Receiver audio can typically be taken from the high side of the squelch control. This audio must be de-emphasized with the controller's optional de-emphasis circuit, which provides a -6dB/octave slope. Optionally, audio can be taken from later in the receiver's audio chain, where it is already de-emphasized. Care must be taken that this source of audio is not subject to adjustment by the radio's volume control. If the receiver audio has not been properly de-emphasized, either in the receiver itself or on the controller board, the repeater will have a very "tinny", unnatural sound to it.

To de-emphasize the receiver audio on the controller board, install a .0068 F capacitor in position C1, change R2 to 51K, and change R1 to 510K. These values should be considered a good starting point. You may want to experiment with the values of C1 and R1 to get better sounding audio. We have had consistently good results with this de-emphasis network.

The receiver must provide a signal present indication (also called COR, RUS, CAS) to the controller. Because of the varieties of polarity and state that this signal can take, we have chosen to implement the controller's signal present input with an opto-isolator (ISO1). The anode and cathode of the LED in the opto-isolator are exposed through a current limiting resistor (R30). This allows easy interfacing to active-high, active-low, and combinations of both to indicate the presence of a received signal to the controller. Clever wiring can allow the user to create CTCSS and COR, CTCSS or COR, etc. configurations. Note that both the CAS+ and CAS- terminals must be connected to something in order for the controller to detect the signal present indication.

Transmitter audio can be fed directly into the microphone input of the transmitter. VR2 is the master level control, used to set the audio level into the transmitter. The transmitter's deviation limiter (sometimes called IDC) should be set such that the transmitter cannot overdeviate, regardless of input signal level. One way to adjust transmitter deviation is to set the transmitter deviation limiter wide open (unlimited), adjust the controller's master output until the transmitter is slightly overdeviating, then set the transmitter's deviation limiter to limit just below 5 KHz deviation. Then reduce the controller's master output until the transmitted audio does not sound compressed or clipped. Transmitter deviation should be adjusted with a service monitor or deviation meter.

Transmitter keying is provided by a power MOSFET (Q6) configured in an open-drain circuit. This can be used to key many transmitters directly. The MOSFET essentially provides a closure to ground for PTT. For other transmitters, the MOSFET can drive a small relay to key the radio. Although this MOSFET can handle several amps, we recommend that no more than 500 mA of current be drawn through it.

The LEDS can be disabled to reduce the power consumption of the controller. Remove jumper JP2 to disable the LEDS.

The repeater's deviation is set with VR2 (the master level) on the controller board and the transmitter's deviation/modulation control. The key to properly adjusting these controls is to remember that the limiter in the tranmitter is after VR2 but probably before the transmitter's deviation/modulation control. The transmitter's deviation/modulation control will set the actual peak deviation, and VR2 will set the level into the transmitter. You do not want excessive limiting on normal speech going through the repeater; it sounds bad and tends to "pump-up" background noise. On the other hand, some limiting is desirable. An oscilloscope connected to the audio output of a receiver tuned to the transmitter's frequency will show limiting as the audio gets "flat-topped" or clipped by the limiter. Ideally, a 4.5KHz deviation signal input to the repeater should result in a 4.5 KHz deviation output, and 5.5 KHz of input deviation should result in just under 5.0 KHz of deviation out of the repeater. A service monitor (or two), deviation meter, and/or a signal generator are necessary to do this job right.

Adjust VR6 (the beep level) to set the courtesy tone and CW tone level.

The easiest way to adjust the ISD2590 input and output levels is to select the simplex repeater mode and record and play messages until the audio sounds right. VR3 adjusts the record audio level into the ISD2590. Adjust this control for the best sounding record audio. VR5 sets the ISD2590 playback level. Adjust this control for best acceptable transmitter deviation. Note that the ISD2590 includes on-chip limiting/compression; this may fool you into thinking that you have the input level set just right when it is really too high. Try recording a whisper, it should play back quietly, also try recording normal speech with large gaps between words in a somewhat noisy environment to listen for background noise pumping. Properly adjusted, the ISD2590 recorded audio should be indistinguishable from normal audio repeated through the system.

VR4 is used to set the receiver audio level, and may not need to be adjusted from midpoint.