[ARC5] Power up Modulators

[David Stinson]

Tom Dawson wrote:

> Looking for caveats and gotcha's regarding powering the modulators and 
> transmitters up after so long in storage.
(and receivers, I assume)

I just built a complete set and it works well,
so here are some tips.

Save yourself some grief:
Take the time needed to thoroughly refurbish the radios;
it will pay dividends.

Check every "bathtub" cap with
a reliable cap checker like the ZM-11.
You will find between one third and two thirds of them bad.
The 5 uFd dyno filter will always be "bad."
For some reason I don't understand, the hardest to
replace- the three-banger .22 uFd- is good 80%
of the time.  I'd try it "in circuit" and watch for
trouble rather than pull it for testing right off.
The one bathtub in the transmitters should be
tested carefully, out of the circuit and under
full voltage for at least a couple of hours.
Leakage in this one causes big headaches,
so you want to be sure it's in good shape.
The modulator caps are hard to check.
Most will be good.  Do it anyway.
You'll be glad you did.

Check the receiver and transmitter variable caps
between the stator plates and the connection bar
to which they are attached.
The connection bar at the end of the stator plates is "press-fit."
Oxidation gets in between the plates and the bar to
which the wire is soldered and makes the connection intermittent.
Take a big soldering gun to it and solder across the gap.

Clean every contact and control you can reach.
Follow the transmitter refurbishing and
output coil polishing procedure I've posted several times.

Check every single solder joint in your transmitter.
Reheat the ones in the osc. section just on general principles.
Reheat any others that look even slightly suspect.

Make sure all your racks and shock mounts are connected
to a common ground system, using short wires.
I used straps to build a "ground plain"
and connected to that.

Don't connect the wire on the receiver rack that carries
sidetone unless you're using an external, isolated
attenuation amp like the original interphone system.
If you don't, the sidetone and PTT "clicks" will bust
your eardrums.  Don't try to use limiting diodes across
the sidetone leads like in a telephone headset;
the reflected impedance in the mod transformer will
foul up transmitter modulation.

Use enough power supply to start the TX dynamotor without
grunting- 30 amps minimum, and that's if you have
some big "computer" capacitors on the output to
supply the starting surge, like a few hundred
microFarads.  Batteries are a pain, but the smaller
lead-acids like those used in UPS supplies will do
if you don't like the cap idea.

Let your transmitters warm up at least an hour
or two and they will be stable.  Even the BC-375
stays put if you let it warm up properly, with
the cover in place.

I'll probably think of more later, but this is a start.

GL OM ES 73 Dave S.