Ray Gianelli
Super Member
I had a Yamaha RX-900U on my bench recently. Belongs to a friend. Pretty easy diagnosis; burned components in the right channel. Shorted outputs, drivers and open fusible resistors. Checked back from the outputs until everything else checked good. Replaced shorted and burned components, and fired it up on the DBT. Major negative offset, and since it's direct coupled like most modern amps, it's bad everywhere.
I checked and rechecked everything, and couldn't find a problem. I finally pulled a transistor that showed some voltage between collector and emitter on diode test; usually I find that when they test like that in circuit they're fine when you pull them. Not this one. Replacement of this transistor fixed the amp.
So this transistor E/B and B/C junctions both tested good. It was only when testing the E/C that it showed continuity, and only on one orientation of the meter leads.
Not the usual failure, but transistors can and do fail in other manners besides short or open. Thankfully I rarely find one that only fails under higher voltage, but leaky can be just as hair pulling.
So, does anyone have any advice on how to find these? The only reason I did was circuit analysis, helping zero in on the area I thought was causing the trouble.
I checked and rechecked everything, and couldn't find a problem. I finally pulled a transistor that showed some voltage between collector and emitter on diode test; usually I find that when they test like that in circuit they're fine when you pull them. Not this one. Replacement of this transistor fixed the amp.
So this transistor E/B and B/C junctions both tested good. It was only when testing the E/C that it showed continuity, and only on one orientation of the meter leads.
Not the usual failure, but transistors can and do fail in other manners besides short or open. Thankfully I rarely find one that only fails under higher voltage, but leaky can be just as hair pulling.
So, does anyone have any advice on how to find these? The only reason I did was circuit analysis, helping zero in on the area I thought was causing the trouble.