[grc] Fwd: An engineering question

[al davis]

On Sat, 12 Jun 2021 16:47:41 -0700
Leigh Robartes via grc <grc at maillist.peak.org> wrote:
> I am having problems getting the remote start on our new(ish) CD players 
> to work. The strange thing is that it will work if the player is not 
> installed in the rack and/or grounded through the unbalanced audio. The 


There seems to be no standard for remote starts.  You really need to
study both ends, and perhaps build an interface circuit to map one way
to the other.

To make matters worse, these things usually poorly documented, or not
documented at all.  Perhaps the maker of either the console or CD
player will help you with this.

Digging in, from the manual of the CD player, it seems that a momentary
contact toggles it on and off.  .. momentary short to pause, another
momentary short to start.  That's asking for trouble.  It's "floating",
whatever that means. That means neither side is grounded, as if they
just brought out the contacts of the pause button.  That too is asking
for trouble.   You really need a schematic.

What "floating" really means here is that you really need a relay
contact.  Nothing else will do.  There is probably some way to wire it
that will explicitly start or stop, but it isn't clear from the
documentation.  Again, you really need a schematic.

That's the CD player ..

Now look at the console.  Here again, it is not adequately documented,
but you have a schematic.  It appears that they use a Panasonic AQW210S
chip as the interface.  The manual shows a D connector for the starts,
with one pin for each and a bunch of pins marked "start common".  This
is NOT floating!!!!!!!!  From the schematic, it looks like that common
is really chassis ground.  The chip says it is a "solid state relay",
and is rather expensive at about $5 each in production quantity.  It
seems odd to me to use such an expensive and hard to get part in this
application, where a 10 cent transistor would do just as well.
Electrically it looks like a MOSFET.  So I think the console's circuit
is that the MOSFET is on when the channel is on, off when the channel is
off. "On" is a MOSFET channel with about 30 ohms resistance.

Other consoles .. Kaatskit uses a BJT for this, similarly connected.
the device is on when the channel is on, it's on pin 5 of the RJ45,
which simplifies the wiring, and is cheap.  Arrakis, when they provide
it, seems to use a pair of relays, in a "form-C", 3 pins for 2 relays,
on the RJ45 pins that are supposed to be used for power.  My point here
is to support the statement "they are all different".

So at least the console isn't really floating.  The change in
behavior with grounding is what I would expect.  It will become even
more strange with 2 CD players, as you unknowingly connect some
internal point in one to the other.  You need to build an interface.

The schematic you attached doesn't tell me anything.  It's probably
wrong anyway.  The cap across a bead looks wrong.  The network of small
capacitors and ferrite beads is probably an RF suppression circuit
and/or a de-bounce circuit.


You need to build an interface consisting of a relay and driver, to map
one kind of control to the other.