[TMC] GPR-90 vs GPR-90RXD BFO Question

[rrkrr]

John,
The GPR-90RXD can be set up for diversity reception, if you have two of 
them.  This requires that one receiver provide the VFO and BFO and the 
other receiver slave to the external oscillator inputs (you still have 
to manually tune the RF front end of both receivers, but they assume 
you're not going to be changing frequencies very often).

The reason for the different BFO coupling network you describe in the 
-RXD is so that the external BFO injection will work.  The cathode 
driven BFO circuit in the original GPR-90 has a low output impedance 
even with the local BFO tube turned off due to the cathode circuit 
components and would significantly load the externally injected BFO 
signal.  The anode coupled BFO circuit  of the -RXD has a high impedance 
to ground with the local tube off due to the RF choke you mentioned and 
would not present a low impedance to ground to an externally injected 
BFO signal.

I have a  GPR-90RXD I can hook up to measure the BFO voltage.  However, 
I have to travel this week, and won't get to it until next weekend, so 
you'll probably have to remind me.

Bob K4ERR

John Poulton wrote:
> I've been working on a GPR-90RXD off and on for the
> last couple of months, and am just now getting near
> the end of the electrical work.  One last problem:
> the BFO injection is there, but very weak.  Tube voltages
> (DC) on the BFO tube differ from the ones in the manual, 
> but don't appear to be so far off as to cause worry.  
>
> But, when comparing the '90RXD to the '90, I noticed that
> the BFO injection is entirely different.  In the RXD,
> the BFO voltage is taken from the plate of the 6AG5 (which
> is loaded with an RF choke), then capacitively coupled 
> to the tap of a tapped coil operating as a step-up 
> autotransformer, and finally coupled from the output of
> the coil to the AM detector plate through 3 pF.  I measure
> about 12 Vpp at the plate of the BFO and about 7.5Vpp
> at the detector plate, after all of the tapped-coil
> and coupling-capacitor shenanigans.  There's also an external
> BFO input that drives the tap of the tapped coil.
>
> In the plain '90, however, the BFO plate circuit is 
> resistively loaded, and the BFO voltage is extracted from
> the cathode of the BFO and applied to the grid of the last
> IF through 15 pF.
>
> So:  does anyone know why the two receivers differ in
> design?  Is there any reason to think than TMC *intended*
> for the BFO injection on the '90RXD to be weak?  Anyone
> have a working '90RXD who can report whether the BFO
> is working correctly?  If so, would you be willing to
> measure the AC voltage at the BFO input jack on the back
> of the radio, with a scope?
>
> I'll be able to read email only sporadically over the
> next two weeks..
>
> Thanks and 73, John K4OZY
>
>
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