[Milsurplus] BC-474 Tips

[David Stinson]

(Copied to the list for general information)
BC-474 tips.

A gentleman wrote:
>  I just got one, in nice condition, ...
> Have you used yours on the air?
Yes, indeed.  I've check into the local 75 meter AM net 
with it and confirmed 10 states on CW over a couple of
weekends last winter. 

The BC-474 is a bit challenging.
Here's some tips.

Be sure your 90 volt receiver negative lead is *not* grounded.
The receiver uses a back-bias resistor in the B- lead before
it goes to ground.  If you ground the receiver B-, the 
audio output transformer primary will open almost instantly.
The majority of these that you find 
have open output transformers for this very reason.
The B- of the transmitter high voltage must also 
float above ground for a similar reason.

If you have a tuner, you can use the rig on 40 meter CW.  
You double in the final and let the tuner take care of any harmonics. 
Doubles the amount of chirp which is inevitable in an MOPA transmitter,
but it's not bad and never gave me a problem.

The local oscillator pulls when you turn up the volume 
due to a badly designed AVC circuit.  
I'm working on a way to fix this without hacking the radio.
Looks like moving one resistor connection might do it,
but don't know yet.

The CAL position- used to net the RX with the TX- tends 
to overload the receiver.  So you have to turn the volume down
to zero beat.  Then, when you turn it up, it pulls the 
receiver back off freq...ack!  It takes some getting used to.

When use CAL to net the receiver, do it with the transmitting 
key closed.  Otherwise, you're netting about a KC off freq.

On the back of the transmitter deck, there is a can electrolytic.
This is connected after the B+ section of the T/R switch.
When you switch from receive to transmit, charging this cap
causes a spark and pitting at the switch contact.
The cap is not needed (it's for smoothing out the hand cranked
genny B+).  Disconnect it and tuck the wire out of the way.

Unlike many mil radios, this one will tune a 50 ohm load.

I have seen three of the RCA microphones that fit this radio.
None of them have worked, no matter how hard you thump them.
I use a mike with a telephone element in it and it works fine.

Good luck with it,
73 Dave S.