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

[gary.biasini at shaw.ca]

Charles,

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.  

https://1drv.ms/u/s!AhXaYQRsUlJ0gvAlzPkC5fr0UP5u-g?e=Kocv1x

https://1drv.ms/u/s!AhXaYQRsUlJ0gvAk82FuOezJdQdwEA?e=d98UMB

https://1drv.ms/b/s!AhXaYQRsUlJ0gvAjY-YgPeZfXgHX_w?e=kzhzoV

I hope that you can better understand the circuit with these pictures.

Regards,
Gary

-----Original Message-----
From: r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> On
Behalf Of Charles Steinmetz
Sent: July 11, 2021 8:19 PM
To: Gary Biasini <gary.biasini at shaw.ca>
Cc: Collins R-390 HF Receiver List <r-390 at mailman.qth.net>
Subject: Re: [R-390] Audio Sub-chassis Break-in relay mod

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


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