[Milsurplus] BC-348 - Power Supply

[Jim Whartenby]

HutchTwo things come to mind:One> Your transformer is in saturation because it wasn't designed to operate with the higher line voltage now in use.  In other words, the transformer core lacks enough iron to prevent saturation at high line voltage.  Along the same lines, the copper wire gauge is too thin and is presenting a larger IR drop to the capacitor charging current.
Two> Your filter capacitor is too large causing a big slug of current and this current is heating the high voltage winding over time.
If you have a VARIAC, try reducing the applied voltage to 115 vac and see if the temperature drops and B+ remains more or less constant over time.  
If your filter capacitor is too big try a smaller value or try adding 100 ohm or so power resistors between the rectifier plates and the high voltage transformer winding.  
The RA-20 power supply uses a pi filter with 8 uF filter capacitors.  The lower cap value allows for a longer conduction period.  Larger value filter capacitors shorten the conduction period with a corresponding increase in capacitor charging current.  Smaller value capacitors are easier on the rectifier tube too.
Let us know what you find that works the best,Jim
I wonder why people argue over the 10% of their differences and ignore the 90% they agree on? 

    On Monday, February 4, 2019, 9:26:58 PM CST, John Hutchins <jphutch60bj at gmail.com> wrote:  
 
 Have made several power supply versions for my BC-348R.   I am close to 
what I want.  BUT, big  BUT,  I have a heat issue. Outside in free air 
the power supply stays on target +/- 2V.   All buttoned up inside the 
BC-348, voltage drops slowly from 225V- to - 190 V  G.T > 1.5  hrs time 
and falling.  Stuck a wire thermal probe into the unit, temperature 
~37-39 C  102F.  I am not surprised the voltage dropped, but had no Idea 
it would be > 10 %.  Any ideas/Suggestions on how to wrangle this issue 
please contact me "off" the list.   I have a pic, windows .BMP file of 
the schematic.

Thanks

jphutch

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