[TMC] GPR-90RXD stuff

[Richard Knoppow]

    All this discussion of the GPR-90 and its variations led me 
to download several manuals from the K4OZ site. I had some but 
decided to create a dedicated folder.
    I looked at e-ham net reviews again, they are interesting 
because those who hate the receiver do so for reasons that 
indicate to me their sets were broken. I thought I saw a review 
that said the BFO pulled but couldn't find it.
     Some thoughts about the changes between the GPR-90 and the 
RXD version. Of course the RXD has the fixed frequency crystal 
oscillator but also has other changes. One is that they got rid 
of the grounded grid first RF. I think this was done to improve 
RF selectivity to prevent cross modulation but am not sure. The 
added pentode amp is almost a tunable pre-selector using the 
"antenna trimmer" to tune the RF stage. Also, the changes in the 
BFO are interesting. The original is a conventional BFO coupled 
to the the third IF stage with a small value capacitor. This 
arrangement is used in several other receivers and is supposed to 
provide some isolation for the BFO. The only receiver where I 
have heard pulling was on my unmodified BC-779 and it was the HFO 
that pulled because of lack of B+ regulation. I installed a VR 
tube for the LO and that cured the problem. I wonder what is 
actually happening in the GPR-90 and if this is an actual 
problem. The BFO in the RXD is connected to the detector via a 
matching transformer. That is the coil being discussed here. One 
change is that the BFO is isolated from the IF output. Since the 
RXD was obviously made for diversity receiving systems with the 
IF going out to an RTTY or SSB converter it may have been 
important that the BFO's on the local receivers not upset the 
system Actually if one is using an SSB adaptor like GSB-1 or the 
MSR series, the use of the local BFO serves no purpose.
    I have read over the documents on the site concerning the 
design of the GPR-90 but they do not answer the question about 
the market. I suspect it was intended to provide TMC with a 
receiver suitable for filling out system contracts. Otherwise 
they would have had to use something like a modified SP-600-JX or 
certain other receivers. I don't have a GPR-90. Eventually, I 
will find one and a MSR-9 (connects to the receiver AVC). Which 
should be the equal of nearly anything. I also suspect TMC had a 
price point to meet, maybe half the price of an SP-600 or 51J3 
and lots less than an R-390.
     K4OZ has lots of inside information but one is still left 
with questions about the design and market.

-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL