[ARC5] IIRC Stinson?s solution to sidetone issue

Mon Feb 11 10:44:12 EST 2019

 MY BC456 is set for 600 ohms to match the receiver impedance so that everything is theoretically consistent.However, as you know , the receiver sensitivity control has no effect on the modulator sidetones output. 
I use an LS-166 with the system, because the heaphones were unbearably loud in tx.After a bit of reserch , I came to the conclusion that the SCR274  system must have relied on the inter-phone system to balance the outputs of tx and rx. The ARC-5 systems did address the volume issue with jack boxes with volume controls as an option. 

To fix this, I did two things.1) used the the J22 jack box circuit as a secondary mater volume which plugs into the rx rack and the phone/speaker plug into the jack box .2) Cut the wire  from pin 1 of the 6 pin connector which sends audio to the rx rack and inserted a 680 ohm 2W resistor ( overkill , but didn't have a 1W) in the line. This attenuates the audio before it gets to the master volume control which helps a lot and eliminates mike feedback. 

On the same 6 pin  connector, pin 5 carries the "key" line. Cut this wire and insert a switch for "spotting" or "net".This disables the rack relays so you can hear the recievr output during tx. I use this to get on frequency with a contact by tuning the transmitter to zero beat the at the receiver frequency.  Flip the switch back for normal operation.
Another helpful thing to have is a frequency counter to get on a specific frequency before going on the air.I use an old portable frequency counter and a wire wrapped the antenna wire at the relay box as a  pickup.
These mods are  all non-destructive and make this equipment easier to use in "Ham mode".

    On Monday, February 11, 2019, 8:17:05 AM EST, Ronnie Hull <w5sum at comcast.net> wrote:  

 I have used Dave’s method in my SCR274N station and use that switch for “netting” and it works well!I do however, want to install a inter phone box. Who has one they will part with?
W5 Sum Ugly Mug
Ronnie
“A 20wpm Extra”

Sent from my iPhone
On Feb 11, 2019, at 7:06 AM, David Stinson <arc5 at ix.netcom.com> wrote:

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(Think I made the attachment too large.  Trying again)

Thanks to Brian Clarke and Mike Hanz for jogging my memory.

I reviewed the diagrams over coffee this morning to blow the dust

off the creeky stuff in my old and rapidly-emptying head.

The SPDT relay(s) in the receiver rack are picked by the PTT buss.  

The Common contact is connected to the Receive Audio busses

and their associated switching.  The Normally Closed contact

is connected to the Receiver’s pin 2, Audio Out.

The Normally Open contact is connected to the Sidetone buss

supplied to the receiver rack.

Thus, in “Receive” mode, the radio’s Audio Out feeds the rack

audio busses and the Sidetone buss is open-circuit.

In “Transmit” mode, the receiver Audio Output is open-circuit

and the Sidetone buss is connected to the Rack Audio buss

with its associated switching (and also out to the RX Control Head).

In order to provide a means to Net the TX-RX, a small toggle switch

on the display stand open-circuits these relays, allowing the receiver

BFO (if properly aligned) to be used for netting.

See photo attached.  The switch obviously mounted in a scrap 

adaptor box is for netting.  Please forgive the “frumpy” look;

in the middle of refurbishing some sets.

Left the Sidetone buss open-circuit in all modes.

Did experiment with Sidetone attenuation schemes, but all seemed

to affect the Modulation envelope to some degree.

I confess I did not try very hard; to do this “by the book” would 

require the addition of an Interphone box to the display.

Of course, I can’t speak for anyone else, but the point of my 

displays is to tell a story to hams in hopes of sparking interest.

Those hams, mesmerized by the latest shiny things, tinkling

bells and flashing lights (more expensive thingies to break

and give the makers a Repair revenue stream)

are only half-curious to start with.

Getting them to stop the “Hamfest Zombie March” long enough 

to talk about the sets is hard and getting harder.   Adding an interphone

box would make the display even more “busy,”  giving it an 

unbalanced look.  

And there is this:  In story-telling, there is an axiom: 

“If a character does not advance the story, 

 the character should be cut.”  

An interphone box would be of great interest to you and me,

but it would not “advance the story” when trying to ignite

a spark in a newbie.  An Interphone box adds a complicating

element that would need further explanation.  Trust me; 

getting them to stand still long enough to talk about the bare set

is hard enough.  Start adding elements and their eyes will glaze-over.

Remember the goal, and Montgomery Scott’s admonition:

“The more complicated they make the plumbing,

the easier it is to stop-up the drain.”

GL OM DE Dave AB5S

<ATAARAnet.jpg>

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