[Milsurplus] MAB circuit

[Ray Fantini]

I think the design is a thing of beauty compared to the hair brain layout of something like a BC-611, fast heat tubes are the key. T102 the output transformer is pure genius eliminating any type of antenna switching between receive and transmit and although I am getting beat up by the failure of the audio output transformer located in T101(A) still a big fan of the radio. The rocks can be obtained from AF4K and I believe they are the same rocks as used in the BC-611
I have never had the vibrator pack for the radio and always used battery packs that I built from stacking nine volt batteries for all the voltages and found that stacks of cells tend to last for two or three years for the amount of time I use the radio. I have had a MAB once before but did not get that up and running liking the back pack appearance of the DAV.
Personal preference for any field radio from WW2 would be a DAV/MAB or early TBX, not an eight because I love the simple clean design and layout of the radios and also like that they are incredibly efficient in terms of consumed power all be it short of output power. One thing I have noticed about both the DAV and TBX is although they appear to have AVC circuts the AVC action sucks. May be that C110 or C111 have issues and kill the AVC voltage but never gotten much love from the AVC on that radio. No big deal talking peer to peer but in a net can be an issue epically with a set like the TBX where you have to keep cranking on the gain control. Other issue I have had with the DAV is that I tend to keep the battery systems connected in the radio and that keeps screen and plate voltages applied all the time so if any of the bypass capacitors (C114-C116) are leaky that can be an issue so found that I have had to change them out.
Never having a vibrator pack wonder how is it controlled? What turns it on and off? Dose it sense current drain on the filament bus? Or do you have to open the radio and switch it on and off?
Have not decided what I am going to do to resolve the issue with this radio yet. Lot depends on if I can find a small output transformer to wedge in their someway or not.

Ray F/KA3EKH


From: milsurplus-bounces at mailman.qth.net [mailto:milsurplus-bounces at mailman.qth.net] On Behalf Of Hubert Miller
Sent: Tuesday, March 27, 2018 9:39 PM
To: milsurplus at mailman.qth.net
Subject: [Milsurplus] MAB circuit

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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