Lexicon LXP-5 Service Manual

LXP-5 Service Manual
Circuit
Description
The LXP-5 power supply provides five regulated DC output voltages:
+5 VDC
+12 VDC
-12 VDC
-7.5 VDC
+7.5 VDC
AC power is provided by an externa! transformer rated at 9 VAC @ 1.5 A. The
transformer output is terminated with a 5 mm/2.5 mm barrel-type connector,
with its mating input jack located at the rear panel of the LXP-5. A .01 uF
capacitor, C25, is connected across the AC input to help prevent noise spikes
from entering the unit. In addition, C25 along with C38 and ferrite beads FB4
and FB5 stop circuit-generated RFI from radiating through the power line.
A pair of voltage doubler circuits develop the unregulated DC voltages for the
± 12 volt supplies. C27 and C28 store the peak level of AC input voltage
during one half of the AC cycle. This stored charge adds with the next AC half
cycle to produce approximately twice the peak AC input voltage across C29
and C40. Diodes CR9 and CR12 are polarized to produce the unregulated
+12 volt supply (approximately +20 VDC) across C29. CR10 and CR13 are
polarized to produce the unregulated -12 volt supply (approximately - 20
VDC) across C40.
A single diode (CR11) is used as a half wave rectifier to produce the
unregulated + 5 volt supply (approximately +10 VDC) across C26 and C39.
The +5VUNREG supply powers two of the front panel LEDs and is monitored
by the LXP-5 Reset and Battery Backup circuits for power up and power fail
conditions.
Voltage regulation is handled by three TO220 packaged ICs:
+12 VDC regulation - U25 (7812)
-12 VDC regulation - U26 (7912)
+5 VDC regulation - U24 (7805)
Current limiting and short circuit protection are incorporated into the internal
circuitry of these ICs.
A ±7.5 VDC supply is created from the ±12 VDC supply by using 1K resistors
in series with 1N755 zener diodes and 22uF capacitors. R59 and CR21
develop the +7.5 volt supply across C90, while R58 and CR19 develop the
-7.5 volt supply across C84. ±7.5 VDC is required by the CD4053 analog
switch (U38) only.
A +4.7 VDC supply is produced from the +5 VDC supply by placing a schottky
diode (CR3) in series with R6 and C5. A charge is stored across C5 which
provides the +4.7 volt supply for an extended time during powerdown due to
the polarization of CR3. This supply is used by a 74HCT14 (U1) to insure
proper operation of the LXP-5 Reset and Battery Backup circuitry when
power failure occurs.
Power Supply
3-3

Lexicon LXP-5 Specifications

General

Brand	Lexicon
Model	LXP-5
Category	Recording Equipment
Language	English