Howdy Ray,

please see comments in <<...>> below:

Been working on a BC-659J( SCR-609/610) and usually don’t work on stuff this old for other people but decided to take this set on. Can see how Galvin later to become Motorola started to develop a lot of what was used in two-way stuff from the fifties in terms of layout and design. The loctal tubes are weird but imagined they thought it was a good idea to keep tubes in place.

AI told me they were developed by Sylvania in 1939 for car radios, and I have to wonder why they never found much application in other military radios. 

<<The loctal tube base did more than lock the tube in the socket.  The tube's metal base and socket locking clamp also provided a lower inductance RF ground return for tube elements, e.g., cathode, suppressor grid return, etc.  Loctals were extremely ruggged and widely used in car radios into the 1950s, undoubtedly the reason Galvin, manufacturer of "The Motorola" car radio, chose them for the BC-620 and BC-659.  Heavy pressure for miniaturization resulted in their being quickly supplanted by miniature battery tubes, such as used in the BC-1335, a superior design which replaced the BC-659. >>

The lack of a T/R relay and the transmitter keying design is more along the lines of a backpack or later handheld radio using the PTT line to turn the transmitter filaments on and off for transmitting. Still working on figuring out how the transmitter oscillator is locked by the output of the discriminator and how they had to use a dry battery pack to get this all to come together.  Weirdly, there is a manufactured potted assembly that takes the place of the battery and is fed by the receiver filament bus that somehow appears odd to me that someone would market an assembly for such a useless radio.  After all, anything that covers that frequency and is FM only would have real limited use.

<<The BC-620/659 were very innovative designs in that first generation of tactical FM radios, though they were big clunkers and therefore on the verge of obsolescence after just a few years.  The use of direct FM via a reactance modulated LC oscillator drastically reduced tube count compared to the common crystal-controlled designs.  To eliminate the frequency drift problem, the transmitter oscillator was frequency-locked to the receiver via its discriminator.  The receiver stayed on during transmit and the transmitter signal was leaked into the receiver front end.  Lowpass filtered DC output from the discriminator was fed back to the reactance modulator, holding the transmitter on the receiver frequency.  The receiver LO was crystal controlled, so good transmitter stability could be attained this way.  To minimize static frequency error, closed-loop gain was maximized by feeding the discriminator output to a DC amplifier which then fed the reactance modulator.  This was a very sensitive point in the circuit, so to avoid power supply vibrator hash inevitably worming its way into the modulated oscillator, the DC amp plate voltage was derived from the 60V tap on the bias battery.   The battery's -4.5V bias voltage served as the AFC voltage reference, among other things.  This same general transmit AFC scheme was later used on the BC-1335, PRC-6, PRC-8, 9,10, RT-66,67,68, and RT-70.  But the BC-620 and BC-659 were the first.  Honor is due.

The required BA-41 bias battery was a logistical problem with these radios and had to be replaced every 6-8 months.  These radios will not work properly without it; the transmitter will quickly drift off frequency, and receive audio output will be maxed out regardless of volume control setting.  This is why a DC/DC converter module was marketed for hobby use of these radios.  I wonder how well it was designed--good regulation and low noise on the -4.5V and +60V is important for stable transmit operation.  A better though less convenient approach is to wire up a string of batteries, which is what we all did in The Olde Daze.>>

That brings me to the question, this radio only has rocks for 27.0 and 38.80 MHz Going to assume that the person who owns it wants to use it so imagine you can play around with 38.80 on low power and talk to things like VRC-12 and PRC-25 sets but have to wonder if there is any given frequency that Military Vehicle people use below 30 MHz? Think channels 295 to 297 may be useful being 29.5 to 29.7 MHz . I think 29.6 is the FM calling frequency but never used that before.

So, anybody got a rock for channel 296? (6.325MHz) have seen these crystals around for years and now finally know what they are used for.

<<We have been using 29.4 Mc as the calling frequency for military FM radios like the BC-659, BC-1335, SCR-608, RT-67, PRC-9, RT-524, and others since the late 1960s.  29.4 Mc has been busy over the last two years during solar max, with RT-524s and the WWII sets on the air regularly.   I would recommend 29.4 for the BC-659 if the owner is a licensed ham.  Look for "channel 249" FT-243 crystals.  

Alas, the BC-620 won't operate on 10 meters without heavy modification.  But back in the late 60s, my buddies and I had them on the air on 27.9 Mc, which I now freely admit since the statute of limitations is long expired.  Good BA-41 bias batteries could still be had back then.  

By all means, put these historic sets on the air and have some fun.>> 

73,

John K9WT

Ray F/KA3EKH

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