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

Thu Nov 8 17:03:44 EST 2007

Hi Jim-
One of the very nice things about the Q model BC-348
is no black beauties! All the caps are metal cased and
well sealed, as opposed to black beauties and paper
caps... Al Klase told me not to worry about thpse, but
to definitely replace the Mica caps...

How would I connect in the filter? It has two leads-
where do they go? Also- how would I go about measuring
Henrys? Seriously- I have no idea how to gauge the one
I have...

Scott

--- Jim Whartenby <antqradio at sbcglobal.net> wrote:

> Visit our web site - See http://www.njarc.org
> _______________________________________________
> 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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