Telex 1505 service manual Microphone Jumpers, Receive Line Input Impedance, Switch Settings

4.3.2 Microphone Jumpers

Depending on which microphone you are us- ing, the following jumpers must be installed:

JP2 - carbon element; JP3 - electret element; JP4 - dynamic element with a low output.

No jumpers are installed for a dynamic micro- phone with a high output.

4.3.3 Receive Line Input Impedance

A 600Ω terminating resistor is provided for each receive line input. Refer to the schematic and Table 1 for the proper jumper installation.

4.3.4 Switch Settings

Switches S1 to S5 provide the means to pro- gram the signal inputs to the output circuits. Any input can be programmed to any output. Multiple inputs can be combined to a single output, or conversely a single input can be pro- grammed to one output. Refer to the schematic and Table 2 for the proper jumper installation. Refer to the schematic and Table 2 for proper switch closures.

4.3.5Parallel Transmit Line Impedance Matching

If the occasion arises where the transmit line output must operate in parallel with another line terminating device, an impedance mis- match to the phone line will result. To maintain a 600Ω termination, it will be necessary to cre- ate a resistive pad on the output of the 1505 line driver. A similar pad must be installed in each parallel device. This resistive pad creates a matching network with the phone line, how- ever it induces a loss in the transmission path which lowers the line level and may affect the ultimate performance.

In order to easily insert the pad, a provision has been made in the circuit to allow the user to al- ter the value of two resistors in the transmit line output. The 1505 is shipped from the fac- tory for single line termination without a pad, resulting in the balanced transmit line output having a 0 Ω resistor in each leg of the trans- former without loss. To add the pad, both 0 Ω resistors must be replaced with a value corre- sponding to the number of parallel lines. Refer to the schematic and Table 3 for proper resistor values and corresponding insertion loss.

Table 2. Output Programming Switch Settings/Associated Switch

Input			Outputs
	Tx Ch #1	Tx Ch #2	Tx Ch #3	Tx Ch #4	Speaker
Rx Ch #1	S-1	S-1	S-1	S-1	S-1
Rx Ch #2	S-2	S-2	S-2	S-2	S-2
Rx Ch #3	S-3	S-3	S-3	S-3	S-3
Rx Ch #4	S-4	S-4	S-4	S-4	S-4
Mic Input	S-5	S-5	S-5	S-5	S-5
Sum Audio In	S-6	S-6	S-6	S-6	S-6

Table 3. Parallel Transmit Line Resistor Network Selection

Tx Ch #	Resistor Designation	Parallel Lines, Resistor Value / Line Loss (dB)
		2	3	4
1	R112, R113	300 Ω / 6 dB	620 Ω / 9.5 dΒ	910 Ω / 12.0 dΒ
2	R119, R121	300 Ω / 6 dB	620 Ω / 9.5 dΒ	910 Ω / 12.0 dΒ
3	R109, R110	300 Ω / 6 dB	620 Ω / 9.5 dΒ	910 Ω / 12.0 dΒ
4	R118, R120	300 Ω / 6 dB	620 Ω / 9.5 dΒ	910 Ω / 12.0 dΒ

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