[Milsurplus] MAB circuit

[Hubert Miller]

The MAB circuit is not actually a transceiver, it is, strictly speaking, a transmitter-receiver, as the two sections
are separate but for the switching and the common. The two sections are switched by switching filament power
to one or the other. But here is something that puzzles me: when the receiver is unpowered, transmitter is
powered, the neg bias from the RF PA is supplied to the AVC and first audio grid. The manual states, "In addition
[ to normal AVC function ], in order to prevent RF feedback from the transmitter into the receiver, grid bias from
grid No. 6  [ i.e. pin 6 ]  of the transmitter oscillator tube, ( V-106 ), is applied to the receiver diode, AVC line and
first audio stage control grid, biasing them beyond cutoff."
However, I point out, when the transmitter is powered, none of the receiver tubes is powered, so what's to "cut
off " ? If there's any kind of RF feedthrough, the rec audio output tube still is NOT given this cutoff voltage. What
gives ?

Also, before looking at the schematic, i wondered, if this receiver, like the RBZ would have AVC to an audio stage
also. It's not called out in the MAB manual, but if you look at R-106, R-107, R-114, you see that with receiver "on",
AVC voltage is divided by 1/5 ( R-106 and R-114 ) and supplied to the det-1st audio tube 1S5 grid via R-107. So,
lacking a volume control knob, there's an additional measure of output limiting by hitting the audio stage also with
some control voltage. I'm not sure this would have a lot of effect, and I'm not sure it was a design intent, or just
the consequence of connecting both diode output and 1st audio grid together for DC via the cutoff supply resistor
R-114.

Ray, I don't know what I'd do about your blown output transformer. There's really no room for anything additional
in the chassis. Maybe something like a JFET + bipolar cascode amp plugging in the rec 3S4 slot and driving the 600R
headphone directly.

My MABs are on 'Channel G', 3725 kHz. In the ballpark, but not the 3885 standard, so I'll need to see if I have crystals
I need. Mine in their postwar use were powered by a battery stack, and the manual says the dry battery pack was
considered only to be used as a secondary backup, so I have to assume the owner discarded the vibrator supply.
-Hue




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