[Milsurplus] GP-7 transmitter?

Mike Hanz AAF-Radio-1 at cox.net
Wed Aug 4 21:08:13 EDT 2004


John McCarty wrote:

>Has anyone ever got one of these up and working? If so, how did you get around
>the 800 cycle power requirement? You can't just remove the original power
>transformers and replace it with a 60 cps unit. The original seem to be just to
>specialized for this rig. Just about every tube in the set has it's own filament
>winding. Out boarding a supply can't be simple either with all the
>half/full/tune switching. Any thoughts???  
>

It ain't easy.  There were a number of 800~ inverter types made during 
the war, but finding them now is a bit of a chore.  Last December I saw 
one in the industrial district in Baltimore, perched on top of a box 
(where I left it) in a partially collapsed building (watched over by an 
amiable homeless person) if anyone wants to try to retrieve it...   :-) 
   Once you get one of them, your work has only begun, unfortunately.  
Fiddling with my GP-7 on the workbench with a dummy load a couple of 
years ago, I discovered several things:

1)  It needs a fairly solid 115vac supply, which means that an oversized 
inverter does better than one designed for the "specification" load of 
500-600 watts steady state.  I suspect that may be because of the 
primary keying design...there's a fairly high charging current in the 
first few milliseconds with each "key down" stroke, looking at it with a 
current probe on the scope.  There are two inverters that fill that bill 
that I know of - the Bendix 800-1A, B, C, or D that was used on most 
Navy aircraft, and the almost identical AAF contracted PU-43/A.  The 
800-1 has a current draw of 75 amperes at full tilt, though it doesn't 
run anywhere near that with a single GP-7 tied to it.  The extra 
headroom seems to help a lot on keying, though.  The TBW MG set may also 
be okay since they use a similar keying system on the TBW/GO-*, but I 
don't have any direct experience with that particular MG set.  I did 
find that without a fairly solid primary voltage, you do get some 
yooping on CW - with a nominal 500w PE-206 for example, the transmitter 
is not as well behaved.

2)  The power factor of the primary feed needs to be adjusted per the 
manual, preferably /before/ it gets to the GP-7.  Later versions of the 
GP-7 had a mod to it that disabled the internal PF capacitor switches, 
in favor of a power factor correction capacitor hung on the main line 
before reaching the transmitter.  Any variation in the power factor also 
seems to contribute to instability in the transmitter note, though not 
nearly as much as voltage droop.

3)  Though not a requirement of the 803, there is a neutralization 
adjustment to be checked from 3MHz on up.  It only compensates for 
capacity change in going from Tune to Operate, but if it's significantly 
off it can cause problems.  The GP-7 shares the characteristics of the 
BC-375 in terms of getting a bad rap if everything is not tuned up to 
par, but its 80 meter performance seemed no worse than the -375 on CW.  
Might be I was just lucky with this particular transmitter, 
though...they're all a little bit unique, like the command 
transmitters.  One will have all sorts of problems, the next perfectly 
well behaved.  The anomalies of a real antenna would likely introduce a 
new set of problems as well...dunno for sure, since I haven't tried it 
with one yet.

4)  Like the BC-191/375, sequencing of the keying relay is critical.  
The timing of each relay contact as it closes, expressed in 
milliseconds, affects the stability of the note in an important way.  
There is a complete subjective description of the sequence in the 
manual, but unfortunately no specific delay specs for each contact 
actuation.  The lack of stable time bases on the oscilloscopes of the 
era probably dictated that omission.  The BC-191 had a tool to help 
adjust the relay contacts properly, but apparently something like that 
was considered beyond the knowledge of the Navy maintainers.

That's about all I can think of at the moment.

73,
Mike





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