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Old 16th Nov 2021, 12:47 pm   #5
Join Date: May 2009
Location: Newcastle, Australia
Posts: 265
Default Re: BC-348-Q. Receiver.

G’day everyone.

First up I would like to thank the people who have responded to my Forum thread.
I am by no means a competent typist so I do understand the effort that you people have made to offer advice on my BC-348-Q.
Thank you!

Second. I apologize for being a little slow in replying to your posts. I have had 5 hospital stays this year and at my age it is not easy to bounce back as I would have as a young fella.

This radio supplies AGC to the control grid (g1) on the 2nd RF stage as well as the IF stages. This radio is not a restoration candidate as it has been modified too much. There is the usual meter put into the front panel (which is not connected) along with a power on light on the front panel (also not connected) and a toggle switch on the front panel for mains switching. There is a mains power supply fitted with associated wiring changes not just dropped in and the output transformer and choke is gone with a conventional speaker transformer fitted.
This radio is not a restoration project. It is a get going and use radio.
I do have an “R” model in almost complete condition that is a good candidate for restoration as well as 4 other chassis in various states of dishevelment.
I have quite a few receivers and I looked up quite a few of the manuals and the screen resistor value between 30 and 100k Ohms is the usual.
I have also looked at the manuals for the other iterations of the BC-348 and none of the other models have the plate and screen connected together. All of the others are more or less conventional.
Most of these radios use screen voltage switching by the associated transmitter to shut them up during transmit. There are 2 terminals on the original connector on the rear of the radio for this function.

Yes it is odd to have it done this way. The valve is not really set up as a triode either is it?
I do have a good manual for this radio and it does show on a clear circuit diagram the plate and screen connected together and both fed from the B+ via a 15k Ohm resistor.
None of the other BC-348 radios are done this way.
“Radio Boulevard” states that there were over 100,000 of the BC-348 radios manufactured for the war effort.
My circuit diagram shows the 2nd RF amp to be set up conventionally.
One interesting thing I have noticed. The coupling cap from the plate of RF amp 1 to the switched coils of the grid circuit of RF amp 2 is 0.01 micro Farad. To me that seems a little large. I would have expected to see maybe 0.001Mfd.
Another difference with this “Q” model; all of the other models the plate of the 1st RF runs through the tuned coils of the input to the 2nd RF stage. This radio has a coupling cap to isolate the B+ from the coils.
One thing I have read somewhere was that these radios were provided with 2 RF stages to prevent re-radiation of the local oscillator signal. This would otherwise have been a problem as the NME would have been able to DF the aircraft using these radios. Word has it that a lot of Allied shipping was lost because of the re-radiation from the local oscillators of National HRO receivers.
I have wondered if the 1st RF amp setup in this radio was a result of research to limit the exit of a local osc. Signal.
I work on the presumption that the people who designed and built these radios were not dummies!

You might have just hit the resistor on the head.
Good old human error!
Reached for a 680k instead of a 68k resistor. Gunna be ****** deaf innit?
68k resistor is well inside the desired range.
While I have felt well enough to do it I have had a bit of a poke around inside it and some of the work has been quite shoddy. Soldered joints terrible and other bodges! Example; the plate and B+ of the audio output transformer were touching because of bare wires from poorly soldered joints.
One thing about this radio is it has a beautiful steel desk top case with louvers on both sides and good quality mesh on the top. Very professional.

Enough of my chatter for now.
Thanks again my fellow enthusiasts!
QQVO6/40 is offline