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-   -   BC-348-Q. Receiver. (https://www.vintage-radio.net/forum/showthread.php?t=185838)

QQVO6/40 13th Nov 2021 3:21 am

BC-348-Q. Receiver.
 
G’day everyone.

BC-348-Q. Receiver.

1st RF amplifier. 6SK7.

All of the circuits I have including the workshop manuals have the plate and screen electrodes joined together and fed from the B+ rail via a 15k Ohm resistor. The radio runs on a nominal 250V B+ supply.

In this radio the plate and screen have been separated. The plate is still powered from the B+ supply by a 15k resistor. The screen is now powered from the B+ supply via a 680k resistor and is bypassed to deck by a styroseal cap value unknown at this stage. Ducon Styroseal caps are generally quite good if a little temperature fragile. The 680k resistor now reads 840k Ohms so it will be replaced.
Valve data for the 6SK7 says nominal 250V plate, 100V screen at 2.4mA.
680k Ohms seems a little high do you think? I was thinking something in the range of 30k to 100k Ohms.
Am I wrong? I have a good VTVM and a good decade box so I can play later to decide when I do eventually power it up.

This radio has been worked on in 1956 (small metal tag attached to the front panel) by I think the Royal Australian Air Force (RAAF). The RAAF receivers and transmitters I have seen so far have been overhauled in RAAF workshops.
Receivers and transmitters owned by civilian airlines were overhauled by Amalgamated Wireless Australia (AWA). Those done by AWA have a small red coloured stamp on the chassis.

This radio has a mains power supply fitted and it looks to be professional in it’s construction so I guess it was done as one of the mods from the RAAF.

I have not powered up the radio at this stage and will not until I have gone right through it. The B+ line reads 500k Ohms to deck using a 250V Megger but of course this does not test coupling caps which can still cause issues.

Question. What are your thoughts on the 1st RF screen wiring changes as it does deviate from the original design of the radio. My usual policy is to return a radio to the original design and remove mods unless It is a quite rational mod that has been done responsibly.

Thanks in advance.
Cheers, Robert.

orbanp1 14th Nov 2021 3:13 pm

Re: BC-348-Q. Receiver.
 
Hi Robert,

The 6SK7 is a remote-cutoff pentode, controlling the screen voltage does control the transconductane of the tube. Look up the data sheet of the tube.
Screen control is generally associated with AGC.

May be they wanted to change the gain of that RF-amplifier stage?
Using a pot there, not as a variable resistor but as a voltage divider, with other resistors, one can implement manual RF-gain adjustment.

I would restore the radio to factory wiring, and then see if it performs as expected.

Your calculations for a separate screen resistor seems to be in the ballpark, in case you want to go that route.

Regards, Peter

Radio Wrangler 14th Nov 2021 3:35 pm

Re: BC-348-Q. Receiver.
 
Odd thing to do.

The suppressor will shield the control grid against capacitive coupling from the anode, but strapping the screen grid to the anode will put some of it back.

The schematic is not too good, the one I found made it look like the second stage had g2 and a shorted, despite a resistor from g2 to HT and a decoupling capacitor on g2.

David

turretslug 14th Nov 2021 5:45 pm

Re: BC-348-Q. Receiver.
 
Could someone doing the overhaul have meant to fit a 68k, but picked up a 680k instead, maybe one with an orangey yellow band as it were? 68k would seem to be more in the ball-park for this application, but once fitted, the radio may have registered "within spec" with the 680k. A reflection of the way valves will shuffle their operating point a bit and carry on even with resistors a long way off spec. perhaps, and of the gain available with two RF stages.

Triode-connection does seem curious, perhaps it is a poorly-drawn or reproduced diagram. The Scott RCH has a 6K7 RF stage with anode and suppressor grid (yes, suppressor) connected together and screen grid used conventionally, maybe there was an advantage in forcing a pentode to be a tetrode, though it seems retrograde!

QQVO6/40 16th Nov 2021 12:47 pm

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.

Peter.
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.

David.
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!

Colin.
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!
Cheers,
Robert.

QQVO6/40 17th Nov 2021 6:22 am

Re: BC-348-Q. Receiver.
 
G'day everyone.

Interesting that on an American forum at the moment is a topic about converting BC-348 receivers to solid state with an online site mentioned with a video on how to do it.
Nah. Not going there even with my radio not good enough to be a restoration project.

All good.
Cheers.
Robert.

FrankB 19th Nov 2021 7:24 am

Re: BC-348-Q. Receiver.
 
I have a BC348Q in my collection in storage. It came from my Elmer William H. Ward W7SS CWO USN SK. (Pearl Harbor Survivor, Purple Heart)
I have read a number of articles on the BC348 series. There are numerous mods out there to "Improve something" in the set.
IIRC The Surplus Schematics Handbook had all the details on the set. The OEM Military manual should be downloadable from the U.S. DOD website.
https://www.radionerds.com/index.php/BC-348
https://www.radionerds.com/images/a/a2/AN-08-10-112.pdf
This latter is the complete maintaince manual. Good pix, drawings and schematics

Now I do remember a couple of things about that set.
Most important was the built in power supply had NO FUSE. Mine has that compartment a charred mess. Stuffed into it was a note to fuse the P.S. or this would happen again.

The other thing I solidly remember was the "canned caps" in the round metal housing with the mica tops were failing- a lot. The paper caps in them had the paper used as a dialectric breaking down. This was from various U.S. ham radio magazines in articles on the BC348 family of sets. (Remember- this was war time production. It was good enough for then to get by). HTH

Keith 1st Dec 2021 12:03 pm

Re: BC-348-Q. Receiver.
 
Hi all,

I have a BC348 which had a missing ID plate when purchased. I know it's not a J, N or Q variant because the CW OSC switch is the toggle type in line with the rotary CRYSTAL IN OUT switch. Does anyone know of a list of variations between the E, M, O, P, S, H, K, L and R types such that mine could be identified and a replica plate made?

usradcoll1 1st Dec 2021 11:53 pm

Re: BC-348-Q. Receiver.
 
Quote:

Originally Posted by Keith (Post 1429000)
Hi all,

I have a BC348 which had a missing ID plate when purchased. I know it's not a J, N or Q variant because the CW OSC switch is the toggle type in line with the rotary CRYSTAL IN OUT switch. Does anyone know of a list of variations between the E, M, O, P, S, H, K, L and R types such that mine could be identified and a replica plate made?

The BC348J,N and Q are the newest models, built by Wells-Gardner.
All the earlier models BC348 E,M,O,P,S,H,K,L,R are built by various firms, RCA, Stromberg Carlson and Belmont. The circuitry varied as well.
They also used some grid-cap octal and UX base valves.
Dave, USradcoll1 I have a few examples of this set. :beer:

usradcoll1 2nd Dec 2021 4:25 pm

Re: BC-348-Q. Receiver.
 
Quote:

Originally Posted by QQVO6/40 (Post 1424997)
G'day everyone.

Interesting that on an American forum at the moment is a topic about converting BC-348 receivers to solid state with an online site mentioned with a video on how to do it.
Nah. Not going there even with my radio not good enough to be a restoration project.

All good.
Cheers.
Robert.

I didn't even bother to look it up. It seems like a lot of foolishness to me, as being a long time collector.
Dave, USradcoll1 :soapbox:

Keith 7th Dec 2021 5:24 pm

Re: BC-348-Q. Receiver.
 
Thanks Dave,
I was hoping there might have been some documentation giving the differences between the various non-Wells-Gardener versions or possibly some distinguishing marks or features. Looks like such information may be lost in the mists of time.


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