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

[Jim Whartenby]

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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