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

Scott Roberts ng19delta at yahoo.com
Thu Nov 8 18:05:52 EST 2007 

That's Them! ;) 

I am planning to change those out- they seemed to be
working for the moment, but I will be replacing them.

Scott


--- Carole White-Connor <carolew at bellatlantic.net>
wrote:

> Visit our web site - See http://www.njarc.org
> _______________________________________________
> Scott:
> 
>     Those aren't mica caps. They're Micramolds.
> They're notoriously
> unreliable and should be replaced regardless of how
> they test. In the
> BC-348, they're accessible so it's not a major
> problem replacing them.
> 
>                                 Joe Connor
> 
> Scott Roberts wrote:
> 
> > Visit our web site - See http://www.njarc.org
> > _______________________________________________
> > 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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