[R-390] Audio Sub-chassis Break-in relay mod

[Charles Steinmetz]

gary wrote:

> Thank you for your insightful response and my apologies for the delay in
> responding.  I am attaching links to the photos as they appear to be too
> large to be attached directly to the message.

Gary,

The pictures don't do anything for my understanding.  I stand by what I 
said in my original reply:

> That is just a gain stage to allow the relay to be driven by a low-voltage,
> low-current source (perhaps even a logic gate).  The input
> (150 ohm resistor) feeds the transistor base, the emitter is grounded, and
> the collector goes to the relay.  There should be some protection (at the
> very least, a clamp diode across the relay coil) to prevent the inductive
> flyback from the coil from destroying the transistor when the relay is
> de-energized.
>
> As you have it drawn, either the diode or the capacitors are polarized the
> wrong way.  The diode would be fighting to develop a negative voltage on the
> positive terminals of the capacitors.  Which of these (the diode or the
> caps) is drawn correctly will determine whether the transistor is an NPN or
> PNP, and also whether the control voltage needs to be positive or negative
> with respect to ground.

I see that the transistor is a TO-3 (relatively high-power) part.  It 
should be a Darlington so it has both high gain (only requiring low 
input current) and high-ish output current capability.

To be useful, it really needs several kinds of protection added so it 
doesn't blow itself up under use conditions.  And, of course, the diode 
or capacitor polarities need to be fixed.  (If it really is drawn to 
match what's in the radio, it has no hope of working.)

If you're getting the impression that I do not think it is well 
designed, you may be on to something....

Best regards,

Charles