[Milsurplus] BC-659 vs BC-1335

[John Vendely]

----- Original Message ----- 
From: "Unserviceable but Repairable" <cosmoline@ba-watch.org>
To: <milsurplus@mailman.qth.net>
Sent: Thursday, May 06, 2004 2:58 PM
Subject: Re: [Milsurplus] BC-659 vs BC-1335


> Alan according to W4MEW, the BC620 (20-27 armor / wb FM) & BC659 (27-38
> attly) carried a a small 22.5V(?) battery to run the AFC-lock on
> transmit.  Well it's presence wasn't well known & when run-down, wud cause
> them to jump freq. w/o warning.

<< Good point about the "bias battery" in the BC-620/659.  These radios
won't function properly without one, in either receive or transmit.  It
produced several voltages used in both receiver and transmitter, including a
coupla negative voltages, 22V, and approx 60V which served as the plate
voltage for a DC amp in the transmitter AFC circuit.  It's likely the reason
for using the battery was the high sensitivity of the AFC circuit to any
vibrator power supply ripple. BTW, technically, the AFC was not a PLL, but a
FLL--a frequency-locked loop.  It was an imperfect AFC system, as some error
in the controlled variable (frequency) resulted due to the finite loop gain.
The DC amp was added to maximize loop gain.  The beauty of the PLL as an AFC
system is that the residual error is in the integral of the controlled
variable--phase--thus allowing zero average frequency error.


> Big reqm't in those days was xtal control & FM.  Contractors worried
> about paying Armstrong for his xtal-controlled phase modulator.  Hence
> 2 alternate modulation spins on SCR500 competitors.


<<Perhaps the most important reason for the alternative modulator in this
application was that, by using direct FM of an AFC'd LC oscillator, plenty
of deviation could be obtained without resorting to the bulky, power-hungry
frequency multiplier chain needed for the Armstrong modulator.  Only one
frequency doubler was needed in these sets, and this was partially to
eliminate pulling problems associated with having the modulated oscillator
on the carrier frequency.

73,

John K9WT