[Milsurplus] RBM - MF question(RU-19 selectivity)

[Hubert Miller]

Jack, thanks for the figures. I always like to see the numbers.
I thought someone ( you? Dave Stinson ? ) posted sometime past that the converter + LF coil just wasn't that great a solution and that the 
regenerative RF stage was more workable.  
Altho I most likely will get rid of the remainder of my RUGF things, I thought about it a lot and I think my approach would be a 2-MOSFET or FET
Franklin oscillator, a 2-terminal oscillator, on a little board nested inside the receiver, connected on the first RF coil, and controlled via a DC lead 
out of the receiver to an external pot. 
I also wondered about changing the bias on the detector tube so it functions fulltime as a product detector and not as a biased detector. This 
means backing off the DC bias. That 'might' offer a little more selectivity, working as a direct-conversion receiver, "I think", but then you have to
have the BFO always on. 
I think some organization in Sweden is offering a cash prize for the first person to solve the RU receiver on ham bands problem.
-H 

-----Original Message-----
From: milsurplus-bounces on behalf Of Jack Antonio
Sent: Tuesday, October 31, 2017 8:00 AM

On 10/31/2017 3:37 AM, Hubert Miller wrote:
> I wanted to know the -60 db selectivity of some receivers, since, 
> AFAIK, that's a good judgment point for overall selectivity.

> 
> RAX  IF=160 kHz   -60 db  -6.8 kHz  +6.8 kHz
> 
> RBM IF=112 kHz  -60 db  -7.5 kHz   +8  kHz   ( switch= Narrow )


I've been playing with an RU-19 recently, and trying to figure out how to use one on the air with my GP-7.

I tried checking into the OMRN net on 3570 with the G range coil set. which was very frustrating. The digital signals at 3576 simply overwhelmed the weaker CW signals. And on AM, it always seems there are strong SSB stations 10 kHz up that wipe out the Sat morning net.

One thing I tried, which helped a little, is to put the receiver in MCW and use the LM as an external BFO. Then tune the receiver down in frequency, to put the QRM down on the upper side of the curve. The CW stations lower in frequency weren't as much of a problem as the digital signals. But still, it wasn't all that successful.

So now, I am playing with a crystal controlled converter using the low range coil set for the RU-19. The converter is a rough equivalent to the Novice Q-5er, which Ken Gordon has the article for on his website.

In doing so, I made some simple selectivity measurements using the O/P coil set for the RU (roughly 200-400 kHz).

The quick and dirty measurements I came up with are

200kc    -6db  2.1 kHz         -20db  3.8 kHz
400kc    -6db  6.8 kHz         -20db 13.0 kHz

Still a bit broad, but much better. The RU-19 only has
3 tuned circuits in the signal path, do I didn't expect it to be as good as a BC-453.

Results so far are promising. Right now I'm seeing what happens if I feed the converter into the loop input, and using the front panel switch to switch between antenna direct and the converter.

If all this is successful, I may try building the converter into a derelict ZB housing to make it look "proper".

One other rather extreme experiment I tried, is to feed the 455kHz IF out of the R-390A into the RU. But that is bordering on the ridiculous.

Jack Antonio
WA7DIA