[Milsurplus] [ARC5] BC-348 PSU questions

Fri Oct 2 13:43:01 EDT 2015

OK, good news - with a known good power supply (B- floating, etc),
the radio comes to life. My usual test for WWV 10Mc with a few feet
of wire as an antenna works. Very nice. Now I can start planning to
reverse some of the mods. I do need the AVC-MVC switch.

The transformer in the original "power supply" is still something of
a puzzle. I have disconnected everything from its leads. Apart from
two black AC input leads, we have:
  - Red1
  - Red2
  - Yellow (possibly red/yellow originally)
  - Green1
  - Green2

All the transformers I've seen like this before have a centre-tapped HT
winding
and a heater winding. This one is different. I measure 6.3VAC across G1/G2
with AC mains applied to Black/Black, as expected. But the DC resistances
are odd:

  - Red1 - Green1 or Green2: 62 ohm
  - Red2 - Yellow: 56 ohm
  - Red1 - Red2: no connection
  - Green1 - Green2: 0.2 ohm

The transformer might be Heathkit. It is marked 13496-N and 949215.

There is no audible hum, no sign of abrasion, no sign of temperature
damage. Conjectures, anyone?

73, ian K3IMW

On Wed, Sep 30, 2015 at 7:52 AM, <antqradio at sbcglobal.net> wrote:

> Curious, I haven't yet received WA5CAB's email that he sent at 1:32 AM!
>
> Anyway, the resistance of the choke in the audio pack without the 0.05 uF
> cap is 240 ohms and I can also verify the 300 ohms DCR of the other audio
> pack choke with the cap.
>
> Jim
>       From: Ray Fantini <RAFANTINI at salisbury.edu>
>  To: "WA5CAB at cs.com" <WA5CAB at cs.com>; "milsurplus at mailman.qth.net" <
> milsurplus at mailman.qth.net>
>  Sent: Wednesday, September 30, 2015 8:12 AM
>  Subject: Re: [Milsurplus] [ARC5] BC-348 PSU questions
>
> 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...?).
> >
> ______________________________________________________________
> Milsurplus mailing list
> Home: http://mailman.qth.net/mailman/listinfo/milsurplus
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:Milsurplus at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
> ______________________________________________________________
> Milsurplus mailing list
> Home: http://mailman.qth.net/mailman/listinfo/milsurplus
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:Milsurplus at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>
>
>
> ______________________________________________________________
> Milsurplus mailing list
> Home: http://mailman.qth.net/mailman/listinfo/milsurplus
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:Milsurplus at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>