Kyocera A-710 puzzle

[resoundpdx]

Hi all - I have been banging my head against the proverbial wall with a Kyocera A-710 integrated amplifier. It originally came to me with a blown output transistor, which was addressed easily enough (though I was surprised that the emitter-collector short did not take out the .22 ohm emitter resistor). Anyway, once it was fixed the owner asked for a complete recap. I first did the power supply and protection circuits (mostly Nichicon PW's) - after which the unit operated just fine. Next I did the right power amp board - the one where I had replaced the output transistors (mostly with Silmic II's), after which the unit again operated just fine. Last, I did the remaining (left) power amp board - the one which had presented no problems before. Now the amp does not work at all ;-(

Within about 2 seconds of switching on the power, the main protection relay trips - which on this unit disconnects the high voltage +/- supplies from the power amp boards. Then the relay turns on again, then off, etc., cycling every couple of seconds. The problem is definitely on the left power amp board, as when I disconnect it the unit works fine (no protection relay tripping). When I look at the output of the left power amp, it is clear what is happening. On power up, the output quickly drifts up (many volts) which triggers the DC offset sense and thus the protection relay. But why that is happening is a mystery. What is odd is that the input to the front end differential circuit also seems to drift and oscillate - but since it is operating off the + supply this may be just because the power is coming and going?

I have rechecked/reflowed all soldering, the polarity of the caps, etc. The two channels match exactly. I also diode checked all transistors (and diodes) - all check good and match the readings on the right channel. I am mystified as to why the left amp went bad after straightforward cap replacements and what actually is going on. These were all new caps from Mouser - is it conceivable that one is bad? Other than swapping them out one by one, how would I figure out which one is the culprit? Any suggestions GREATLY appreciated.

Thanks in advance-

 

[resoundpdx]

Oops - I thought I had posted this, but apparently not. Here is the resolution to the problem, in case it helps anyone.

There is a temperature compensating diode in the bias circuitry that is mounted to the heatsink between the pairs of output transistors. It looks like a small transistor but with only two legs. Each leg is connected to the power amp circuit board with a short black wire (pretty much same as transistors). I tried to avoid disconnecting and then reconnecting all these wires when I tipped the circuit board away for cleanup and recapping. No difficulties encountered on the right channel. But on the left channel the diode wires broke at the circuit board. No problem - when done with the recap and ready to reposition the board I resoldered them. To make sure I had orientation correct I copied the right channel. (The two amplifier channel assemblies are identical.).

Well- it took me hours to deduce this by comparing the signals/voltages of the two channels at every component, in the one second intervals I had before the protection circuit shut the amp down - it turns out that the two diodes are NOT the same. Although all markings, mounting, etc are identical, on one the left leg is the anode and on the other it is the cathode, and vice versa. So i had inadvertently reconnected the diode on the left channel backward. I still can hardly believe it but i rechecked both diodes repeatedly. And when i reversed the one on the left channel it resumed working perfectly.

 

[cfinchrist]

It sounds fabulous with my KEF R3 Meta's as well as a lesser speaker I tried it with. Above average sound from this Integrated amp for sure.