[ARC5] BC-348 audio mod

Tue Sep 4 13:46:13 EDT 2012

-----Original Message----- 
From: arc5-request at mailman.qth.net
Sent: Tuesday, September 04, 2012 2:07 PM
To: arc5 at mailman.qth.net
Subject: ARC5 Digest, Vol 104, Issue 4
Thanks Dave for the info on the little DC-DC bucker down..I have a 
requirement for about 4.5 v for a solid state bfo for the ARC 5 46145 BCB rx 
and this sounds ideal for using the A line (mine are 12v)..I note the 
switching freq of the device I'm looking at is 150khz and the rx I.f. is 239 
khz..I doubt if I'll be as lucky as you with the 915 in the 348 but for a 
few bucks we shall try it anyway..the audio mod for the 348 is very neat and 
no mod to the rx at all..also that you can use it with that way reduced B+ 
which cant be bad for the caps in the 348..I have an idea for replacing the 
HRO-60s 2 6v6s and p/p output transformer with 2 10w solid state audio 
modules..tiny little things..imagine the heat eliminated and the reduced 
strain on the pwr tfr..with apologies as always to the purists..hi!...there 
is also a tiny solid state HV choke replacement device that could be put in 
the 60..hi!..these brilliant little devices are here to help us..73 de 
Pat/EI7CN
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Today's Topics:

   1. 2E22 tubes (mac)
   2. BC-348 Reduced B+ Report (David Stinson)
   3. Re: [Milsurplus] BC-348 Reduced B+ Report (David Stinson)

----------------------------------------------------------------------

Message: 1
Date: Mon, 3 Sep 2012 22:25:10 -0700
From: mac <w7qho at aol.com>
Subject: [ARC5] 2E22 tubes
To: milsurplus List <milsurplus at mailman.qth.net>, ARC-5 List
<arc5 at mailman.qth.net>
Message-ID: <A0DC2073-69B3-40E3-8C29-2886EB14046E at aol.com>
Content-Type: text/plain; charset=US-ASCII; format=flowed

Short thread on these a few days ago.
320970554992

Dennis D.  W7QHO
Glendale, CA

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Message: 2
Date: Tue, 4 Sep 2012 07:40:08 -0500
From: "David Stinson" <arc5 at ix.netcom.com>
Subject: [ARC5] BC-348 Reduced B+ Report
To: <milsurplus at mailman.qth.net>, "ARC-5 List" <arc5 at mailman.qth.net>
Message-ID: <E781625DDF4D43E7A8FEFAF39F15B465 at CompaqSR5710F>
Content-Type: text/plain; format=flowed; charset="iso-8859-1";
reply-type=original

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

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Message: 3
Date: Tue, 4 Sep 2012 08:07:02 -0500
From: "David Stinson" <arc5 at ix.netcom.com>
Subject: Re: [ARC5] [Milsurplus] BC-348 Reduced B+ Report
To: <milsurplus at mailman.qth.net>, <boatanchors at theporch.com>, "ARC-5
List" <arc5 at mailman.qth.net>
Message-ID: <97EC0A5014A24AE49B656EFCC14A4FF1 at CompaqSR5710F>
Content-Type: text/plain; format=flowed; charset="iso-8859-1";
reply-type=response

One of our members asked:

> Dave do you think any of those "boosting and bucking" regulators can
> be modified to
> output 90 or 100 volts?

No need to modify.  Here ya go:
http://www.ebay.com/itm/170876068976

I'm using one of these for B+ and one of the small
"buck" regulators to power a battery set from a single
12-VDC gel cell.  Works great.  No RFI problems,
at least so far.

NOTE:  There is one down-side.  These are non-isolated-output.
Your -12 volts in is your -90 volts out.   So you can't use them
if your require floating-B-minus, like in a BC-474.
There are a few isolated-output DC-DC parts, but they are
more expensive and fewer available.  The makers tell me
more and less expensive isolated-output modules are coming.

73 DE Dave AB5S

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End of ARC5 Digest, Vol 104, Issue 4
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