Telos Zephyr User Manual page 214

Power Supply Detail
The power supply circuit should provide a minimum of 48 volts (50- 53 V typ.) to pin 8
of J1, the S- Interface connector. It should maintain this minimum voltage even under
loaded conditions (phone or NT1 attached). Diode D18 is used to prevent a surge of
current INTO the Zephyr should a self powered NT1 or ISDN phone be plugged in. The
cathode of D18 should be toward the connector.
The Maxim chip (U5) monitors the output voltage and adjusts circuit parameters to
keep it within range. The control (or "gate") voltage to Q1 is provided by pin 6 of U5
and turns on the FET causing current to flow through the inductor L1. The gate voltage
is then removed and the inductor will try to maintain the current flow, charging up C1
with a burst of current. The voltage on C1 is kept from leaking back into the charging
circuit by the zener diode D1. The gate control signal switches at a frequency of
approximately 50 kHz. Some causes of improper output voltage are:
•
Leaky Capacitor C1
•
Bad Maxim chip U5 (gate signal will be missing or low in amplitude).
•
Blown or leaky Q1 (low or missing current flow through L1 with the gate signal
present).
•
Improper values for R1 and R2
•
Diode D18 backwards or missing
S-Interface Detail
The Mitel MT8931 (U2) is responsible for changing the digital coded audio signal
to the ISDN S/T signaling protocol, as well as performing some of the ISDN
handshaking with the central office (sending SPIDS, receiving TEI's (Terminal Endpoint
Identifier), etc.). The S- Interface is characterized by a 4- wire connection (polarized
receive and transmit pairs).
The Mitel chip transmits on pins 3 and 6 of the connector J1. With no NT1 connected,
and the U- Interface unplugged, the "Info 0" waveform (see Fig. 1) should be present on
each of these pins.
If this signal is not present reboot the ISDN software from the front panel. If the signal
still does not appear probe pin 26 of the Mitel chip. If no signal is present, replace the
214
Section 11
"INFO 0" Waveform
TECHNICAL INFORMATION