SECTION 4 — SERVICING
4-1 General Information
TheTru-VUisdesignedtoneedaminimumofmain-
tenance for long trouble-free operation.Should repair
be necessary, the technician should first read the in-
formation in the manual concerning the circuits in
questionandshouldfollowproperproceduresfortest-
ing and replacing semiconductors.Logitek engineers
arereadilyavailableat the factory to providetechnical
assistancebothduringandafterthewarrantyperiod.
**********WARNING*********
The inside of the Tru-VU contains no controls or
user serviceable parts. The cover should only be re-
moved by competent technical personnel capable of
servicing this type of equipment.Hazardous voltages
exist on the underside of some circuit cards.
***************************
4-2 Fuse
The primary winding of the power transformer is
fused with a 12amp slow-blow 20mm fuse housed in a
fuseholderin a slide-outdrawerunder the powercord
ontherear panel oftheunit.Itshould only be replaced
with fuses of the same type and current rating. The
power cord must be unplugged from the rear panel to
gain access to the fuse holder.
**********WARNING**********
Do
NOT
replacethefusewithoutfirstdisconnecting
the meter’s power cord from the wall socket, as dan-
gerous voltages are present which may cause electri-
cal shock. Always use fuses of the same value and
type as specified in this manual.Do
NOT
use fuses of
ahighervaluethanthosespecified,asashockhazard
and a fire hazard may result.
****************************
A blown fuse is most often, although not always, a
symptom of another failure. Related circuitry should
always be carefully examined after any fuse failure,
especially if a replacement fuse also blows.
4-3 Handling of CMOS Integrated Circuits
All logic components in the Tru-VU meter are
CMOS type integrated circuits. Even though all cir-
cuits have built-in static discharge protection, special
care must be taken in the handling of these devices,
particularly in dry environments.
*******IMPORTANT NOTE********
Do
NOT
remove or handle CMOS integrated cir-
cuits except in a grounded environment which is free
of the risk of static electricity. Store such circuits on
conductive foam or in anti-static containers. Do
NOT
store on styrofoam or other plastic sheets. Improper
handling may damage these devices.
****************************
4-4 Access to Circuit Cards
The internal components of the meter are ac-
cessed by removing the eight screws that attach the
top cover and removing the cover.
4-5 DSP Control Card
Filtered +9 VDC from power connector P3 is fed to
the+5VDCregulatorIC5viaconnectorP4.+9VDCas
well as -9 VDC from P3 is also passed to the input
cardsviaconnectorsP1andP2.The+5VDCoutputof
IC5isclampedagainstovervoltageandvoltagerever-
sal by zener diode D1.Distributed power is filtered by
C1, C2, C3, C4 and C5.In addition to powering all cir-
cuits on this card, IC5 also supplies +5 VDC to the in-
put and display cards via connectors P1, P2, P5 and
P6.
On power up, the DSP is kept in reset by IC1 until
the power has stabilized, ensuring a good power on
reset.IC1 also contains a watchdog timer that will re-
set the DSP if its ST pin in not taken low at least once
every 150 mSec.Thispinisnormallypulsedlowevery
5 mSec by a DSP write to address 0.
The DSP (IC4) is clocked by a 12.288 MHZ crystal
oscillator consisting of Y1,C6,C7 and an IC4 internal
driver.IC4, pin43 is a buffered output of the oscillator
andisusedasthemasterclockfortheanalogtodigital
converters on the analog input cards.
The first thing the DSP (IC4) does after reset is
copy its program from EEPROM IC3 to its internal
memory. Note that the 8-bit data bus from the EE-
PROM is connected to the middle byte of the DSP’s
24-bit data port.
Addressdecoder IC2 is usedto reset the watchdog
timer IC1 and supply the serial bit clock and load sig-
nals to the LED drivers on the display cards.
The front panel PPM/VU slide switches are con-
nected directly to the low byte of the DSP address
port.
Serial data is sent synchronously (data,clock,load)
to the LED driver chips via display connectors P5 and
P6 in 16-bit strings.Data is placed on the top bit of the
DSP data port and clocked into the display drivers via
awritetoaddressdecoderIC2.OtherwritestoIC2will
load the 16-bit strings into the left or right side display
driver.
Two channel audio data is fed synchronously
(data,clock,frame sync) to serial ports on the DSP via
input connectors P1 and P2.If the control card is only
driving one display then the DSP IC4 will only have
one serial port which will be connected to P1. In this
caseonlyleftsideconnectorsP1andP6willbeactive.
Page 8 ** Tru-VU Instruction Manual **
