[Milsurplus] RCH Receiver

[Robert Nickels]

WA5CAB at cs.com wrote:
> No.  Your point could be valid if the RCH were like the Command Set 
> receivers which have no audio gain control.  But look at the RCH 
> schematic.  Box stock audio circuit for almost all WW-II ground and 
> shipboard receivers except that some separate the audio and RF gain 
> pots (like the TCS).  The audio gain pot sits between the output of 
> the 6H6-GT detector and ground.  The wiper goes to the grid of the 
> 6SQ7-GT first audio.  The cathode of the 6SQ7 goes to ground through a 
> resistor.  No sort of feedback or other connection to anywhere else in 
> the circuit.  With the audio gain pot fully CCW, it wouldn't matter 
> what the condition of the capacitor across the cathode resistor was.  
> No input equals no output.  The only way in which there could be input 
> would be for the grid not to be grounded with the pot fully CCW.
>
>
A little follow-up on this...

Robert,  the ground connection is intact and ohmmeter checks on the dual 
pot are as you'd expect.   I finally printed the schematic I'd 
downloaded from BAMA and taped all six pages up so I could follow the 
wiring and verified both the AF and IF/RF sections of the pot are wired 
correctly and see no signs of modification.  However, contrary to your 
very logical observation above, the audio bleed-thru is still quite 
loud, and is unchanged even with a clip lead from the  grid of the first 
audio tube directly to ground.  In fact, audio can still be heard, 
although lower in volume and with distortion, when the 6SQ7 first audio 
tube is removed from its socket!   With the tube in, the audio level is 
unchanged from zero to about 9 or 10 o'clock, then it increases to what 
sounds like the specified 2 watts.  By the way this bleed-thru is only 
audible in the MOD position, with the BFO on the audio indeed goes to 
zero at full CCW rotation of the gain control.   So it seems there is a 
secondary path by which audio is reaching the audio output stage.

And so, head-scratching continues!

73, Bob W9RAN