[R-390] Power line Filter

[Flowertime01 at wmconnect.com]

Rad Smith,

The biggest concern about replacing the power line filter is the fact that 
the stock filter passes enough current to ground to trip the new ground 
fault breakers. If you run the receiver on an old style breaker with a good 
ground wire there is no problem.

There are several computer line filters than can be found free or hacked 
out of computer power supplies that will set behind the hole left when you 
remove the standard filter. These will filter and fit. But it is unsightly.

You can remove the filter, melt out the solder to get the "can open" and re 
pack it with a couple caps that have a small enough value so as not to 
upset the GFI breaker.

You can run the receiver from an isolation transformer.

You can run the receiver from a small UPS. Its input will not trip the GFI 
and its output will accept the line losses of the receiver filter. Do your 
homework and consider the caps in question are pulling power even when the 
receiver is off.

You can run the receiver without the filter. Or gut the filter and run the 
can empty.
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Are there any concerns about removing the back panel for this job?

Only wire harness wire length. Better to stand the receiver on end, Pull 
the VFO for a bit of knuckle room and just do the rework with the panel in 
place.

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I would probably replace the selenium rectifier with an appropriate silicon
bridge and would like to know about any dropping resistor needed and others
issues.

Just put the new bridge in and worry not about any change in values. 
Remember the load is the relay coils. These will pull what ever power they need. 
As current is drawn, voltage will drop across any resistance (forward drop of 
diodes) in the circuit. A bridge will have a higher voltage. but the load 
is the same and thus the current will be lower and the power will be as it 
was across the coil to begin with. Other loses in the circuit will be less and 
this is OK. Consider the varnish insolation on the coil wire winding. As 
long as you do not exceed it break down value the coil will be OK. The voltage 
rise on the 24 volt power supply transformer will be no problem for the 
receiver. Lots of bridges have been installed with no other changes. They only 
get used if you are doing calibration and break-in. Not a large duty cycle.

Have fun

Roger AI4NI

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