[NJARC] BC-348Q Power Questions(2 of them)
Carole White-Connor
carolew at bellatlantic.net
Thu Nov 8 18:02:01 EST 2007
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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