Re: GEC BRT400D Communications Receiver
Hello Ian,
I restored a BRT400K a few years ago.
According to my copy of "A guide to Amateur Radio" by Pat Hawker, the BRT400 was in production from 1947 to 1967. My 1961 Radio Communication handbook has an advert for the BRT400K.
The BRT400K had mostly miniature valves such as 6BA6, 6BE6, EL84 as opposed to the mostly B8B based valves in the earlier versions. I think the older valves were out of production by the time the K came along.
These are good sets. The detector has particularly low distortion, making it very good for listening to AM.
The power supply uses small value oil filled paper capacitors rather than the usual electrolytics for smoothing. To reduce the HT ripple it has a smoothing valve V13 (EL84 on a BRT400K) which sits across the HT rail and is fed with a sample of the HT ripple. The valve gain is adjusted to cancel out the hum. I have not seen this done on any other radio.
My example had a lot of electrically leaky metal cased capacitors. Ones to watch out for are C112 which is in the grid circuit of the smoothing valve. If this leaks the valve draws an enormous HT current. On my example it had melted the wiring harness under the chassis, along with much other damage.
Most sets have a capacitor between the 1st AF stage and the output valve (often referred to as THAT capacitor). If it leaks then this valve also draws a huge current and damages other expensive components.
On the BRT400 series there are three capacitors C103 , C109 and C110 which can cause this sort of failure. The leakage from all three adds together. The output valve grid resistor is 4.7M ohm just to make things worse!
I would strongly recommend changing all these capacitors for piece of mind. At least measure the signal grid voltages of the two power valves and check that they are close to 0V.
Good luck with your restoration.
Andrew
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