Concept:
Modify a Station Control Module (SCM) for better duplex use when building a repeater from a station.

Description:
A technician may want to instantly disable the transmitter to check for desense.  This modification rewires the "Line Disable" switch to allow disabling of the PTT and enhance the use of an outboard repeater controller.  The modification will also eliminate an outside source from inhibiting the PTT.  A pilot lamp (LED) can also be added to indicate when the transmitter is active.  Also, you may desire to have the transmitter channel element keyed with the exciter.  This prevents local oscillator leakage that often can be heard for a good distance from the repeater site.  From the factory, the SCM provides a 200 mS. delayed drop-out of the keying of the exciter PTT.  Sometimes it is desired to have the transmitter shut down immediately when the PTT signal disappears or when the PTT Disable switch is actuated so modification to eliminate this delay has been included.

Keying Switch and LED Modification:
1. Completely remove Q14 and Q17 from the board.  This disables the ability for an outside source from inhibiting the PTT.
2. Remove CR8 and CR14.  This allows the antenna switch transistor to be used to enable the TX channel element.
3. Connect a wire from the normally open unused pole of the Line Disable switch to the Key Inhibit input.  This routes switched ground from the Line Disable switch to the Inhibit input to keep the module from developing PTT.
4. Drill the front panel and wire a Green LED through a 2.2 k resistor from the collector of Q18 to A+.

PTT from the outboard controller is connected to pin 14 (Local PTT) on the card.  This is done on the backplane to allow the card to be pulled if needed.

The above modifications will disable the ability for the Station Control Module to create a PTT by using the "Line Disable" {Now "PTT Disable"} switch to provide a Key Inhibit input thus keeping Q9 and Q10 from creating Keyed A+.
The added LED will indicate when the exciter is keyed (On Air Lamp).

Keyed Exciter Channel Element Modification:
Q16 can be used to control the keying of the exciter channel element for the benefits stated previously.
Simply remove or cut out CR8 and CR14 as stated earlier.  This allows the channel element enable line to be shorted to ground to enable it, and not allow the local PTT input to key the antenna switch transistor until Keyed A- is developed so proper disabling of the element occurs when the PTT disable switch is engaged.
Use pin 15 (Ant Sw / Audio Mute) on the backplane to supply a ground to enable the Channel Element by running a wire from there to the transmitter interconnect pin 19 (F1 Channel Element Select/Enable).

Elimination of Delayed Keying:
One original function of the Station Control Module is to provide a 200 mS. delay of unkeying the exciter.  This delay is to allow time for the Reverse Burst to pass before dropping the RF carrier.  When the station is used in amateur service it may be desirable to eliminate this delay drop-out so instant unkeying and disabling of the transmitter will occur.
Simply remove or cut out C8 and delayed PTT will no longer occur.

The module can be run in the PL Disable mode full time if necessary.
Re-label the Line Disable switch to indicate "PTT Disable."
The PTT switch (top switch) will work as original.
 

This modification was prepared by Kevin K. Custer  W3KKC, July 31, 2001
Comments welcome, email kuggie@kuggie.com

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