Comark IOX Series Reference manual

Technical Service Bulletin 030328
Technical Service Bulletin 030328
IOX / DCX Troubleshooting Table
This service bulletin provides a troubleshooting table for the Comark IOX and DCX series of
UHF television transmitter. The information contained in this table is the compilation of inputs
from Comark customer service representatives, field service engineers, design engineers, and
customers. If you would like to make an addition or correction to this table, please send your
information to the Comark Customer Service department via e-mail to
[e-mail]
Symptom Problem / Solution
480VAC Three Phase Power
"Soft Start" failure prevents
application of high voltage. Missing phase in 480V feed to beam supply primary. Trace three-phase voltages back from beam supply disconnect with voltmeter to find where missing leg disappears.
Possible partial failure of CB3 motorized breaker. Check for presence of three-phase 480V
at output terminals of CB3 when closed. Replace CB3 as necessary.
Possible failure of HV rotary switch or outdoor disconnect. Check for 480V at output
terminals of suspected device. Replace faulty components as necessary.
To prevent damage to CB3 breaker, HV rotary switch, step-start contactors, and other 480V
devices, check tightness of terminal connections at regular intervals as part of a
preventative maintenance program.
Beam voltage OK when HV is
isolated, but sags significantly
(>5kV) when IOT is
connected. Possible positive
grid current alarms or severe
distortion of transmitted
waveform.
Missing phase in 480V feed to beam supply primary. Trace three-phase voltages back from
beam supply disconnect with voltmeter to find where missing leg disappears.
Possible partial failure of CB3 motorized breaker. Check for presence of three-phase 480V
at output terminals of CB3 when closed. Replace CB3 as necessary.
Possible failure of HV rotary switch or outdoor disconnect. Check for 480V at output
terminals of suspected device. Replace faulty components as necessary.
Beam voltage is 2/3 of
nominal value e.g. 22kV
instead of 33kV.
Missing phase in 480V feed to beam supply primary. Trace three-phase voltages back from
beam supply disconnect with voltmeter to find where missing leg disappears.
Possible partial failure of CB3 motorized breaker. Check for presence of three-phase 480V
at output terminals of CB3 when closed. Replace CB3 as necessary.
Possible failure of HV rotary switch or outdoor disconnect. Check for 480V at output
terminals of suspected device. Replace faulty components as necessary.
Beam voltage is unstable, bi-
stable (jumping between two
values).
Possible high voltage breakdown of 60 ohm filter resistors in HV beam supply. Extinguish
high voltage and inspect beam supply resistors. Replace resistors as necessary.
Beam will not come on due to
cabinet interlock alarm, yet all
safety devices connected to
cabinet interlock line are in
proper positions.
Possible over-temperature condition in step-start resistors. Step start resistor temperature
sensor is also part of cabinet interlock line. Condition typically occurs after frequent HV on-
off cycles in a short period of time. Allow step start resistors to cool down for ten minutes.
Interlock will clear itself as resistors cool.
Step-start overheating may also be due to open phase leg on K32 step start contactor.
Remove 480V and connect voltmeter across contactor terminals for one phase. Reapply
480V and read voltage drop across contactor while engaged. Repeat measurement for all
three phases. Look for non-zero voltage drop on one phase leg. Replace K32 as necessary.
Note: other devices in cabinet interlock line are HVPS door, HVPS ground hook, HVPS oil
level switch, HV key interlock system.
High voltage is stable in beam
mode but drops out with
cabinet interlock alarm after
Possible over-temperature condition in step start resistors due to high resistance condition
in one or more legs of K32 step start contactor. Step start resistor temperature sensor is
also part of cabinet interlock line. Remove 480V and connect voltmeter across contactor
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Technical Service Bulletin 030328
several minutes in RF mode.
terminals for one phase. Reapply 480V and read voltage drop across contactor while
engaged. Repeat measurement for all three phases. Look for non-zero voltage drop on one
phase leg. Replace K32 as necessary.
Amber three phase power
alarm. HPA controller screen
displays “waiting for three
phase” message.
Possible three-phrase power problem. Check status of HVPS ON/OFF switch on AC
distribution panel on HPA cabinet. Check for 277V line-neutral on all three phases with
voltmeter at AC input to transmitter. Check for signs of incorrect phase rotation if there has
been a recent AC blackout
Possible jammed three phase detector relay contacts. If three phase power is OK from
previous step, place temporary jumper across terminals TB1 - 8,9 on three phase detector.
If three phase alarm clears, replace three phase detector.
Possible absence of +24V from HPA controller power supply. Remove HPA front panel and
check status of +24V LED on HPA power supply card (far right). Check continuity of small
green pico fuses at rear of card. Replace fuses as necessary.
Remove HPA power supply card and measure resistance between ground and second-
from-bottom pin on right side. Resistance should be at least 235 ohms. If resistance is less
than this value, +24V line is shorted somewhere in HPA cabinet. Locate and eliminate short
before replacing fuses and reinstalling card.
Fuse failure is sometimes caused by damage to HPA controller card 452117-01, in turn
caused by a destructive failure of one or more snubber networks 452251-01, 452252-01 on
relays in HPA relay panel assembly. Be sure to indicate software revision level of board
when obtaining replacement HPA controller board (e.g. v2.12, v3.01). Does not apply to
451144-01 type HPA controllers.
Early vintage IOX transmitters should be checked for presence of snubber networks on
relays K2, K10-12, K15-18. Contact Comark if snubber networks not present.
Loud humming coming from
480V contactor(s) in rear of
HPA cabinet.
Contactor faulty. Unit must be replaced.
Ratcheting or machine-gun
sound coming from step-start
relays during turn-on.
Intermittent activation of step-start relays. Possible intermittent contacts in step start time
delay relay K31-pins 1, 2 or auxiliary contacts on K1-pins 43, 44. Determine and eliminate
source of intermittent contacts. Replace faulty components as necessary.
Pumps trip thermal protection
on motor starters. Each pump
runs approx. 20 seconds and
shuts off.
Three-phase electrical problems. Verify 480V phase-phase voltage on all legs of feed to
transmitter / pumps. Check balance of current draw on all three phases will clamp on
ammeter. For certain three-phase imbalances, the pump thermal protections sometimes
prove to be more sensitive than the three-phase detector relay.
Control System
HPA control panel buttons
intermittent or insensitive.
Buttons must be pressed very
hard to register command.
Condition worsens with time.
Contact Comark for a replacement unit PN 452103-02. Be prepared to provide panel serial
number, revision level of panel and HPA brain (CPU) board. (CPU code and Front Panel
code.)
Does not apply to 451144-01 type HPA controller (non LCD screen version).
HPA controller(s) do not
respond or respond very
slowly (>1 min) to beam ON
command or RF ON
command from system
controller in exciter cabinet.
Possible incorrect RF system pattern. Any HPA not called for in the current RF system
pattern will not respond to an ON command from the system controller. The determination of
the current pattern is based on the position read back of the RF system switches, NOT on
the commands issued by the pattern select buttons. Therefore, an HPA not called for in a
given pattern will have its control inhibited, even if that pattern were entered by manually
actuating the RF system switches one at a time. This effect does not occur when an HPA
controller is in internal control mode.
Possible partial logic/communications freeze-up of PLC controller in system cabinet.
Recycling power to system controller in exciter cabinet will restore proper operation to
control system. A permanent solution may be had by upgrading to a 452117-01 HPA CPU
card with code version 3.10 or later. Contact Comark for further details.
Does not apply to 451144-01 type HPA controllers (non-screen).
HPA falls to stop mode after
brief AC interruption (~5 sec)
Software bug in HPA controller, which manifests itself only under certain external conditions.
Problem is non-existent at many sites, but evident at small minority of sites. Cause of
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Technical Service Bulletin 030328
and fails to respond to
commands from exciter
cabinet or remote control.
External HPA control can be
re-established by cycling HPA
controller from external to
internal to external mode.
variability from site to site unknown. A permanent solution may be had by upgrading to a
452117-01 HPA CPU card with code version 3.10 or later. Contact Comark for further
details.
Does not apply to 451144-01 type HPA controllers (non-screen).
RF output from cabinet
disappears as HPA is steered
to magic tee reject load. HPA
controller spontaneously
drops from RF mode to
standby mode.
This is normal. IOX/DCX system controller will inhibit RF output of any HPA not called for by
current RF system pattern. The determination of current pattern is based on position read
back of RF system switches, NOT on commands issued by pattern select buttons.
Therefore, an HPA not called for in a given pattern will have its drive inhibited, even if that
pattern were entered by manually actuating the RF system switches one at a time. This
effect does not occur when an HPA controller is in internal control mode.
Transmitter system drops to
standby mode five seconds
after being switched from
local to remote control.
Failsafe interlock not satisfied. Remote control panel input IN15 must have +24V applied to
satisfy failsafe interlock.
Control system behaves
erratically. (erroneous faults
that will not clear, blower will
not come on, control freeze-
up)
Possible failure of 452117-01 HPA brain (CPU) board. Verify failure by swapping board with
known good unit from spares or other HPA in multi-tube systems. Contact Comark for
replacement board. Be sure to indicate software revision level of board when obtaining
replacement HPA controller board (e.g. v2.12, v3.01). Does not apply to 451144-01 type
HPA controllers (non-screen).
High voltage isolation relay
will not remain in position,
instantly relaxes when HV
connect/isolate button is
released.
Incorrect selection of HV isolation relay type in HPA controller configuration. Consult Service
Bulletin 040704 for more information on configuration of HPA controller backplane and
selection of correct HV isolation relay type.
HPA controller screen
indicates NOVRAM errors or
failure.
Possible absence of +/- 12V from HPA controller power supply. Remove HPA front panel
and check status of +/- 12V LEDs on HPA power supply card (far right). Check continuity of
small green pico fuses at rear of card. Replace fuses as necessary. Check for failure of
power supply itself. Does not apply to 451144-01 type HPA controllers (non-screen).
Possible failure of HPA controller backplane board. Consult Service Bulletin 040704 for
more information on troubleshooting / replacing backplane board. Does not apply to 451144-
01 type HPA controllers (non-screen).
HPA controller screen
indicates “anti-fault.” HPA control system has observed a condition inconsistent with current operating state (e.g. RF forward power reading while transmitter is only in start mode). Anti-faults are typically
due to stuck / malfunctioning sensors. The most common anti-fault is due to a stuck vane on
an airflow sensor, thereby registering airflow even when blower is extinguished. Does not
apply to 451144-01 type HPA controllers (non-screen).
Any of following conditions
after installation of new HPA
controller backplane:
HV isolation relay fails to stay
engaged. FBI supply fails to
turn on in start mode. Out of
tolerance alarm for filament,
beam, bias, or other operating
parameter even though levels
are correct.
HPA controller backplane has not been programmed correctly. Tube type, HV contactor
type, FBI supply type must all be programmed upon installation of a new HPA controller
backplane. Requires special password. Contact Comark for more details.
Does not apply to 451144-01 type HPA controllers (non-screen).
LCD screen on HPA controller
is to too bright, too dark. Possible misadjustment of screen contrast control. Locate the contrast trimpot on the display driver PCB (R6). Use this trimpot to adjust LCD screen contrast. Replace LCD screen or
front panel if satisfactory adjustment is not possible.
Multimeter selector button on
HPA metering bridge does not
work properly. Multimeter
remains stuck on top metering
selection.
Possible damage to K3 on 450226 Multimeter PCB Assy. Lift HPA metering bridge and
inspect K3 for crushed pins or partial ejection from socket. K3 is easily crushed by
downward pressure applied to HPA metering bridge. Avoid stepping on HPA metering
bridge when on top of HPA cabinet. Remove alignment tabs for pins 4, 5 on orange
connector at TB-1 to minimize crush hazard.
Panel View screen on
Possible corrupted programming in system controller. Restore screen programming from
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Technical Service Bulletin 030328
(millennium-style) system
controller dark except for
“type 633 error” message.
memory via Panel View Screen. Contact Comark for procedure.
Input IN12 does not reset
HPA alarms on remote I/O
block.
Typographical error on certain versions of remote control pinout listings. HPA fault reset
may be on input IN11 with system fault reset on input IN12. Documentation incorrectly
shows IN12 as system/HPA fault reset and IN11 as unused. Contact Comark if doubts
remain.
Incandescent bulbs in system
controller frequently burn out. Replace incandescent bulbs with LED replacements. Suitable replacements are as follows:
(upper white buttons) = LEDtronics FF200-21W-028B
(lower buttons) = LEDtronics GF200-21W-028B
(lower keypad) = LEDtronics BPF120-OUY-028V
This entry applies only to non-millennium system controllers (non-Panel View).
High Voltage Arcs & Crowbars
Crowbar fires upon
application of high voltage. Arc has occurred in high voltage circuit. Isolate high voltage from IOT by activating HV ISOLATED mode and repeat test. Consult table entries below.
Crowbar immediately and
consistently fires upon
application of high voltage,
does not fire in HV
ISOLATED mode.
Possible high voltage arc to ground through solid material such as HV wires, IOT input
cavity, or AC isolation transformer to FBI supply. Inspect red HV wire for pinhole burns,
especially where wire outer surface makes contact with cabinet ground. Once short circuit
develops in solid material, damage is irreversible and crowbars occur immediately. If
transmitter withstands HV for even a second, even once, before crowbarring, arcing is most
likely across air (corona) or a vacuum (inside IOT). Consult section on arcs through air or
vacuum, immediately below.
Note: An immediate and consistent arc that is absent upon cold turn on, but appears after
15 -30 minutes of warm up time, is characteristic of a insulation breakdown in the AC
isolation transformer to the FBI supply. Swap transformers between cabinets in multi-tube
transmitters or test HV standoff with a hipot test set to confirm failure. Consult Service
Bulletin 030614 for more information on troubleshooting with a hipot test set.
Crowbar fires after random
period of time between 2 sec -
30 minutes after application of
high voltage, does not fire in
HV ISOLATED mode.
High voltage arc to ground through air or vacuum. Arc may be internal or external to IOT
e.g. in the junction box or high voltage compartment.
Inspect all HV circuits for oxidation due to corona or carbon marks due to arcing. Look for
any sharp edges liable to create corona. Clean dust from all HV standoffs and bushings.
Test HV circuits with hipot test set for leakage current and corona. Consult Service Bulletin
030614 for more information on troubleshooting with a hipot test set.
It is also possible to have a crowbar falsely triggered by the induced current from a static
electric discharge from the HV wire outer jacket to ground. (jacket arc). This type of arc will
leave no traces and does not permanently damage the HV wire. Jacket arcs are mostly
likely to occur wherever the HV wire approaches, but does not touch, ground – especially if
there is a sharp protrusion (screw head, cabinet seam, etc.) in the vicinity. Re-route HV
wiring as necessary to eliminate arcing. Crowbar frequency increasing during dry weather or
other climatic changes is a sign of arcing external to the IOT due to corona or jacket arcing.
Raise bias voltage to most negative setting and reapply beam without RF drive. If high
voltage holds with full negative bias, crowbars are most likely internal to IOT. Slowly bring
bias voltage more positive until normal idle current is re-established. Re-apply RF drive.
Read table entry concerning arcs internal to IOT, immediately below.
If arcing continues despite full negative bias setting, lower beam voltage to lowest setting
and reapply high voltage. If high voltage holds with lowest beam voltage setting, arcing is
probably due to corona, jacket arcs, or a spuriously firing crowbar. Re-inspect all HV circuits
external to IOT for source of arcing. Test HV circuits with hipot test set for leakage current
and corona. Consult Service Bulletin 030614 for more information on troubleshooting with a
hipot test set.
If arcing continues despite full negative bias setting and lowest beam voltage setting, test
HV standoff of IOT and AC isolation transformer with hipot test set. Replace isolation
transformer, IOT input cavity, or IOT itself, as necessary. Consult Service Bulletin 030614
for more information on troubleshooting with a hipot test set.
Crowbar fires after application
High voltage arc to ground through vacuum inside IOT. The presence of ion current after arc
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Technical Service Bulletin 030328
of high voltage, does not fire
in HV ISOLATED mode.
Arc known to be internal to
the IOT.
event serves as a good confirmation that arc is internal to IOT.
Note: an ion current reading of as low as 5uA may cause an immediate crowbar upon
application of high voltage. Do not attempt to re-apply high voltage until the ion current
reading has fallen back to zero.
Note: certain brands of IOT do not have an ion pump and therefore do not produce an ion
current reading.
Arcing inside IOT may be due to the following causes:
Excessive RF drive (or excessive sync stretch in an analog transmitter)...especially in
an aging tube with marginal cathode emission.
Excessive IOT filament voltage (often accompanied by negative bias current).
Sudden change in tube operating power level in older tube (tube has acquired
"memory" of previous power level). This condition usually clears within a day at the
new power level.
Transient overdrive conditions caused by intermittent RF connections, disconnection of
AGC feedback cable, or other drive level instability.
Brand new tube is clearing internal burrs, surface irregularities (first month of
operation).
Attempt to "nurse" IOT back to normal by raising bias voltage to most negative setting,
lowering beam supply to lowest tap setting, and reapplying beam without RF drive. If high
voltage holds, gradually bring IOT back to normal operating parameters by lowering bias
voltage five volts and/or raising beam voltage one tap setting every ten minutes until normal
levels are re-established. Activate RF at 10% power and increase power 10% every ten
minutes until 100% operation is restored.
If arcing continues despite full negative bias setting and lowest beam voltage setting, test
HV standoff of IOT. Replace IOT input cavity, or IOT itself, as necessary. Consult Service
Bulletin 030614 for more information on troubleshooting with a hipot test set.
Crowbar immediately and
consistently fires upon
application of high voltage,
even in HV ISOLATED mode.
Probable spuriously firing crowbar. Clean dust from crowbar assembly legs, large external
resistors on side of crowbar assembly and repeat test. If crowbar continues to fire, measure
thyratron filament voltage with voltmeter at base of tube (carefully set aside crowbar cover
without disconnecting fan to do this). Filaments should be 6.3V +/- 0.2V. Move tap on
isolation transformer in LV compartment to adjust filament voltage, as necessary. Consult
Service Bulletins 030605 and 46744354-194 for further details.
Possible insufficient high voltage rise-time due to damaged filter resistors and/or capacitors
in HV beam supply. Crowbar will spontaneously fire if high voltage ramps up too quickly.
Inspect beam supply filter components for signs of damage. Check output signal for
excessive 120 Hz ripple (field rate) for confirmation of diagnosis on IOX (analog)
transmitters. Replace filter capacitors as necessary.
If beam supply shows no signs of damage and crowbar continues to spuriously fire with a
filament voltage < 6.3V, thyratron tube should be replaced.
Instant trip of CB3 motorized
breaker upon application of
beam mode command.
Crowbar does not fire.
Possible arc inside transmitter and malfunctioning crowbar. Attempt to establish high voltage
with HV isolate relay in HV ISOLATED position. Attempt to establish high voltage with HV-
lead disconnected before crowbar (remove J3 from crowbar and suspend in HV
compartment away from all metal surfaces). If high voltage is established, in either case,
without trip of CB3 breaker, first determine why crowbar is not firing, then search for cause
of arc.
If high voltage does not hold with J3 disconnected from crowbar, search for possible short
circuit in HV conduit leading to transmitter. Disconnect HV- and HV+ return wires at beam
supply and attempt to establish high voltage. If CB3 breaker still fires, consult entry on beam
supply short circuit, below. If CB3 breaker does not fire, replace HV wires in conduit to HPA
cabinet.
Possible short circuit in beam supply. Visually inspect components inside beam supply.
Search for arc marks or creepage along fiberglass resistor support board. Individually isolate
HV filter capacitors and attempt to re-establish high voltage. Remove diode transpack from
oil tank and verify diode action with multimeter (one-way conduction). Replace fiberglass
resistor board, shorted HV cap, diode transpack or other damaged components as
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Technical Service Bulletin 030328
necessary.
After arc event(s), IOT Idle
current is checked and found
to be very low (<200mA).
Note: idle current is the beam
current with RF drive
extinguished.
Possible low beam voltage due to missing phase / damage to CB3 motorized breaker, rotary
switch, or other three-phase component. If beam voltage is low, trace three-phase voltages
back from beam supply disconnect with voltmeter to find where missing leg disappears.
Possible cathode emission temporarily disrupted by full dissipation of arc inside tube due to
malfunctioning crowbar. Test operation of crowbar with high voltage isolated from tube and
verify crowbar failure. Determine and eliminate source of crowbar malfunction. Consult
Service Bulletin 990611 for more information on crowbar test procedures. IOT cathode
emission should eventually recover and idle current return to normal after several hours or
days of operation in beam-only mode. Contact tube manufacturer for recommended cathode
re-activation procedure. Never apply RF to the IOT while it is in this reduced emission
condition.
After arc event(s), IOT Idle
current is normal but gain
(output power) is very low,
sync is excessively
compressed in NTSC Tx.
Tube cathode emission temporarily disrupted by full dissipation of arc inside tube due to
malfunctioning crowbar. Test operation of crowbar with high voltage isolated from tube and
verify crowbar failure. Determine and eliminate source of crowbar malfunction. Consult
Service Bulletin 990611 for more information on crowbar test procedures.
IOT cathode emission should eventually recover and idle current return to normal after
several hours or days of operation in beam-only mode. Contact tube manufacturer for
recommended cathode re-activation procedure.
Never apply RF to the IOT while it is in this reduced emission condition.
Crowbar events frequently
interrupt AC power to entire
transmitter and/or entire
building.
Possible incorrect setting or failure of CB3 motorized breaker. Verify trip setting on CB3
breaker body is set to MIN while any other breakers upstream in AC system are set to MAX
(where applicable). Verify that CB3 breaker and motorized actuator are not jammed or
otherwise physically damaged. Replace CB3 as necessary.
Possible use of incorrect fuses or magnetic breakers in building AC distribution system.
Comark specifies BUSS FRS-R type fuses for main Tx feed and each HPA branch feed to
prevent spurious AC trips during crowbars. Contact Comark or original installation AC
distribution diagram for more details.
Ticking or snapping sound
coming from high voltage
compartment at regular
intervals. Crowbar does not
fire and transmitter operates
normally.
Static electric discharge from the HV wire outer jacket to ground (jacket arc). This type of arc
will leave no traces and does not permanently damage the HV wire. Typically, this type of
arc will trigger and fire the crowbar, but this is not universally true depending on the location
and size of the discharge.
Jacket arcs are mostly likely to occur wherever the HV wire approaches, but does not touch,
ground – especially if there is a sharp protrusion (screw head, cabinet seam, etc.) in the
vicinity. Re-route HV wiring as necessary to eliminate arcing. Clean dust from all HV
standoffs and bushings.
Crowbar only partially
discharges HV upon test
firing. HV drops, but not
completely to zero (< 5kV).
Thyratron tube does not have sufficient internal gas to fully conduct arc to ground.
Possible thyratron filament voltage too low. Measure thyratron filament voltage with
voltmeter at base of tube (carefully set aside crowbar cover without disconnecting fan to do
this). Filaments should be 6.3V +/- 0.2V. Move tap on isolation transformer in LV
compartment to adjust filament voltage, as necessary. Consult Service Bulletins 030605 and
46744354-194 for further details.
Possible faulty thyratron or thyratron at end-of-life. If filament voltage OK and problem
persists, replace thyratron tube.
Crowbar does not fire in
response to test button. Possible thyratron filament voltage too low. Measure thyratron filament voltage with voltmeter at base of tube (carefully set aside crowbar cover without disconnecting fan to do
this). Filaments should be 6.3V +/- 0.2V. Move tap on isolation transformer in LV
compartment to adjust filament voltage, as necessary. Consult Service Bulletin 030605 for
further details.
Possible faulty triggering circuits in crowbar, especially U5 thyristor on crowbar low voltage
board or T2 toroid on crowbar high voltage board. Attempt to test fire crowbar in start mode
with HV compartment door open. NOT TRIGGERED LED on far right of upper HV section
should extinguish as test fire command is received. If LED does not change state, trace EXT
trigger circuit in crowbar LV and HV PC boards. Replace faulty components.
If filament voltage OK and trigger command being received, replace thyratron tube.
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Crowbar triggers and fires in
response to test button (as
evidenced by voltage falling
rapidly to zero), but crowbar
counter does not advance.
Possible failure of HPA brain (CPU) board. Verify failure by swapping board with known
good unit from spares or other HPA in multi-tube systems. Contact Comark for replacement
452117-01 board. Be sure to indicate software revision level of board when obtaining
replacement (e.g. v2.12, v3.01). Does not apply to 451144-01 type HPA controllers (non-
screen).
Crowbar will not report ready
status: one or several green
LEDs are extinguished on
crowbar HV section (upper
section).
Problem with high voltage PCB (upper section) of crowbar, possibly latch-up of U5, U6, or
U7 ICs on crowbar high voltage board. Remove cover to HV section and touch chips with
fingertip as crowbar warms up. A latched-up chip will be extremely hot to the touch. Chips
U5, U6, U7 should all be replaced with retrofit daughter boards. (as of 10/2003)
Check for presence of U5 and U6/U7 retrofit daughter boards on HV board. Obtain Comark
Service Bulletin 020709 describing retrofit.
Consult Service Bulletin 46744354-194 for further details.
Crowbar will not report ready
status: All LEDs on HV
section (upper section)
crowbar are on.
Possible intermittent connection. Break Molex connections and inspect for partially recessed
pins. Check crowbar ready status input to HPA controller at connector A7J4 - 10 (rear of
HPA controller) for presence of +24V relative to ground.
Place jumper between pins P1 - 3, 4 on crowbar Molex plug. If problem disappears, problem
is low voltage PCB (lower section) of crowbar, possibly failure of U12 IC. Replace faulty
component or PCB (see SB46744354-194 page 4).
NOTE: If crowbar assembly revision (451227-01) is prior to REV G review SB020709.
If problem remains, place jumper between pins A7J4 - 9, 10 at rear of HPA controller. If
problem disappears, problem is intermittent connection in HPA harness. If problem remains,
replace HPA controller CPU card.
Crowbar requires full 10
minute warm-up period after
intermittent AC interruption.
Possible failure of C14 capacitor on crowbar low voltage board. Replace faulty component.
Crowbar counter advances
hundreds of counts after
crowbar assembly is
replaced.
Spurious crowbar counting due to crowbar assembly being disconnected while HPA
controller is still powered up. Always remove power to both crowbar assembly and HPA
controller using front panel CROWBAR and CONTROL POWER breakers whenever
crowbar assembly is replaced / serviced.
High voltage drops out with
crowbar triggered alarm.
Crowbar does not fire and
motorized breaker does not
trip.
Intermittent connection in crowbar NOT TRIGGERED line. Consult schematics 451262 and
450349 in crowbar chapter (8) in HPA manual. Check for intermittent connection in NOT
TRIGGERED fiber optic cable between upper and lower sections or J1 Molex connector.
Trace logic signal through HV and LV PCBs. Any interruption of the NOT TRIGGERED
signal will cause this condition.
Check for presence of U5 retrofit daughter board in U5 socket on HV board. Obtain Comark
Service Bulletin 020709 describing retrofit.
Consult Service Bulletin 46744354-194 for further details.
Instantaneous body, beam, or
positive grid current alarm
upon application of high
voltage.
Possible grid neutralization problems with Comark IOT. Consult manufacturer instructions
for proper adjustment of neutralization circuits
IOT Focus and Body Current
Body current alarm. Crowbar
does not fire. Motorized
breaker does not trip.
Possible grounding of IOT collector or collector return lead. Check for grounded metal
objects touching collector. Check integrity of video bypass capacitors from collector to
ground. Note: does not apply to tubes with grounded collectors.
Possible arc in beam supply. Occasionally, in the case of mild short circuits in the beam
supply, the body circuit will alarm before the motorized breaker trips. Visually inspect
components inside beam supply. Search for arc marks or creepage along fiberglass resistor
support board. Search for moisture or other paths to ground in HV air compartment of beam
supply. Replace damaged components as necessary.
Possible arc internal to beam supply capacitors or C1 in high voltage compartment (white
0.1 uF cap) due to intermittent connection. When a capacitor develops an intermittent
connection, high voltage will (internally) flash across broken connection, thereby causing an
inrush transient (body current alarm), but voltage across capacitor quickly stabilizes, thereby
REV H - 10 January 2007 7

Technical Service Bulletin 030328
preventing a total HV collapse (crowbar). Replace suspected capacitors to prove diagnosis.
Possible transient trip at turn-on due to inrush current in C1 in high voltage compartment
(white 0.1 uF cap). C1 is not required for ATSC service and may be removed in all DCX
transmitters. C1 is required in most IOX transmitters for video line-rate filtering and may not
be removed. Problem is extremely rare in IOX transmitters, but may be alleviated by re-
routing C1 ground wire to HV+ line returning to beam supply.
Tube withstands high voltage
w/ normal idle current, but
trips on excessive body
current (only) when RF is
applied. Crowbar does not
fire.
Possible improper video bypassing of collector. Check integrity of video bypass capacitors
connected between IOT collector and ground. Turn off HV and discharge collector before
performing check as a safety precaution.
Possible problem with magnetic focusing circuit. Check for proper focus current and focus
voltage.
Focus current or focus
voltage erratic, unstable. Possible dried out or burst electrolytic capacitors. Replace electrolytic capacitors in supply. Possible faulty regulator chip IC1. Replace IC1.
Whistling sound (like tea
kettle) coming from collector. Localized boiling of coolant inside IOT collector water jacket due to excessive focus current or insufficient cooling. Lower focus current. Check for adequate cooling flow to collector.
Disassemble & clean or replace clogged cooling system components, as necessary.
RF Output Level and IOT Tuning
Beam on, RF mode on, but no
RF at output. All LEDs on
driver status panel either red
or extinguished.
Failure of driver power supply in HPA cabinet.
Check for presence of AC at power supply input. Check status of circuit breaker on AC
distribution panel.
Check for proper operation of internal power supply fan. Temporary relief for faulty fan may
be had by adding external fan.
Check power supply output unloaded with voltmeter. Either isolate bad (shorted) load or
replace faulty power supply unit.
Output power level is
unstable, bi-stable. Power
drops in level and exciter
output is raised to
compensate. Gain returns
suddenly causing forward
overpower alarm
Cold solder joint or other intermittent connection in drive stage. Check coaxial drive cables
for recessed or misaligned inner pins. Mechanically vibrate drive cables, IPA splitter plate,
IPA combiner plate, and observe effect on output power level. If suspected, diassamble IPA
combiner or splitter plates and inspect for signs of damage or cracked solder joints.
Little or no output power from
IOT. High power from
circulator reject port at IOT
input. Poor null of input return
loss for IOT.
Possible incorrect input tuning. Attempt to tune IOT according to Service Bulletin 030528 or
030615. If IOT input will not tune correctly, possible damaged input connector to IOT inside
input cavity. IOT input cavity must be replaced.
IOT tuning very sensitive to
mechanical shock, especially
input return loss.
Input drive cable or coupling loop loose inside input cavity. Repair may be possible on-site
with partial cavity disassembly. Contact Comark or tube manufacturer.
Transmitter metering circuits
sensitive to opening and
closing of HPA doors.
Metering indications change
more than 5% according to
door position.
Possible RF leak from IOT due to poorly seated cavities and / or missing finger stock. Check
IOT cavities for proper mechanical seating. Measure ambient radiated power with ANSI-type
radiation meter. Ensure that radiation level is below ANSI standard.
Each HPA cabinet has flat
frequency response when
measured individually, but
combined system response is
tilted.
Incorrect intercabinet RF phasing. Intercabinet phasing off by 15 or more RF wavelengths.
Add or subtract drive cable length to one HPA cabinet to improve frequency response.
HPA cabinet phasing
trombone or RF attenuator at
end of adjustment range.
Re-center adjustment range by adding N-barrel(s) to drive path opposite phasing trombone
(i.e. path with attenuator) or N-attenuator pads to drive path opposite attenuator (i.e. path
with trombone). Consult system RF flow diagram to determine proper location of pads or
barrels.
REV H - 10 January 2007 8

Technical Service Bulletin 030328
Rippling of IOT swept
response (greater than +/-
0.5dB). Ripples do not
change in frequency as IOT is
retuned.
VSWR at IOT output or on RF sample cable to analyzer. Add 6dB attenuator pads on each
end of sample cable. Check VSWR performance of RF dummy loads. Note changes in
frequency response as tube output is steered from combiner reject load to system test load
(where applicable). Also, verify integrity of ballast load at input to balanced RF mask filter
(where applicable).
It is normal to see VSWR ripples less than +/- 0.5dB in amplitude.
Drive power higher than
normal, beam current and
output power lower than
normal.
Possible mistuning of IOT input cavity and input tuner (where applicable). Retune IOT as
necessary. Consult Service Bulletin 030528 or 030615 for more details.
Drive power and beam
current higher than normal,
output power lower than
normal.
Possible mistuning of IOT output cavities. This is especially true for NTSC/PAL, where
almost all of energy is concentrated at vision carrier frequency. A frequency rolloff at vision
carrier will have a profound effect on measured IOT gain and efficiency. Retune IOT as
necessary. Consult Service Bulletin 030528 or 030615 for more details.
Output power and beam
current drop after first minute
of operation.
Possible loose stackpole resistor. Check mechanical tightness of stackpole resistors in HV
compartment. Replace fiberglass center rod as necessary. Consult Service Bulletin 030331
for more details.
Output power drops 20-30%
on one HPA. Drive power also
reduced by similar value.
Remaining HPAs at full
power.
Possible failed IPA amplifier. Check driver status panel for alarm indications or extinguished
LEDs. Disconnect feed to single IPA amplifier at RF splitter plate, record drop in power, and
reconnect feed. Test each IPA amplifier in turn with this procedure. If particular IPA causes
little or no drop in power, amplifier may require replacement. Kill RF drive while
making/breaking each RF connection as a safety precaution.
Cold solder joint or other intermittent connection in drive stage. Check coaxial drive cables
for recessed or misaligned inner pins. Mechanically vibrate drive cables, IPA splitter plate,
IPA combiner plate, and observe effect on output power level. If suspected, diassamble IPA
combiner or splitter plates and inspect for signs of damage or cracked solder joints.
Arc Detectors (output cavities)
Transmitter fails to come
ready for beam mode due to
blinking arc detector alarm.
Arc detector has failed auto-test during transmitter warm up.
Possible faulty arc detector photocell or burned out test bulb (inside output cavity). Test arc
detector functioning with arc detect test button on HPA control panel. If arc detectors fail test
(blinking red LED), remove photocell and bulb assembly from output cavity and repeat test.
If bulb does not light during test, replace bulb. If no voltage is reaching bulb during test,
verify wiring to bulb, verify HPA controller fuses, and replace HPA brain board as necessary.
If bulb is lighting during test, replace arc detector photocell.
The HPA controller records the results of the last arc detector test in memory. Issue
following commands on HPA LCD control screen to access stored arc detector test results:
Information Access > System Operations > HPA Maintenance > Messages > Arc test.
Screen will display three numbers: OFF resistance, ON resistance, and threshold
resistance. For a properly operating photocell and bulb combination, the OFF resistance
should be > 2 Megohm and the ON resistance ~ 1kohm.
Does not apply to 451144-01 type HPA controllers (non-screen).
Transmitter drops to start
mode from operating state
due to arc detector alarm.
Arcing in output cavities due to one of following causes:
Damaged or loose finger stock in output cavities. Disassemble and inspect output
cavities for visible signs of arcing and damage. Inspect RF output stack for
damaged watchband spring bullets.
Dust, moisture, or other foreign contaminants in output cavities. Disassemble and
clean output cavities.
Excessive aural ratio (NTSC/PAL only). Check aural ratio with spectrum analyzer.
Aural carrier must be at least 10dB below peak visual carrier. Adjust aural ratio
with exciter potentiometer B4-17 as necessary.
Incorrect output tuning response (typically too narrow). Check output tuning
response. Retune as necessary. Consult Service Bulletin 030528 or 030615 for
further details.
REV H - 10 January 2007 9

Technical Service Bulletin 030328
Undesired second harmonic mode in RF output stack. Check for presence of high
second or other harmonic levels at IOT output directional coupler. Offsetting cavity
tuning doors or changing orientation of low pass filter may solve problem. Contact
Comark for procedures.
Possible spurious trip of arc detector due to incorrect threshold in HPA controller. The HPA
controller determines the arc detector trip point based on photocell performance during the
self test routine executed when the HPA enters standby mode. If the self-test has been
performed with a malfunctioning arc detector photocell or bulb, the trip point setting may not
have adequate margin for normal operational variances.
Possible spurious trip of arc detector photocell caused by fluorescence (glowing) of output
window ceramic due to electron bombardment. Very rare problem. NEVER attempt to view
output ceramic while tube is operating (X-ray hazard). Contact tube manufacturer for further
instructions.
Transmitter fails to come
ready for beam mode due to
yellow arc detector interlock
alarm.
Arc detector interlock loop interrupted. Check integrity of arc detector connections, magnet
cart focus coil connection, input cavity lid position (where applicable), tube socket sensor
position (where applicable).
Reverse Power
Sudden reverse power alarm.
Multiple HPAs trip
simultaneously in multi-tube
system. Body current alarm
may also be present.
Arc or dead short in RF output system. Infinite VSWR at IOT output gap disrupts normal
operation and causes beam to scatter, thereby also causing excessive body current. Switch
transmitter to dummy load to determine if arcing is occurring in antenna & tower
transmission line or in transmitter RF system. Inspect transmission line for localized hot
spots. Disassemble and inspect any suspect areas of transmission line for internal damage.
If search for damaged component unsuccessful, disconnect HPA reverse power RF sample
cable(s) and allow transmitter to operate into arc for five seconds. Listen for origin of arcs.
Disassemble RF output system and inspect for damaged components.
Sudden reverse power alarm
on single HPA cabinet. Body
current alarm may also be
present.
Arc or dead short in RF output system. Infinite VSWR at IOT output gap disrupts normal
operation and causes beam to scatter, thereby also causing excessive body current. Inspect
HPA output stack and coaxial line for localized hot spots. Disassemble and inspect any
suspect areas of transmission line for internal damage. Concentrate search on watchband
spring bullets, loose bullets, and the harmonic filter.
If search for damaged component unsuccessful, disconnect HPA reverse power RF sample
cable and allow transmitter to operate into arc for five seconds. Listen for origin of arcs.
Disassemble RF output system and inspect for damaged components.
Reverse power readings do
not agree between HPA and
system power meters.
Assuming meter calibrations have not been disturbed, some reverse power meter
disagreement is normal, especially in analog IOX transmitters. This is because the majority
of the power in an analog (NTSC/PAL) signal is concentrated at the visual carrier frequency.
Accordingly, the reflected power reading depends heavily on the VSWR level at the visual
carrier frequency, which is very “location-dependent” in an RF system. It is possible for one
HPA to have a VSWR null at the visual carrier frequency while another HPA has a
maximum. It is also typical for these nulls & maxima to change frequency as the RF system
pattern is changed. As long as all reflected power readings are below 2%, there is no reason
for concern.
Filament, Bias, and Ion (FBI) Supply
Bias current alarm. Bias
current is negative. Contamination of IOT grid with emissive material boiled off cathode. Filament voltage too high. Slightly reduce IOT filament voltage. Report problem to tube manufacturer before
taking action.
Bias current alarm. Bias
current is positive. IOT grid being driven positive due to RF overdrive at input. Find and eliminate cause of RF overdrive. Check output tuning response (too wide response = lower gain). Crowbar may
also fire in extreme cases of transient overdrive.
Lower drive power to 75%. If positive grid current remains, imminent IOT failure due to
internal structural damage is probable. Contact tube manufacturer for further instructions
and to line up replacement IOT.
If positive grid current is instantaneous upon application of high voltage: possible grid
neutralization problems with Comark IOT. Consult manufacturer instructions for proper
REV H - 10 January 2007 10