Babcock Project (Help!)

[Stephen D]

I am restoring a re-cased Babcock BCT-2 transmitter (Tube), a BCT-3 receiver (tube), and a magic carpet receiver (transistorized).

The transmitter is finished and produces a nice lookin carrier on 27.255 mhz and a tone when the button is operated.

I then decided to see if the transistorized receiver would work. Hooked it up per the instructions and it worked fine….for a short time. I then probed the detector transistor (got a signal)…and each amplifier stage (got a larger signal). Then I got to the “transistor/relay” control stage which across the relay I still get the carrier signal (with tone if the button is pushed). There is a diode it this stage that I believe converts the tone to dc for the relay. (But I am not sure how the last stage works!)

I would be very appreciative if someone with experience could explain how these receivers drive a relay off of a tone? Thanks in advance for your time!

[Phil_G]

Hi Stephen, welcome aboard

In a static state T4 is baised on the boundary of conduction by R7 & R9, its input is capacitively coupled so when the audio tone swings negative it will switch on and operate the relay. Positive swings at its base will hold it off and C11 (although smaller value than expected) will smooth out the audio half-waves into something more like DC. With a crystal earpiece (high impedance) you should hear a loud hiss (white noise) at T4 base when the transmitter is off, silence when the transmitter is on, and a strong tone when the tone button is keyed. The diagram shows no electrolytics, these are the first to go, I would check these are indeed absent, if not change them. Otherwise could be low-gain trannies, broken resistors, etc. Check the values of R7 and R9, I would have expected either a 1k R9 or a 680k R7
Jay might know better, and may have service documents
Cheers
Phil

[Stephen D]

Phill,
Thank you SO much for the explanation! I tried to upload pics/schematic, but am a bit daft when it comes to this! Thank you for your time for this explanation! I think with this, I should be able to figure out the issue! It is a real joy for me to figure out how these work!

Thanks again,
Steve

[Stephen D]

Here are a few pictures of the Babcock tube transmitter/receiver I have been working on.

The transmitter is a Bct-2 transmitter re-cased to sit on the ground (it came that way). I cleaned everything, built a power supply for the “B” battery (12V inverter, bridge, a few smoothing capacitors, and a voltage divider), checked voltages, checked tubes and it produces a great looking carrier wave with tone.

Right now the receiver is on life support until the batteries get here. It has had the same treatment as the transmitter but is going to get a batch of 9V lipos for the “b” so hopefully it can fit in a model. The receiver works really well. I have not tested it for range yet (gotta get the batteries to make it portable).

I learned a lot from this:
1. People were very determined in the 50’s.
2. These take a lot of batteries…and a large model!
3. Some of the actuator designs were brilliant and probably could not be produced today.
4. My workbench is very messy, it was clean when I started!

Stephen, I have resized your pictures to a more suitable size for the forum i.e. below 1MiB, Cheers, Tobe

[Tobe]

A messy bench is sign of proactive action!