[ARC5] [Milsurplus] BC-348 PSU questions

Wed Sep 30 07:10:58 EDT 2015

Ian,
I saw another post regarding a possible swapping of the center tap with one of the high-side secondary leads of the power transformer.  Good point!  I'd never seen that before and hadn't thought of it.  Do check for that.
There was a modification described in the Surplus Radio Conversion (Perversion) Manual Volume 3, which "supposedly" eliminated the negative-floating requirement by adding a cathode bias resistor to the audio output stage.  You might check to see if one has been added; I think the description called for something between 250 and 390 ohms but I don't recall exactly.  If it's there, it's not original.  The cathode should be grounded.
This mod did not address the negative bias for the front end (mixer), which comes from the same source as the bias for the audio stage.
As the article on Henry Rogers' radioblvd states, the BC-348 has suffered from a lot of put-down by hams over the years.  99.99999% of the bad-mouth is totally untrue and unwarranted.
The biggest issue with almost all the BC-348s (and a few 12-volt BC-224 versions) I've encountered is:
MISALIGNMENT OF THE CRYSTAL FILTER!
The procedure in the BC-348 manual is extremely confusing.  Disregard it.  Use a signal generator to find the exact frequency of the filter crystal.  Then align all the IF transformers EXACTLY to that frequency.  The improvement in performance is dramatic.
The Army Signal Corps manual calls for finding the crystal frequency, then "detuning" the generator by 10 KHz, and aligning the IF to the "detuned" frequency.  This does not make sense; if you do this, you'll lose 50% of the sensitivity of the receiver when the filter is switched in, and the selectivity is poor as well.  If someone knows "Why" this procedure was specified, I wish they'd explain it.  Apparently the military techs followed it "because it was by the book" and the result was poorly performing receivers; as I said, nearly every one I ever saw that came "straight from the military" was misaligned like this.
There is one US Navy manual (actually has an AN or Army-Navy number) which DOES have the correct crystal filter-IF alignment procedure spelled out in a special note.  I forget the number but it's cited on Henry's web site, also is (I think this is correct) referred to on "James A. Moorer's BC-348 Page," another outstanding web site. 
And if you want a truly excellent manual for your receiver, you can download it there, but the files are HUGE because of the super-hi-res scans.  Jim offers CD versions of the manuals for sale as well as printed, comb bound copies.  I highly recommend them.  I bought a BC-348-B manual from Jim, and it came with a CD containing the B manual plus ALL the other manuals; this for $15 USD at the time (about 2007).
For a power transformer, look for something that will yield 220 volts DC from a full-wave center-tap circuit.  About 480 volts AC across the full secondary would be ideal.  If your transformer has higher voltage on the secondary, use a choke-input filter circuit; you don't want to put more than 220 volts DC into the radio.  A little more HV won't "kill" it, necessarily; but there's no point in generating the extra heat resulting from too much current draw with higher voltage.
My "receiver roster" includes the B-model (1939 contract, 1.5-18 MHz in six bands, no low freq band) running on AC power, a Q-model with dynamotor power, and an R-model with AC.  Of these, the Q is my favorite.  Plus, the J-N-Q models are considerably easier to work on; and many of these models were overhauled by the military or a contractor well into the 1950s, incorporating higher-quality capacitors that often do not need replacing today.  I have also encountered not as many "drifted-in-value" resistors in the J-N-Q series.
73
Mike
WA4DLF
      From: Ian Wilson <ianmwilson73 at gmail.com>
 To: Mike Everette <radiocompass at yahoo.com> 
Cc: "kgordon2006 at frontier.com" <kgordon2006 at frontier.com>; ARC-5 List <arc5 at mailman.qth.net> 
 Sent: Tuesday, September 29, 2015 8:50 PM
 Subject: Re: [ARC5] [Milsurplus] BC-348 PSU questions

Mike - thank you for the link: hadn't seen that one before. Tons of good stuff there!
I rebuilt the power supply, replacing diodes, capacitors, etc. Kept the transformer andchoke. Then I dismantled it and measured the power transformer HT windings. Clearlysomething is wrong. Am going to test using a PSU I have used before (with floatingB-) and then look in stash of iron for a suitable transformer.
73, ian K3IMW

On Tue, Sep 29, 2015 at 4:26 PM, Mike Everette <radiocompass at yahoo.com> wrote:

Ian et al,
A really good source for info on the BC-348, including a proper AC power supply, is the following web site:
www.radioblvd.com
There is a very comprehensive article here on rebuilding/restoring the BC-348 (and many other types of receivers).
A lot -- probably the vast majority of -- AC supplies "hamstered" into BC-348s back in the day were not well designed and were poorly constructed. (I speak from direct experience; I've worked on a bunch of BC-348s-- many had some crazy hack-jobs done on them before I saw them -- for hams and Civil Air Patrol members over the years.)  It sounds as if your problem could indeed be a rectifier diode (if silicon diodes are used), or just a bad filter cap.  Maybe the filters -- one or both -- are grounded to the chassis instead of to the power transformer center tap. 
NOTE WELL: For an AC supply in this receiver, the negative side of the supply must be left floating.  It goes to ground through a bias filter network which is part of the audio output transformer (I hope yours still has the original transformer...?).  Study the power supply schematic on the web site, read the info, and most if not all your questions will be answered.
The bias produced in the receiver is for the audio output stage, and for the mixer.
NOTE: The negative lead for the B plus does NOT come out to the connector on the back of the radio.  The connection is made to the terminal strip on the dynamotor unit, inside the receiver.  In the original dynamotor supply, the negative side of the dynamotor output was left floating and brought out to one of the terminals.
Good luck with it.  The BC-348 is a great receiver.  You'll love it!
73
Mike
WA4DLF
      From: Kenneth G. Gordon <kgordon2006 at frontier.com>
 To: Ian Wilson <ianmwilson73 at gmail.com> 
Cc: ARC-5 List <arc5 at mailman.qth.net> 
 Sent: Monday, September 28, 2015 1:50 AM
 Subject: Re: [ARC5] [Milsurplus] BC-348 PSU questions

On 27 Sep 2015 at 20:07, Ian Wilson wrote:

> The center-tap of the transformer and the -ve side of the caps all go
> to one point. This point connects to the B- connection (not the chassis) of the
> receiver.
> 
> There is a hand-written note that says 'loud hum' so perhaps there is more
> to what's going on here than just the PSU wiring. Will look tomorrow in more
> detail.
> 
> 73, ian K3IMW

Didn't you say it uses solid-state rectifiers, i.e., diodes?

If so, undoubtedly one is bad.

Ken W7EKB
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