[ARC5] An AN/ARC-2A Repair

[David Stinson]

The AN/ARC-2A has been on the bench for a couple
of weeks.  Operation on CAL, CW and MCW was
normal, but on VOICE, when the microphone was
un-keyed, the audio output was blocked- almost
like the AVC was seriously railed; it took several
seconds for the audio output to recover to useable
levels.  

Thanks again to Robert Downs, the ARC-2 manual
helped find the "Bad Boy."  From page 40,
paragraph 4-61 of AN 16-30ARC2-12 Service
Instructions:

	"... When the push-to-talk button on the
microphone is depressed, the cathode circuit of
the  r-f power amplifier tubes is completed, and
260 Volts dc is fed to the remainder if the
transmitter tubes making the transmitter
operative. *This high voltage is fed also to the
cathode circuit of the receiver noise-limiter
tube, cutting off this tube and the output of the
detector stage to the audio amplifier stages
during voice transmission.* ... (emp. mine)

A look at the circuit, figure 4-13 on page 44 (see
attached), shows that the blocking voltage to the
12H6 Noise Limiter cathode is supplied through
R-200, a 10 Meg-Ohm resistor.  "If the band is
blue, that's trouble for you."  Sure enough, it
measured 17 megs, so the stage was not cut-off,
leaving the Detector and Audio stages open to
blocking.  A new resistor and the recovery delay
is more shallow and down to about a second.
Likely another part in the circuit is out of spec
or leaking, but we finally have this nasty gremlin
on-the-run.  Thank You, Father.  This one was a
hair-puller.

GL OM ES 73 DE Dave AB5S


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