[Milsurplus] possible modifications for RBC receiver?

[Kenneth G. Gordon]

On 18 Sep 2016 at 18:34, George Babits wrote:

> Dave,
> 
> The RBB and RBS are grest receivers.  Why pervert them with a product 
> detector?   Just turn off the AVC, turn the BFO on,  and  you can copy SSB 
> just fine.  Over the years I have picked so many receivers that have been 
> ruined with crappy product detector conversions that I just can't imagine 
> why people do that.  Is it just so the S meter will read (maybe) on a SSB 
> signal?

No. In order for THAT to work, one also has to pervert the AGC system. The AGC voltage 
must be taken out of the IF chain before the detector so that the BFO doesn't lock the AGC 
voltage high. All AGC systems of the day took the AGC voltage off the detector.

Actually, a GOOD product detector results in far more "recovered audio", greater apparent 
receiver sensitivity, and reduced noise than the old diode detector ever could.

Furthermore, a GOOD single-triode product detector will operate as a very good AM 
detector when the BFO is turned off. Although I haven't examined that detector (used a lot 
by Collins and Heathkit) in any detail, I suspect that with the BFO voltage turned off, the 
detector acts much like an infinite impedance detector for AM. But that is only a WAG.

That single-triode PD is simple and works very well. Most other PDs I am familiar with are 
MUCH too complicated. One of the early PDs shown in handbooks of the period, uses 
THREE triodes. Another uses a 6BE6 mixer tube. I don't like either one.

After all, all a PD is is an audio-frequency mixer. In fact, the Pullen Mixer makes an 
excellent PD.

Anyway, I modified an almost new BC-779 many years ago with that single-triode PD, and it 
made an immense improvement in the receiver. I was amazed at the improvement. It 
became my favorite receiver.

But as you say, George, most modifications were never thought out thoroughly and just 
plain stink. And this includes many which were published in the ham literature.

The OT who designed the gear to begin with were no dummies. They knew what they were 
doing.

Ken W7EKB

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