[R-390] R390-A/URR antenna trimmer question

[Roger Ruszkowski]

Franco Mengacci,

You did not mention what books you have for reference.
Find a copy of TM 11-5820-358-35 and the Y2K manual as PDF files at
www.r-390a.net/

You are going to divide this problem into three parts.
Part one
Inject 455 KHz into the IF deck J518 at 150 micro volts CW.
Set the receiver to MGC bandwidth to 2KHz, BFO off.
Set the IF gain for -7 volts on the diode load.
Hang 600 ohms across the local audio out on the back panel.
You need the local output to be zero dead level flat with the generator on CW.

Turn the generator modulation on and add 30% modulation.
Now the local audio output needs to be 27 DB 1/2 watt 17.34 volts AC across the 600 ohm load.
(two 1200 ohm 1/4 watt resistors in parallel)

You have to get the IF deck and Audio deck up to this spec and stable 
or the receiver will never pass end to end test.
You may have to do some tube swapping in the IF deck to get these numbers.
You have got to get the IF stable and quiet.
This division of the receiver lets you work on these sections and get them good then you build on this.
If you do not 1/2 watt Modulated output and a good flat zero CW response then you have to work on the IF deck.

The gain should be flat and steady not dropping in and out with time and temp.
If you see these problems then you know you need to work on the IF deck.

You did not mention your caps.
Do you have all the big black or brown plastic caps out of the IF and RF decks.
These particular caps are know to be going bad after 50 plus years and causing problems.

Part two.
With the receiver in MGC you inject 10 micro volts into one balanced antenna pin.
And short the second antenna pin to ground. Use a .1 cap between the signal generator and the receiver
just for DC isolation.

You need the same 1/2 watt output for the AM modulation but will have 7 DB of noise 
with the generator set to CW for a difference of 20 DB between CW and AM modulation.

If you do not have this then you need to work on the RF deck.
The receiver uses the second mixer and OSC under 8.0 Mhz.
The band switch changes the octaves as .5 1. 2. 4. 8. 16
If an octave is out work on the RF transformers.
IF a single megahertz or crystal set of megahertz is out you work on the crystal oscillator deck and the crystal.
Once you get the whole receiver working well in MGC you go to part three

Part Three
Getting the AGC to work and play well.
Open the AGC  load jumper on the back terminal board.
Hang a volt meter on the AGC load and determine if the voltage is stable and constant. 
If the generator input is stable and the AGC voltage is noisy then look between the detector and the AGC terminals.

If the MGC is Good and the AGC voltage is stable then you have to look into the
down stream AGC circuit. Likely a bad coupling cap.

Cecil pointed out the mica caps to you. In addition  the receiver uses a bunch of
ground lugs with small bolts and nuts. These ground points are starting to oxidize.
Just loosen the bolts and nuts and retighten the connection.
A lot of small low level noise goes away as this procedure is applied to as many of the lugs as you can find.

Good luck.
There is nothing in these receivers that can not be fixed.
Yours just wants some love and care to get it back up to specifications.
Did some one mention C553 in the IF deck to you?
This blocks DC from the mechanical filters.
If it shorts you will loose at least one filter and almost always all four filters
as you kill them one at a time trying the other band width switch positions.
Put a nice new quality 600 volt .01 cap in the place of C553 if it still has the old plastic 
cap in the IF deck.

Roger Ruszkowski  AI4NI  33C4H  68-75