[Milsurplus] RCH Receiver

[Robert Nickels]

W7QHO at aol.com wrote:
> A litle probing around with a scope should provide some clues......
>
Indeed, and to conclude this chapter of the tale of the RCH  that 
wouldn't shut up...

Dave Stinson was correct in that it was a bad bypass cap, but not one of 
the decoupling caps - the common screen supply which I figured to be the 
culprit turned out to be clean.  It was the cathode bypass cap on the 
6SQ7 detector/first audio stage that was  letting the cathode bounce all 
over the place.  This tube or it's miniature or loctal equivalent was 
commonly used as 2nd Detector and first Audio in receivers as it 
contains both a high-mu triode and a pair of diodes, sharing a common 
cathode. 

But wait a minute...don't the books (at least the Ghirardi and Johnson 
book that I consulted) say that cathode bias cannot be used for the AF 
stage, and don't all the typical receiver schematics show the cathode of 
a 6SQ7 tied to ground with either fixed or self-bias at the grid?   
Well,  the RCH designers went with a separate 6H6 Detector and only use 
the diodes in the 6SQ7 to rectify the AVC voltage, so they were able to 
go with cathode bias, which (when properly bypassed) works just fine. 
But with the bypass open, rectified output from the AVC diodes was 
making it's way into the plate, regardless of what was happening with 
the triode grid.

Thanks the the help guys, always good to have to have to think a bit.   
It's a nice receiver, hard to beat tuning with gears.

73, Bob W9RAN

PS:  While there is no "Placed in Service" date on the nameplate, the 
following callsigns/frequencies are recorded in pencil on the log chart:
WAR  3347 kc, A9USA/17 3257 kc, and NDQ on 2268, 2884, 4010, 5790, 2792, 
and 464 kc.