[R-390] Break-In Function

[Drew Papanek]

Roy Morgan wrote:

<snipped>

>  - The grounding of the antenna inputs depends on the condition of the
>antenna relay contacts and also the condition of the antenna relay supply
>rectifier and filter cap (there *is* a filter cap, right?)  Low relay
>voltage may cause high closed contact resistance.

The R-390A antenna relay is powered from the 25 VAC supply via a selenium 
full wave bridge rectifier.  There is no filter capacitor.  It is fairly 
common for that rectifier to fail either open or in an increased resistance 
mode.

If relay armature pulls in and makes solid contact with coil's iron core, 
original rectifier is at least temporarily functioning well enough.

The rectifier can be replaced with a silicon full wave unit.  The style 
rated at 4 amps or so having a mounting hole and all 4 terminals exiting at 
1 end conveniently bolts on to the original bracket.  Silicon has a lower 
forward voltage drop and so relay coil will see higher voltage than with 
original rectifier.  If that is a concern, you can add series resistance. No 
list members have reported problems with excessive coil heating when using 
silicon replacement.   To read more on this topic, goto r-390a.net          
Click on References, Pearls of Wisdom, Power Supply.  You will find a 
collection of power supply related postings gleaned from this list over the 
past few years.

(Aside: 25 amp "block" style bridge rectifier makes a good replacement for 
selenium in older car battery chargers.  Mount to metal case for 
heatsinking.)

Drew

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