[NJARC] BC-348Q Power Questions(2 of them)

Thu Nov 8 07:52:15 EST 2007

Scott
Expect to see a B+ of approximately 350vdc (250 X 1.414) if you are
just using electrolytic capacitors as the filter circuit. A choke input
(8H to 12H as a guess but check the RCA Radiotron's Designer Handbook,
4th edition) filter will reduce this voltage closer to the 250vdc you
want.

It would be a good idea to replace all of the black beauty capacitors
in the set.  Some are hidden in IF cans and in the RF and oscillator
compartments if I remember correctly.  Even if you checked these caps
for leakage, I wouldn't trust them.  They will eventually fail.
Jim

--- Scott Roberts <ng19delta at yahoo.com> wrote:

> Visit our web site - See http://www.njarc.org
> _______________________________________________
> Hi Jim-
> Thanks for the reply- it will help quite a bit. I am
> actually using two transformers- one is a 500 VAC with
> 250VAC center tap, which will have the two 500 leads
> rectified through a pair of 1N4007 diodes, and a pair
> of 47mf/450 caps, to produce 250 VDC to B+, with the
> center tap going to B-. The other is a 24VAC which
> will be providing the heater current.  I have a filter
> in place, but not in the circuit yet- I've been told
> it may not be necessary to use it.
> 
> As to the output tube bias, all the mods that had been
> done were removed- I took the radio back to mostly
> original wiring: the only thing I did not put back in
> was the wire from the input wiring block to the fuse,
> which was for 24VDC from the aircraft to supply the
> dynamotor.
> 
> Scott
> 
> 
> --- Jim Whartenby <antqradio at sbcglobal.net> wrote:
> 
> > Visit our web site - See http://www.njarc.org
> > _______________________________________________
> > Greetings Scott
> > You didn't say how you plan to power the set.  Are
> > you are going to use
> > the dynamotor or are you going to use a transformer
> > / rectifier and
> > filter to supply the 250 odd volts of the plate
> > supply?
> > 
> > Assuming that you are using the latter, how you are
> > producing the 24vac
> > to power the tube filaments?  Perhaps you got lucky
> > and found a
> > transformer with both a 24vac and 175vac or so high
> > voltage winding?
> > 
> > In any event, to answer question one: I think that
> > the dynamotor is in
> > parallel with the tube filaments.  It would be
> > wasteful to replace the
> > dynamotor load with a resistor.  All that will do is
> > add heat to the
> > receiver and abuse the 24vac transformer winding.
> > 
> > Question two:  Both the 24vac and 250vdc are
> > isolated from both the
> > neutral and hot side of the AC line by the power
> > transformer.  You are
> > therefore free to set any point you wish at ground
> > potential using the
> > green ground wire.  So go ahead and ground the
> > chassis.
> > 
> > One word of caution, well several words.  Check out
> > how the audio
> > output tube is biased.  If the previous owner added
> > a cathode resistor
> > to derive the negative bias on the control grid, you
> > can connect the
> > negative 250 volt return wire directly to chassis
> > ground.  If the set
> > is not so modified i.e. it is still stock, then the
> > negative 250 volt
> > return wire goes to a power resistor before it goes
> > to chassis ground.
> > 
> > If either of these two options are not follower, the
> > output tube will
> > have excessive plate current and will most likely
> > take out the audio
> > output transformer.  The only up side to a bad
> > output transformer is
> > that you get to eliminate that pesky 600 ohm to 8
> > ohm line transformer
> > since your chance of finding a NOS replacement audio
> > output transformer
> > is pretty slim for a 70 year old receiver.
> > Regards,
> > Jim
> > 
> > 
> > --- Scott Roberts <ng19delta at yahoo.com> wrote:
> > 
> > > Visit our web site - See http://www.njarc.org
> > > _______________________________________________
> > > Well, this should be easy...
> > > 
> > > I'm still repowering the BC-348Q, and in doing so,
> > > configuring it back, for the most part, to
> > original. I
> > > have reconnected all the tube heaters as they
> > were,
> > > and am finishing off the 24V side of the house. I
> > am
> > > coming up with a question about power reduction...
> > > 
> > > When the aircraft power (24VDC) comes into the
> > set, it
> > > goes through a fuse, and then to the AVC-Off-MVC
> > > switch, which also controls power to the set, by
> > > switching on and off the power to the Dynamotor.
> > Also
> > > connected to the switch are the dial lamps: 2 No.
> > 44
> > > bulbs(6.3V, .25A), connected in parallel to the
> > > dynamotor, and in series with each other and a 200
> > ohm
> > > rheostat and 60 ohm resistor to ground.
> > > 
> > > My first question is, Since the dynamotor is
> > removed,
> > > do I need to dissipate part of the power which
> > > operated it through a resistor to ground, in its
> > > place?  My thinking is, I have 24VAC going through
> > 2 x
> > > 6.3V (total 12.6 V) #44 bulbs, and a rheostat and
> > > resistor, and I don't want to cook any of them. I
> > was
> > > wondering if  the current not used by the removed
> > > dynamotor would be a problem, or not, and if
> > putting a
> > > resistor to ground in its place would alleviate
> > any
> > > such symptom.
> > > 
> > > My second question is related to line current.
> > When I
> > > received the set, it had a 110AC 3 wire plug in
> > place.
> > > I wish to reuse this same wire & plug. But my
> > wonder
> > > is this- I have the 24VAC grounding at points, and
> > the
> > > 250VDC I'm sure grounds over somewhere, so is it
> > ok to
> > > ground the chassis itself with the third wire? I'm
> > > pretty sure the other internal groundings are
> > fine,
> > > but I wondered about the ground wire on the plug-
> > I
> > > don't want a hot chassis...
> > > 
> > > 
> > > Thanks!
> > > Scott
> > > 
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