[grc] Fwd: An engineering question

[Tim Požar]

You may have to avoid grounding one side and just using dry contacts for 
switching.

Tim

On 6/14/21 4:45 PM, al davis via grc wrote:
> 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.
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