[Boatanchors] Power supply troubleshooting

[Dave Maples]

Eugene: I have some questions:

1. The 7812 and 7912 regulators are only designed to deliver about 1 ampere
of power.  Are you sure that the regulators supply EVERYTHING in the
amplifier, or do they only supply preamp circuits, with the output and
driver stages supplied directly from the electrolytics themselves?

2. What kind of output stages are there?  Are they discrete transistors, or
are they some kind of large integrated circuit pack?

3. Do you have a schematic for this thing we can look at?


Most likely things to fail are (a) high-powered solid-state output devices,
(b) power supply diodes, (c) lower-level solid-state devices, (d) 7812/7912
regulators.  Without a schematic, here are some high-level suggestions:

1. Check the output devices first.  Assuming they are discrete transistors,
typical failure is an emitter-collector short; this can exist even if the
emitter-base and collector-base junctions look OK.  Generally I pull these
out of circuit to test them with an ohmmeter; however, you might be able to
do a preliminary check still in circuit.  You shouldn't see a low resistance
from emitter to collector in either direction; if you do, pull the device
and check it again.  If you still see it, that device is defective, and its
partner probably is as well (because when it went it put incorrect voltages
on its mate).


If the output devices aren't discrete transistors but are some kind of
integrated circuit, you obviously can't do this.  Let us know better what's
in there.


2. Isolate the power supply diodes from each other and check them.  Lifting
the lead of each diode connected to the respective electrolytic works great;
it leaves the bridge together as far as the transformer is concerned which
will be important in a minute.  Power diodes usually fail shorted.  If you
find a bad diode, replace it but leave it disconnected from its
electrolytic.

3. Tack-solder or clip-lead the bridge back together with the leads toward
the electrolytics floating, put power from the transformer through the
bridge, and make sure the bridge works correctly.  Put a capacitor across
the bridge output.  If you measured 30 VAC across the transformer you should
measure about 44 VDC across a charged capacitor.

4. While the diodes are disconnected from the electrolytics, measure across
each electrolytic with an ohmmeter.  If you found defective solid-state
devices in the amplifier and replaced them I suspect you'll see both of them
take a charge.


You also didn't mention if you had a couple of variable low-voltage power
supplies you could use to substitute for the internal supply.  If you do, it
might make "divide and conquer" a bit easier, because you could substitute
for the internal supply and meter the current going in, etc.

Thanks,
Dave WB4FUR