[Milsurplus] [ARC5] BC-348 PSU questions

[Ray Fantini]

I have done a couple BC-348Q series in my time and did not use the original output transformer mostly because when I come across the sets they have already been hacked up and often have the transformer removed. I just use the original resistor that's in series with the B-  and have never had any issues with hum or AC noise. Typical filters will use a small choke and capacitor input filter on the output side of the rectifier but no choke in the B- I think the biggest problem today is finding appropriate size power transformers and filter chokes being that at one time the stuff was around fairly common but today is harder to come by.  Have started looking for junk vacuum test equipment and useless radios like the AN/URR-35 to use as donor sets to harvest power transformers and other useful parts for vacuum tube projects.

Ray F/KA3EKH

-----Original Message-----
From: milsurplus-bounces at mailman.qth.net [mailto:milsurplus-bounces at mailman.qth.net] On Behalf Of WA5CAB at cs.com
Sent: Wednesday, September 30, 2015 1:32 AM
To: milsurplus at mailman.qth.net
Subject: Re: [Milsurplus] [ARC5] BC-348 PSU questions

Two comments - the metal box that most hams always call the "output transformer" is properly called the "audio pack".  In the BC-348-JNQ, it contains the audio output transformer and the B- filter choke.  I don't know what the DC resistance of the choke in this pack is as I don't have any left and didn't think to measure it when I did.  However, it may not be the same as the one in all of the other models.  The choke in the JNQ pack does not go to chassis.  It goes to two 50 ohm resistors in parallel, both of which go to chassis.  The bias for the 6K6 audio output tube is taken across the choke and resistors.  Connected across the two resistors alone is one of the grids in the 6SA7 1st DET/HF OSC.

In all other models of the BC-348 and in all models of the BC-224 the Audio Pack also contains a .05 ufd 400 V capacitor.  The other end of the choke goes to chassis, and the grid bias for the VT-152 Audio Output tube is taken across the choke alone.  The mixer does not have fixed bias.  The resistance of the choke is 300 ohms.  And the capacitor is switched across the AGC line when the BFO is turned on.

Unfortunately, the choke in all other models except JNQ goes to chassis internally (inside the audio pack).  So you can't use  these audio packs in a JNQ.  I have never found time to open up one to see whether or not the choke could be easily un-grounded so that you could use the pack in a JNQ.  I have many of the audio packs for the HKLR and the EMOPS but none left for the JNQ.

Robert Downs - Houston
wa5cab dot com (Web Store)
MVPA 9480

In a message dated 09/29/2015 20:24:38 PM Central Daylight Time, k1lky68 at gmail.com writes: 
> >>
> >>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...?). 
> 
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