[Milsurplus] Need BC-348 Expert!

[Ray Fantini]

The 1948 "CQ Surplus Conversion" Manual has a lot about modifications and changes for that radio along with the September 56, February and March 59 issues of CQ Magazine. Lots of Hams at the time were obsessed with playing around with the front end in trying to reduce noise. I don't play around with the primitive version of the radio and tend to prefer the Q series with more modern tubes but know on that version people played with the screens, feedback and replacing 6SK7 tubes with lower noise 6AC7 tubes. I myself have never had an issue with noise on the higher frequencies so always kept that stage of the radio stock. The most useful changes I have found for Ham and SWL operation are the addition of an AC power supply, replacement of the audio output transformer to a low impedance transformer and addition of an S meter. Some of the other common modifications included things like separating the gain control for separate AF/RF gain, adding a triode stage in the first audio for more gain and installing a product detector. I have a page about modifying the BC-348 at:

http://staff.salisbury.edu/~rafantini/bc348modifications.htm

Ray F

-----Original Message-----
From: milsurplus-bounces at mailman.qth.net [mailto:milsurplus-bounces at mailman.qth.net] On Behalf Of Michael D. Harmon
Sent: Sunday, August 25, 2013 11:10 PM
To: milsurplus at mailman.qth.net
Subject: [Milsurplus] Need BC-348 Expert!

I'm rebuilding a BC-348-P which had been modified (poorly) sometime in 
the past.   I'm using the TO 08-10-119 manual downloaded from Andy 
Moorer's BC-348 site.  With a date of 15 DEC 42, it's not the latest BC-348-P manual I have, but it's by far the best print quality.  Both the parts list and the schematic show a 100K, 1/2W resistor (Ref # 48-5) between the screen and cathode lines on the 2nd RF tube (VT-86).  
However, this resistor is NOT shown in the wiring diagram - not the main chassis, the terminal strips nor the tube shelf.

At first, I thought the discrepancy was due to an engineering change that either added or deleted this resistor, but every schematic and parts list I have shows this resistor, and none of the wiring diagrams show it.

The parts list describes the resistor's function as 'bias control'. The other BC-348 manuals (B, H, K, L, R, S) don't show this resistor.  In fact, one of the manuals I have shows this resistor X-ed out, as well as the 3.5K 'noise compensator' pot (Ref # 58). I've been somewhat ambivalent about whether to leave the 'noise compensator' pot connected or not.  It was designed to reduce the RF gain as the frequency went up on each band, but I'm pretty dubious about the benefit of this feature when used in ground-station applications.  In fact, I can see that this 'feature' might be at odds with getting the best performance out of this receiver.

What do you experts think?  Should I leave the 'bias control' pot out?  
What about the 'noise compensator' pot?   FWIW, I'm not trying to build 
a museum piece.  I just want a BC-348 that performs well.

Thanks!
Mike, WB0LDJ
mharmon at att.net