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

[Pierfrancesco Mengacci]

Hi, Roy
Your patience and completeness are commendable; the information is
perfect!!!

A) Measurable resistance toward ground (chassis), when on AGC and receiver
disconnected from power net: 
Your datum, V201 pin 1 = 940 K; my datum = 980 K             Your datum,
E208 = 490 K; my datum = 505 K                  Your datum, E209 = 1.45 M;
my datum = 1.45 M                      Your datum, E211 = 900 K; my datum =
835 K                  Your datum, TB102 (3-4) = 390 K; my datum = 380 K

B) I tried with a 9.0 V battery connected to AGC rear test point( minus to
ground; function on AGC, receiver unpowered) At all control grids I read a
voltage variable from -7,5 V of V 201 to near
-9 V on IF tubes); this corresponds with good approximation to the diagram,
when I take into consideration the various resistive partitions associated
with the multimeter internal impedance (10 Mohm)

CONCLUSION
The AGC circuit of my R390/A-URR is behaving almost correctly, the anomaly
is to be found in some other zone..
 
ERROR on the TM11-856A Manual.
I have controlled the diagram page 183 , figure 104: IT DOES NOT CORRESPOND
TO THE GENERAL CIRCUIT of the following pages (V201 RF section). In
particular, E208 is not connected to the control grid of V201 but to the
union of E213 and R233 (470Kohm). In addition,   R201 is not 22K but 270 K.
This gives reason of "theoretical" discrepancies


SOUL
I think the "anomaly" o "trouble" or "vice" of my radio  (i.e.the
unpredictable  and slow sensitivity reduction/variation  via antenna only)
HAS A SOUL:
When it has understood that I was tracking it with the help of the friends
of the R390 forum, it has disappeared or concealed  itself, may be it has
committed suicide!!!
The fact is that in the last few days I do not have experienced the problem
(i.e. I get at least 40 Db on calibrator position, on all bands, and a
sensitivity in the range (approximate) of 1-2 microvolts on all bands).

What I did in the meanwhile was to remove an reinstall for the 10th time the
first RF coil of band 16-32. I suspect there was a random contact between
some pin of the coil (those on chassis side) and conductors nearby

Thanks to everybody!!!!!

Franco Mengacci (I live in Alessandria, near Milan/Genova and Torino).

-----Messaggio originale-----
Da: Roy Morgan [mailto:k1lky68 at gmail.com] 
Inviato: martedì 5 maggio 2015 01:14
A: Pierfrancesco Mengacci
Oggetto: Re: [R-390] R: Strange variation of sensitivity AGC resistance
toward the chassis


On May 4, 2015, at 8:45 AM, Pierfrancesco Mengacci <pf.mengacci at outlook.it>
wrote: 
> 
> 
> 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

Franco,

I did measure some restances on my R-390A today.

(The one you mention above I did not measure.)

Here are the numbers I got:

TB-102 pins 3 and 4  390 K (AGC connection on rear of radio)

J512 pin 4 (AGC output from Audio deck): 537 K  (I had this connector as
J502, but that may have been a mistake)
J512 pin 13 (“AGC Switch”)  on the very high range of my meter I got 8 meg
and rising, On a lower range (20 megohms) I got greater than 20 meg.

V 201 pin 1 (RF amp grid):  940 K
E208  490 K (there seems to be some error about the diagrams at this point)
E209  1.45 meg
E210  1.45 meg
E211  900 K

Right now I don’t have handy the measurements that are expected from the
Pearls of Wisdom document, but I think these numbers are about right.

I hope these help you sort out what’s happening.

Roy


Roy Morgan
k1lky68 at gmail.com
K1LKY Since 1958