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

[gary.biasini at shaw.ca]

Charles,

Again, thanks.

I think I have the diode backwards.  It looks like a band on the diode
pointing to pin 1 of the relay.  

The good news is, good design or bad, I have no use for the break-in relay,
so I think I will leave in place and move on to the rest of the radio.  You
may have noted that I have replaced at least 2 of the paper caps that are
visible in the picture and the rest of them and the tantalum cap have also
been replaced so, for now at least, I am done with the audio module on this
receiver.

Best 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: September 5, 2021 8:03 PM
To: r-390 at mailman.qth.net
Subject: Re: [R-390] Audio Sub-chassis Break-in relay mod

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


______________________________________________________________
R-390 mailing list
Home: http://mailman.qth.net/mailman/listinfo/r-390
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:R-390 at mailman.qth.net

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html