[R-390] R-390A s/n improvement 6HA5 mixer

[Jim]

Years ago I changed the mixers in a 75A3.  The results were less than 
satisfactory.
The lesson I drew from it was that disturbing the gain distribution of the 
radio led to poor results.
Unless one is prepared to completely redesign the signal path, they are best 
left alone.

Jim

-----Original Message----- 
From: Bob Camp
Sent: Sunday, May 15, 2016 8:32 AM
To: Larry H
Cc: R-390 Forum
Subject: Re: [R-390] R-390A s/n improvement 6HA5 mixer

Hi

Any time you increase the gain of the front end stage(s) in a radio, you 
will degrade it’s overload performance. The IM2, IM3, and blocking
numbers all will get worse. The real way to drop in the 6HA5’s would be to 
re-wind the IF coils to a lower inductance / lower impedance.
That way the gain would stay the same and the noise figure would not be 
impacted (much). Next step would be to take a look at issues
like AGC and it’s “taper”. Mixers generally aren’t a big deal there. RF 
stages (when swapped) generally are.

Bob

> On May 15, 2016, at 12:02 AM, Larry H <dinlarh at att.net> wrote:
>
> I mentioned in my original post on 4-19-16 that with 6HA5's in the 1st and 
> 2nd mixer places, the usual sensitivity measurement (S+N/N) nolonger 
> worked.  This was only a problem in MGC mode.  Well, here's what happened. 
> The gain of the 6HA5 is considerably more than a 6C4, so when 2 or 3 
> 6HA5's are used, the signal level at the 4th IF is way too high  (even 
> with the IF gain turned all the way down).  This causes the 4th IF to go 
> into saturation way too early and turn into a limiter (thus reducing the 
> audio output and the ability to obtain a 10 db increase in S+N for the 
> measurement.  The solution I chose is to reduce the gain of the 2nd IF by 
> adding an 18K ohm res in the cathode circuit unbypassed.  This reduced the 
> overall gain to where the usual sensitivity measurement (S+N/N) now works. 
> This also makes it easy to set the overall gain to 1uv input = 7v on diode 
> load.  All is now good again.  The only measurement I've made so far is on 
> 9.9 mh and it's .65 uv.  The measurement on 9.9 mh with the 3 - 6C4's 
> using this method was 1.5 uv.   I'll report more measurements once 
> completed.
> I tried numerous different ways to reduce the gain of the 6HA5's, but 
> could not find a way without eliminating the gain in S/N ratio they 
> provided.
> Regards, Larry
>
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