[ARC5] BC-348 Reduced B+ Report

[David Stinson]

I recently resurrected a very nice BC-348Q for a friend.
It had a "drop-in" AC supply with an external "in the cord" 
AC power switch, designed so you could remove the 
AC supply and "drop-in" a dynamotor for original-config.
A cap had shorted on the B+ buss and the unfused supply
cooked-off.   I first got the set running well on dynamotor,
then rebuilt the AC supply, retaining the original circuits and 
the ability to swap back to dyno.  I did move the 
audio output transformer tap to the low-Z post.

I decided to lower the B+ to help preserve the set.
About 150-170 volts seemed to be a good compromise
between performance and perservation.  
The rebuilt supply uses a 24-VAC transformer to feed
the A-buss and another small transformer
to a floating-B-minus bridge rectifier, 
supplying about 160 VDC B+.

RF sensitivity and AVC action did not seem to suffer, 
but of course audio output dropped to 
"loud 600-ohm headphone" level.
I disconnected the wire to the lower headphone jack and,
in this line, inserted a little audio amplifier I salvaged from 
some cheap "computer" speakers.   
The little amplifer board fits nicely under the power/IF deck.   
The board needs 9 volts DC to function.  
There are some very useful and very cheap DC-DC 
"boosting and bucking" regulators, and even cheaper
linear regulators, available on Ebay these days.*

I ran a diode from the 24-VAC buss to one of the 
DC-DC "buck" regulators and used that to power the board.
No cap needed as filtering is already on the board.
I've had no RFI troubles from one of these little regulators.

Aligned the set at the lower B+ level and away we go.
The BC-348 now has 600-ohm headphone level on 
the top phone jack and load 4-ohm speaker level
on the bottom phone jack.  
Modification to the radio:  Moved one wire.
It sounds great. Even the usually-defunct crystal filter
works nicely on this one, slicing SSB signals well.

Go you-know-where and search
DC-DC step up (or boost)
DC-DC step down (or buck)
Check the "seller's other items" for 
linear regulators.
Cheap as dirt- some for under $2-
 and lots of useful applications.

GL OM ES 73 DE Dave AB5S