[TenTec] Omni-A not powered up for 7 years...

[rohre]

Rufus, and the group,
OK, I know what to do if it was AC operated tube radio.  See no reason not to
try something like this for 12VDC.

First, I know dipped tantalum electrolytics that have been prone to fail after
long periods of disuse, if you apply full DC to them immediately. 
Particularly, if they are 15 volt units being used in a 13.+ volt supply line.

Like the tube set rejuvenation, I would make up a special DC power cord with a
bulb (lamp) socket for a candelabra or auto bayonet bulb in series with one
side of the DC line.
I would pick something like a 6 volt bulb, say a tail light or stop light
bulb, and put in series with the DC line to limit voltage and current when the
rig is first turned on.  Now you will have to look at the schematic, for if
there are voltage regulators either discrete or chip, you will not have enough
voltage for some of them to turn on, but if the DC goes straight in, you could
do this.

Apply the reduced voltage and see what the rig does,  if any electrolytics
short they immediately brown and emit smoke, if the dipped tantalum type. 
Others might fail more spectacularly, such as tubular electrolytics that blow
up. Wear safety goggles! But at reduced voltage they might hiss first. 
Anyway, after a period of forming at reduced voltage you could increase the
voltage.  That is the process for very old radios.

Unless you have some electrolytics that have been really stressed, like the
marginal design case I cited; you might just go for fusing the line with the
minimum fuse size, and apply full power with radio out of case, on the bench,
where you could monitor ripple and voltage buses.

Before applying power, especially since this one was in marine environment,
open it up, look for corrosion, leaking electrolytics at rubber seals, white
or colored content leak, etc.  Replace any bulging ones, and if you are really
paranoid, clip each one in turn and test either on a capacitor checker, or
with an independent DC Supply, checking for leakage current thru the cap, if
you put it across a supply. You only have to clip one wire, leaving the other
in the circuit.  Use a clip lead to the cut wire, of the cap to the test
supply.  Most modern DVMs have a current scale.  Charge up the cap, and see if
it leaks over some seconds.  After each test, bend the cut wire back to the
circuit cut wire, and simply overlap splice them with solder.

The same voltage limiting the input leads could be done by using a power
rheostat, (wire wound pot) able to take the full current of the radio.  Place
it in series and gradually bring up the voltage, pausing in a series of steps
of a few volts until you are at full power.  Listen to the receiver for
excessive hum, which is a clue to dried up electrolytics in audio circuit. 
Measure ripple with AC volts scale of a DVM, on each electrolytic plus
terminal, and compare to specs for a low ripple power supply or other known
supply in your shack like an Astron, etc.

Then there are those who just apply full power and go for the smoke!

Hope this helps,
Stuart K5KVH
rohre@arlut.utexas.edu

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