ASTRON.2
From: N7PTM@N7SEE

ASTRON power supply modification:
 
This mod prevents RFI and unstable voltages while transmitting HF.
 
There have been several reports that Astron power supplies can blow
fuses, power transistors, and generally become unstable due to RF
feedback problems when these supplies are used with HF transmitters.
ICOM recommends that the mods shown in this diagram be made to
Astron power supplies to prevent RFI and unstable voltages while
transmitting.  This modification can be added between the output of
the supply and the transceiver, thus no internal mods to either unit
are needed.
 
 [+]------------------------------------------------------[+]
                        p/s output            * .01uF/500vdc
               * 1000uF  -------------to chassis gnd     TO radio
               5vdc    * .01uF/500vdc
                        [-]-------------------------------[-]
 
Note the location of the + side of the 1000uF cap.  (ASCII art
doesn't allow the proper display of the --------- symbol.)  One
leg of each .01uF cap is common with the chassis ground.  (Diagram
didn't say what chassis, though)
 
There also has been a message posted to the ham BBS's detailing a
cure for the blown fuse action seen on some Astron power supplies.
That message is quoted below:
 
"If you have been having trouble with Astron power supplies blowing
fuses, pass transistors, and the 723 voltage regulator chip on a
regular basis, then try this simple remedy:
 
"Put an electrolytic cap 100mf @ 25 volts from the GATE to the
CATHODE of the Over Volt Protection SCR.
 
"At first I thought this problem to be bad switches, causing such an
enormous spike, that it was not being regulated and therefore
telling the SCR to "KILL" the AC input fuse.  But then, with it
happening more and more, it would have put a strain on the various
other components.  The 723 voltage regulator can be abused only so
much !  The output can be shorted many times and all that usually
happens is the supply folds back and takes the drive away from the
pass transistors.  What really happens when the SCR FRIES is, the
output of the supply is being forced to OVER CURRENT and VOLTAGE
FOLDBACK in such a manner, therefore "KILLING" the input fuse.
That's whe we should only stick to the recommended value."
 
(from AF MARS "Region 3 Communicator")