[R-390] Power supply MOD

[nryan at mchsi.com]

Hi, Jim,

That indeed is part of a mod to the PS.  The resistor's purpose is to drop B+ voltage to the correct level.  Reason it's there is because the two 25Z5W tubes were removed and diodes installed in their place.  Voltage off of diodes is higher than that off of rectifier tubes, thus the addition of the resistor.

I think this was a commonly applied mod in Navy rigs.  Personally I favor retaining 26Z5Ws.  If you feel similarly, ensure that the filter caps are not leaky so that they do not shorten the rectifier tubes' working life.  The tubes are very rugged and will last long under normal conditions.

Enjoy your rig!

Norman, KG4SWM

Any activity becomes creative when the doer cares about doing it right, or doing it better. -- John Updike


----- Original Message -----
From: Jim Green <jagreen3 at sbcglobal.net>
To: r-390 at mailman.qth.net
Sent: Sun, 22 Apr 2012 17:07:17 -0500 (CDT)
Subject: [R-390] Power supply MOD

Naturally I am not the first to have worked on this R-390. I found a  
MOD (Modification) in the Power Supply.

V801 is missing from it's socket. A wire is coming out of the center  
hole leading to a big 30 Ohm 18 Watt wire wound ceramic resistor. The  
big 30 Ohm is in the circuit between pin 6 of V801 and pin 8 of V801.  
I can see no other changes. It looks like this big resistor just drops  
the 285V Pin 6 voltage down to the 240V Pin 8 voltage. All the  
operational pins on the V801 socket are common to the same pins on V802.

This doesn't seem like a very good idea for some reason. Can someone  
give me a clue what the tech was thinking?

Jim
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