Yamaha DSP A-990

I'm hoping that there may be somebody with a 5 channel amplifier Yamaha
DSP A-990. I like my old machine, 6 years old, a fantastic sound and
video processor. Happened to short the outputs and blew most of the 14
power darlington transistors. I have repaired with newer equivalents
and all e-c-b voltages are OK, but 5 seconds after start, there is a
safety relay that switches power off. I can't figure out why and what
the sensors react on. Complicated electronics. Now I just wonder if
someone happen to have such a machine and if there is also someone who
have schematics. Yamaha don't not respond positively on my request, say
they have no service on such a obsolete amplifier.

Thanks!

 

Looks like the following site has free info on your unit:

http://www.eserviceinfo.com/

 

Usually looking for a DC offset to protect the speakers. Maybe also
oscillation. A scope should tell all.

 

Not finding this model number, at least not on the Yamaha's US tech site.

When the outputs blew, they probably took out one or more resistors,
sometimes a 330 ohm or so which is between the emitters of the two driver
transistors, or a series resistor going to the base of an output transistor,
usually about 4.7 ohms, an emitter resistor, usually .22 ohms at 5 watts
dual package, or, and this seems most likely to me, a resistor supplying B+
or - to the collector of a driver transistor. Usually in the one to ten ohm
range.

Mark Z.