[R-390] 51J AGC and PTO questions

[David Wise]

> From: "Tom M." <courir26@yahoo.com>
> 
> Anyone ever slowed down the AGC action on your 51J?  Mine 
> is really fast and it has an annoying pumping action to it
> (don't know if it has any mods).

Kurt already answered this, but here's my two cents.  You can
certainly increase the capacitance.  How much depends on what
resistance makes up the R part of the RC time constant, and
your personal preference.  Experiment.  But it's likely to slow
down the attack, as it does in the R-390*.  If the 51J has a
separate-channel AGC IF amp, it's probably possible to work
around the problem with diodes like the R-390* Lankford mod.
But to get the ultimate fast attack, you must introduce a PNP
emitter follower and a negative power supply.  Be sure to
protect the input junction against reverse breakdown.

> Also, did anyone notice that the PTO endpoint error varies with line
> voltage on the 51J?

<Spock> Fascinating... </Spock>  No other comment, sorry.
I'd better check this on my R-390A, because I'm trying to
get flatter frequency/warmup by varying the heater current.
What a joke it would be if that threw out the endpoint.

> I set mine spot on about 5 years ago.  It is now 4 kcs long 
> on 120VAC and 2 kcs long on 108 VAC (using a 12V bucking transformer box).
>
> This PTO moves faster than kudzu weed, but I guess one 
> could use a variac to a limited degree to tame the PTO
> endpoint until such time you could dive in again.
>
> Thoughts?

No positive ones.  My guess is that it's the core;
its permeability is changing as it continues to age.
Collins worked very hard on the R-390 core,
and was just beginning to get acceptable results
when the Final Engineering Report was written.

I take it that the endpoint adjustment is hard to
reach.  Too bad we don't have a voltage-controlled
inductance.  Hmm.  It's only a few MHz, it might
be possible to wire up a wideband op-amp as an
inductance simulator.  WWII design with space-age
prosthetic, I like it just for the gee-whiz factor.
The bionic radio... "We have the technology" :-)

I'm not going to work it out, but I
wonder if you could get similar relief with a
varactor diode.  It would replace part of the fixed
capacitance, and you could trim it from the outside.
My guess: the temperature coefficient changes would
scuttle you if the corrector stack didn't.

By the way, I'd like to align my terminology.  Which
way do you-all call "long" and which "short"?

Regards,
Dave Wise