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

[Charles Steinmetz]

gary wrote:

> On a Teledyne Systems audio sub-chassis on an R-390A of mine there are some
> mods of unknown purpose.  I am attaching a schematic of the mods and 2
> pictures - I don't know what the Motorola part is (a transistor?) so it is
> shown as a circle on the schematic.  Essentially, the original connections
> to the coil of the break-in relay K601 (terminals 1 and 7) are removed and
> parts as noted on the attached schematic are added to each side.
>
> Does someone recognize this modification or can you reverse engineer the
> purpose?

Pictures didn't make it.

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.

Hope this helps,

Charles