[Shop-talk] Audio receiver repair?

[Randall]

> If I found the 
> offending 
> part I could replace it, but unless there's spooge all over a 
> cap, I'm 
> not sure I'd know what "leaky" was.

Generally, the term "leaky" refers to a capacitor that is passing DC current (which they aren't supposed to do), rather than a
"spooge" leak.  Depending somewhat on the cap and the meter, you can usually get a good idea by disconnecting from the circuit and
then "trying" to measure the resistance of the capacitor, making sure to observe polarity.  Start by shorting the capacitor legs
together (to make sure it is discharged), then connect the meter across the cap.  You should see the resistance reading start out
low and then rise rapidly to infinity or nearly so.  Also do a visual for any signs of distortion, like end caps that bulge
outwards.  

Or, as noted on the web site, there is something to be said for just replacing the power supply caps.  $60 seems like a lot of money
though, unless you are interested in it as a collector's item.  Old stereo receivers are pretty cheap around here; $10-$15 at a yard
sale will get you one that just needs cleaning and maybe a switch or control replaced.  

> Can these types of problems be tested with a multimeter or does it 
> require a scope?

You can do some checks, like the above check for capacitor leakage and a rough check for whether a transistor is shorted or not.  A
bipolar transistor out of circuit should act like it contains two diodes, meaning you can see it conduct (resistance less than
infinity) for one polarity applied between the base and emitter, or base and collector; and not conduct (infinite resistance) for
the opposite polarity.  For an NPN transistor, it should conduct when the base is positive to the emitter; or the collector is
positive to the base.  For PNP, it's emitter positive to the base, or base positive to collector.

This works with most meters, not necessarily all of them.  If the meter shows open in both directions on both pairs of leads, then
it's worth checking the transistor with some other method.  You may have a meter that uses less than 0.7 volts to measure
resistance.

Like Peter said, though, I would certainly start at the power supply, since the problem affects both channels equally.  If one or
both of the caps are bad, the diodes may be popped as well.

Randall