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Old 13th Nov 2021, 4:21 am   #1
Join Date: May 2009
Location: Newcastle, Australia
Posts: 310
Default 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.
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