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

[Scott Roberts]

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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