the (a) tranny that was in the place of original 2sa1216 was now a wasted wire link.

Does anyone have a schematic for this amp as the Bc550 bc549 bc639 trannies are allover the board not the normal mirror image.

please please help such a good amp be a shame to bin it.

Also cheched idle drivers ok,checked bias pots and surrounding parts all okay have not found a duff part,the output stage i understand have other components inside on the die so could be tricking the meter into false good readings.....

Bernhard 20th November 2005 11:42 AM

I would first put all original parts in place and make sure the behavior is the same as it was. (fails after 10 minutes)

It looks like one device fails when it gets warm, so measuring can't help you.

Better first swap output transistors between left and right channels.
If still the same channel fails, do the same with driver transistors --> BCxxx ---> replace all resistors that get warm or are mounted with a distance above the pcb.

If you can watch fading LED before failure, you can try to freeze Transistors locally with cold spray and find the bad one.
The fading LED should light up again when you freeze the right device.

homertooties 20th November 2005 01:03 PM

Many thanks for reply will have to invest in some of that freezer spray ...cheers

anatech 20th November 2005 03:52 PM

Hi homertooties,
I have had good luck finding thermal defects in transistors. Measure the gain and warm the part with a heat gun out of circuit. Non destructive and easy to do. The reading will drift, but a sudden change (open or leaky) will show up.

I only do this because I'm tired of buying freeze spray. I still do, but this is far less expensive.


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