[R-390] AGC performance

[Roger Ruszkowski]

Eric,

You wrote,
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 AGC performance.   It seems that my R390A needs a relatively high value of 
input signal before its AGC activates, and the Carrier Meter moves.  I note that 
in the AGC circuit description in the technical manual it states that the AGC 
circuit operates only for signals in excess of 5 microvolts – this to me seems 
to be quite a high signal level  for a communications receiver,  and in general 
listening to non local stations, the R390A does not use its AGC circuit.  I have 
zeroed the Carrier Meter , and the IF gain pot is set to  midway, so it is not  
a case of my 390A having excessive IF gain.  Is this normal , or do I have an 
AGC fault?

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Your R390/A is about typical and normal.

Remember the R390 and R390/A were built for the signal corp to do RTTY communication links around the world for the US military.
They were not expecting to do any weak signal work. The idea was nice transmitters and excellent antennas with good receivers at fixed sites operating on fixed frequencies.
It was just simpler to make the receivers full frequency coverage from .5 to 32 megahertz than just the military allocated frequencies. AM broadcast was not a real consideration. CW was just to accommodate a fall back position if the RTTY modulation could not get through. SSB was yet to be invented.

Only in later years were the receivers diverted to perverse applications in weak signal work.

There has been a lot written on the R390 and R390/A AGC circuits. Most is dislike for the time constants. Some stages have been omitted from the AGC circuit. The RF gain has been split between the IF and RF stages with separate gain controls. The resistors have been changed in the AGC circuit to rearrange the gain. All personal preferences and what you want to hear from your receiver.

One mod is to pull the 2MFD AGC cap in the IF deck. Insert an 8 pin octal socket in the chassis and mount an 8 pin octal 2 crystal oven can in the socket. The oven can, can then be populated with AGC caps of your choice. The MED and SLOW AGC caps can be both placed in the can. You can also squeeze in a resistor or two. This then lets you do changes to the AGC by just unplugging the oven can and changing parts in it until you reach a set of values you like.

There is an AGC jumper on the back panel. You may think external DC amp that boost the AGC derived voltage to a higher voltage. The Audio (line or local)  has also been rectified using a bridge and caps on the back panel and then feed as AGC into the AGC pin on the back terminal board. This higher voltage level and possible voltage doubler with selected AGC time constance allows lots of flexibility for AGC in the receiver. The line output can be utilized with variable line again to drive the AGC. The limiting resistors on a terminal board behind the front panel above the dial counter can be by passed on the terminal board (R111 - R115) to provide a higher power  audio signal to convert to AGC if you need it.

Roger AI4NI