[R-390] R: Strange variation of sensitivity AGC resistance toward the chassis

[Pierfrancesco Mengacci]

Hi, Roy !

Thanks for help. 

In theory, the value I measure (rear AGC plug, toward ground), does
correspond to the figure which comes out from the receiver diagram: 

It is series and parallel of the following resistances:

"Res1" = [R546 (180K) in series with R545 (100 K)] = 280 K ;  "Res2" = [R544
(2.7 M) in series with about 20 K(resistance of B+ line)]; the parallel of
these two resistances is "Res3" = 1/(1/280 + 1/2720) = 254 Kohm

Referring to the control grid of V506-A (pin 2 of AGC time constant tube) ,
its resistance toward ground , which I call "Res4", is the sum of "Res3" +
R547(220K) = 254 + 220 = 474 Kohm

But, in addition, at the control grid of V506-A is connected also the
resistance of the AGC circuit inside the RF deck, which I call "Res5", =
R234 (1.5 M) + R201 (270K) = 1770 Kohm

(coil resistances are minimal and they have been omitted)

In conclusion:

The resistance toward ground of pin 2 (control grid of V506 - A) comes out
from the parallel of "Res4" and "Res5"  =    1/(1/474 + 1/1770) = 374 Kohm


This, seems to me, ought to be ALSO  the theoretical value of the AGC
circuit resistance, when measured at contacts 3-4 of  TB102 on  rear panel
(if there are NO Extra Losses or bad capacitors or bad insulation...).

By the way, I measure 380 Kohm, which is rather close to thae above
"theoretical" value (if I'm not mistaken)

Anyway, I wait for your effective measurement!!

Ciao, Franco Mengacci

-----Messaggio originale-----
Da: Roy Morgan [mailto:k1lky68 at gmail.com] 
Inviato: lunedì 4 maggio 2015 14:08
A: Pierfrancesco Mengacci
Cc: r-390 at mailman.qth.net
Oggetto: Re: [R-390] Strange variation of sensitivity AGC resistance toward
the chassis


On May 4, 2015, at 6:12 AM, Pierfrancesco Mengacci <pf.mengacci at outlook.it>
wrote:

> Just for scruple, does someone know which is the “normal”  value of 
> the AGC circuit resistance toward ground , measured at the rear 
> contacts of TB 102 (n° 3 or 4), when the receiver is set on AGC 
> function and disconnected from power supply ? (AGC delay setting: 
> medium)

No, but I can get to a receiver today and measure it.  I’m pretty sure this
has been reported on the list, and is likely to be found in the Pearls of
Wisdom.

http://www.r-390a.net/Pearls/index.htm

The IF deck section is:

http://www.r-390a.net/Pearls/IF_deck.pdf

Note that this is some 500 pages long.

> In my case, I read 380 Kohm (when the negative terminal of the 
> ohmmeter is connected to the chassis, it has a minor influence...). 
> This reading takes even 5 minutes to stabilize.

I think you have bad capacitor(s) in the AGC line, or leaky wires.  All your
reported symptoms point to this.

Roy


Roy Morgan
k1lky68 at gmail.com
K1LKY Since 1958