For full format paper (including table, picture, graphic etc) Please open at IOT TROUBLE
Trouble Shooting and Repair
Installation diffi culties:
[Q] What do I do if the IOT is diffi cult to seat in the socket or the locking handle does not lock easily?
[A] This can be caused by unlubricated o-rings, damaged RF contact springs or mechanical interference. Follow
the suggestions in the sequence listed below:
• If the o-rings are new, lubricate them with some coolant or other acceptable lube (use no Silicone. based
material). The mating surfaces on the IOT can also be lubricated.
• It is acceptable to press down on the attached split pole pieces (mounted on the IOT) with
approximately 200 lbs.
• Ensure that the RF contact springs are not damaged. Replace any if even slight distortion is noted.
• If the IOT still won’t socket lower the water jacket assembly by loosening the 8 nuts holding it to the
bottom of the lower magnet plate. Back the nuts off to the last 2 or 3 threads. The IOT should be easy
to lock at this point because the o-rings will not be in contact with the IOT. After locking the IOT,
evenly tighten the 8 nuts to pull the water jacket onto the IOT.
• If after lowering the water jacket the IOT still won’t socket, try a dry run with the jacket down and the
RF contact springs removed. If the IOT then sockets, replace the springs with new ones and be very
careful to lower the IOT straight in.
• If the IOT won’t socket with the RF contact springs removed and with the water jacket lowered, call
customer service (See page 5).
Tuning diffi culties:
[Q] How can the input cavity response be moved to the proper frequency?
[A] Check that the shorting pins are properly mounted in the positions noted in the manual (See page 6).
[Q] Why does the input circuit tuning change when the power is increased?
[A] When properly matched, it is normal for the tuning to shift down in frequency as the RF drive is increased.
After tuning the input and output cavities at low power (10 to 50 Watts), raise the power and retune the
input and output respectively. After running several minutes at rated power, the output should be fi ne
adjusted to account for the heating of the IOT and cavity elements.
[Q] What can be done when the output is diffi cult to tune or the response cannot be found after setting
the controls according to the Output Cavity Tuning Guide?
[A] Check that the coarse tuning controls are set to the values recommended for your channel (See Output
Cavity Tuning Guide - page 8).
Check that the correct load blocks for your frequency are in the secondary cavity (See Output Cavity
Tuning Guide – page 8). One can see the load blocks by looking through the load coupler port. This
should be required only if the transmitter channel assignment has been changed. For channel 69, check
that the 3⁄4” output load coupler extension is removed. All other channels require the 3⁄4” load coupler
extension, P/N 725280.
Check that the cavity coupler (iris) is not binding and can be rotated by the adjustment knob located on
the underside of the cavity assembly. Determine that the adjustment control reads zero when it is turned
to its stop.
Check that the front and rear indicators for both the primary and secondary cavities to ensure that the
tuners have not been offset. The front and rear settings should be the same.
Revision D: February 2003 Page: 29
[Q] I can’t adjust to a smooth double hump response.
[A] Check that the system is being operated into a clean 50 Ohm load. Sweep and tune the line/load if
necessary.
Trips or other faults:
[Q] I am getting a lot of faults? What can I do?
[A] Work systematically, The fi rst step is to isolate the problem. Beam overloads, crowbar trips, grid current
overloads etc. can all be caused by faults in the IOT, the input cavity, the HV wiring harness, or the power
supplies. One technique is to sequentially remove components from the circuit. It is helpful to remove the
input circuit from the IOT and check for the fault without the IOT connected, then remove the high voltage
wires from the input circuit etc. To complete such operations consult with the transmitter manufacturer
[Q] What is the normal heater resistance?
[A] Check for the following values:
• The normal hot heater resistance is ~1 Ohm; less could indicate that the IOT has lost vacuum or that a
partial short exists within the IOT.
• The normal cold heater resistance is ~0.1 Ohm; more could indicate an open heater or poor contact in
the circuits.
• The resistance between the grid contact fi ngers and the grid bias supply lead should be ~5 Ohms; much
more or less indicates a fault in the wiring or the “video” fi lter components.
[Q] I am getting excessive negative grid current and crowbar trips. What is happening?
[A] First, to prolong the life of the IOT, it is important to properly manage the cathode current. It is good
practice to determine the nominal heater current, for best IOT life, for each particular combination of
equipment and operating power level. This is best done in the fi eld by measuring some important parameter
such as power output, linearity, sync compression; and for digital applications, shoulder levels or any other
signal degradation while the heater current to the IOT is reduced (see AB24 applications bulletin at the end
of the Trouble Shooting section). At some point, while lowering the heater current, there will be a noticeable
reduction in power output or increase in distortion. Safe operation may be performed at a heater current
slightly higher than that point at which performance appeared to deteriorate (see Figure 5 in Theory of
Operation Section). However, the operating power on the cathode should never be set lower than 70 Watts.
Second, if lowering the heater current doesn’t improve the situation, make the following tests:
• Check the resistance between the grid and the cathode. Normally the resistance on the IOT between
the grid and cathode, or heater contacts, is very high. Any value greater than 10k Ohms is acceptable.
Lower values (especially < 1k Ohm) indicate that the grid to cathode insulator has been contaminated
and so has surface leakage. This condition will result in negative grid current even if bias is applied
without the heater on. See the corrective action section below.
• A large capacitor is connected between cathode and grid within the input circuit. The capacitor is
protected by a 200v MOV. With 150 volts applied between grid and cathode, leakage could indicate
bad video fi lter components or grid to cathode insulation failure.
• Grid emission can occur if the surface of the grid becomes an emitter due to contamination by the
cathode. To avoid this failure ensure that the heater current is optimized according to the procedure
above. Also, the heater must never be operated above the current level noted on the data sheet unless
Revision D: February 2003 Page: 30
it is reaching the end of it’s life and the above adjustment procedure is followed. If grid emission is
occurring the grid current will go more negative with HV applied and generally also more negative with
drive.
• If the input circuit (removed from the IOT and with the open end supported away from ground) has
a fault to ground the 3 most likely offenders are the leads (check for carbon or dust on the insulation),
the high voltage blocker disk, or the 40 kV grid to ground capacitor. Note that low level leakage to
ground can show up as negative grid current.
[Q] Why do I have low emission and limited output power?
[A] This can indicate normal end of cathode life, which should not occur below 30 or 40k hours. If the IOT
has tripped off with a crowbar or beam overload it is possible that the cathode has been contaminated with
material from an arc (Arcing through the tube is always an indication that the protection circuitry failed. This
must be repaired to prevent further damage to the tube). In such a case the emission will usually recover over
a several minute period. If the arc was very bad (check for crowbar failure), a day may be required for
recovery. During the recover period don’t push the IOT hard or additional arcing may occur.
[Q] What is causing arcing in the output cavity?
[A] Output cavity arcing is usually indicative of improper tuning, output line reflfl ections, worn contact fifi ngers,
or cavity modes. The usual failure mode of the photocells is safe (resulting in false trips); so the fi rst step
should be to check or replace the cells. Most of the other faults will result in a swept output response that
is not a smooth double hump. If the response can not be corrected by normal adjustment of the tuning
controls and the line sweeps clean, output cavity maintenance is indicated.
[Q] How can unstable power output be corrected? The input level is stable.
[A] Unstable power output has often been caused by RF feedback into the transmitter power supplies or RF
leaking at the IOT or cavity contacts (fi nger stock damage). Move the DC supply umbilical cable while
watching the power supply and power output meters. Variation while moving the umbilical generally
indicates RF leakage of some type. Similarly, move or tap on the input circuit to see if any power variations
result. If so, check that all cables are properly connected. Check the heater, cathode and grid fi nger stock
contacts. Any broken contacts can cause RF leakage. Output cavity leakage can be caused by damaged
fi nger stock in the cavity doors.
[Q] On NTSC service, the picture is torn, streaky or has ghosts.
[A] A “torn” or “streaky” picture may indicate a “glitch”, which is a frequency and power dependent power
consuming mode in the IOT. Always check the drive to the IOT for the anomaly to ensure that the IOT is
the culprit. To determine if the IOT is glitching, look at a differentiated ramp test signal. A discontinuity
that moves with magnetic focus current is a glitch. The IOT focus current should be set to a level where
glitching does not occur. The focus current can safely be adjusted +/- 2 amps from the value specifi ed on
the IOT data sheet to see if the fl uctuation can be altered. If the IOT is in warranty (or otherwise) this
condition should be reported to the factory, because it is not normal. The heater current calibration should
also be checked.
[Q] I have had several crowbar tripoffs. The tube will not hold-off high voltage.
[A] First, the crowbar circuitry needs to be thoroughly checked and repaired. Arcing has occurred through the
tube. After you are sure that the protection circuitry is working correctly, tap the high voltage down to the
lowest value possible and apply high voltage to the tube. Applying voltage may have to be tried several times.
Once the tube can hold the high voltage let it stabilize for a period of time (1/2 hour). Check the ion pump
Revision D: February 2003 Page: 31
current. If there is more than normal current, leave the tube at this voltage until the current comes down.
Move up to the next higher voltage tap and repeat the previous step until the tube will hold off the full rated
voltage. Take your time. This process cannot be hurried.
Once the tube will hold off the rated high voltage, turn-on a small amount of RF. Increase the RF slowly. If
the system trips-off, back down to a lower value of RF and let the system stabilize. Slowly increase the RF to
rated power. This process should not be hurried - the best results come from patience.
If the tube still cannot hold off high voltage at any of the lower power supply taps or if the tube cannot run
with RF, contact Eimac Field Service.
[Q] I recently had a problem with one socket of my three socket IOT transmitter. When I took the failing
socket offl ine and increased the power in the remaining two sockets, they started having tripoffs.
What is happening?
[A] Any change in the transmitter’s operating parameters changes the tube operating environment once the tube
has reached an operating equilibrium. Changing output power causes this equilibrium to change. The
key is to make changes slowly, allowing the tube to reach new equilibrium slowly. The normal tendency of
immediately trying to bring your transmitter up to full power may actually take longer than using the slow
approach. Increase power in slow steps and allow at least an hour before increasing another increment. If
any tripoffs occur, slow the process down.
This gradual process should also be followed when it comes time to later reduce the power after your third
socket is brought back on-line.
If the power is being signifi cantly lowered (i.e., half power), the high voltage should also be reduced to lower
levels. For instance, if the high voltage is normally 34 kV and the power is reduced to half power, readjust
the high voltage to 27 kV. Then adjust the quiescent (idle) current to obtain the desired intermodulation
distortion values desired. Heater current should also be adjusted and monitored according to AB24,
included in operators manual.
If power is being increased signifi cantly, the high voltage, quiescent current and heater current should be
adjusted accordingly.
Taking corrective action:
• If trouble shooting indicates that repairs to the input/output circuits or other hardware is required contact the
factory because many fi eld replaceable parts and assemblies are available with installation/assembly instructions.
• If the IOT is causing crowbar trips, check the crowbar circuitry for proper operations and then operate the tube
at reduced voltage and power for a period of time. Then gradually increase it back to full power.
• If a small DC high voltage power supply (no more than 15 Joules of energy storage) is available, the IOT can be
checked / high voltage processed independently of other components as follows:
1. Remove the input circuit
2. Connect jumpers between all contact points on the IOT’s gun (grid, cathode, heater, ion-pump)
3. Connect the power supply to ground (clamp on the input circuit clamp ring) and the grid (clamp on
the raised seal ring surface on top of the gun). If possible connect the negative lead to the grid and the
positive to ground.
Revision D: February 2003 Page: 32
4. Slowly raise the supply voltage, resetting after trips, until 45 kV DC can be reached and held with no
trips for some length of time.
• If excessive negative grid current is causing trouble it is essential to determine the source of the current. First,
determine if the leakage is in the IOT or circuit components. If the circuit or wiring is at fault, take normal
corrective action. If the current “leakage” is in the IOT, fi rst check the calibration of heater power (an over
powered heater will lead to premature IOT failure). Then, determine if the grid-cathode insulator is at fault by
checking the resistance. ----Firing the crowbar will not pass current through the IOT-----
• If it is determined that true grid emission is at fault, every effort should be made to ensure that the heater power
is as low as it can be. Lower the heater voltage in .25 V increments until the signal can no longer be acceptably
corrected. Over a period of time the negative grid current should diminish (see AB24 applications bulletin).
• If the negative grid current was not noticed until trips occur frequently, the recovery process can be accelerated
by cleaning the grid of the contamination (see next page). This is accomplished by applying DC power to the
grid. The procedure listed below can be used, if required, but it should be noted that the requirement to “grid
scrub” is not normal and indicates some sort of fault that should be corrected. Once the grid has been cleaned
monitor the operation for a tendency toward negative grid current. If the IOT starts again toward negative
current implement heater current management as above.
NOTE: This procedure will cause the tube to become “dirty” (foreign material in the
tube). The tube will have diffi culty holding off high voltage at the end of the
scrub. Tap down to the lowest high voltage setting and bring up HV very
slowly.
Revision D: February 2003 Page: 33
Grid cleaning procedure: (used to remove contaminate from the grid surface)
• All the work here is to be performed without high voltage applied: shut it off and lock it out!
• With the system off and cool connect a power supply between the grid and cathode as shown below.
• Switch on the heater, and allow it to reach operating temperature.
• Set the scrub supply voltage at ~ 24 volts (a 24 volt battery pack can be used).
• Momentarily (~ 0.5 to 1 second) close S1.
• Note that the ion current may jump up.
• Allow the ion current to drop back to the original level (plus 1 μ amp max.).
• Adjust the voltage and repeat the procedure until the grid dissipation = 35 to 40 watts. (Grid dissipation = total
(E*I) loss minus the 5 * (I2*R) loss).
• Repeat closing S1 for longer intervals until it can be closed for 2 minutes w/o a gas burst.
• Return circuit to normal confi guration.
• Switch on heater and bias and confi rm reduction / elimination of negative grid current.
• Run at less than normal high voltage and at 50 to 80% power out for several hours.
• Set all back to normal.
Figure 11: Grid Scrub Schematic
Trouble Shooting and Repair
Installation diffi culties:
[Q] What do I do if the IOT is diffi cult to seat in the socket or the locking handle does not lock easily?
[A] This can be caused by unlubricated o-rings, damaged RF contact springs or mechanical interference. Follow
the suggestions in the sequence listed below:
• If the o-rings are new, lubricate them with some coolant or other acceptable lube (use no Silicone. based
material). The mating surfaces on the IOT can also be lubricated.
• It is acceptable to press down on the attached split pole pieces (mounted on the IOT) with
approximately 200 lbs.
• Ensure that the RF contact springs are not damaged. Replace any if even slight distortion is noted.
• If the IOT still won’t socket lower the water jacket assembly by loosening the 8 nuts holding it to the
bottom of the lower magnet plate. Back the nuts off to the last 2 or 3 threads. The IOT should be easy
to lock at this point because the o-rings will not be in contact with the IOT. After locking the IOT,
evenly tighten the 8 nuts to pull the water jacket onto the IOT.
• If after lowering the water jacket the IOT still won’t socket, try a dry run with the jacket down and the
RF contact springs removed. If the IOT then sockets, replace the springs with new ones and be very
careful to lower the IOT straight in.
• If the IOT won’t socket with the RF contact springs removed and with the water jacket lowered, call
customer service (See page 5).
Tuning diffi culties:
[Q] How can the input cavity response be moved to the proper frequency?
[A] Check that the shorting pins are properly mounted in the positions noted in the manual (See page 6).
[Q] Why does the input circuit tuning change when the power is increased?
[A] When properly matched, it is normal for the tuning to shift down in frequency as the RF drive is increased.
After tuning the input and output cavities at low power (10 to 50 Watts), raise the power and retune the
input and output respectively. After running several minutes at rated power, the output should be fi ne
adjusted to account for the heating of the IOT and cavity elements.
[Q] What can be done when the output is diffi cult to tune or the response cannot be found after setting
the controls according to the Output Cavity Tuning Guide?
[A] Check that the coarse tuning controls are set to the values recommended for your channel (See Output
Cavity Tuning Guide - page 8).
Check that the correct load blocks for your frequency are in the secondary cavity (See Output Cavity
Tuning Guide – page 8). One can see the load blocks by looking through the load coupler port. This
should be required only if the transmitter channel assignment has been changed. For channel 69, check
that the 3⁄4” output load coupler extension is removed. All other channels require the 3⁄4” load coupler
extension, P/N 725280.
Check that the cavity coupler (iris) is not binding and can be rotated by the adjustment knob located on
the underside of the cavity assembly. Determine that the adjustment control reads zero when it is turned
to its stop.
Check that the front and rear indicators for both the primary and secondary cavities to ensure that the
tuners have not been offset. The front and rear settings should be the same.
Revision D: February 2003 Page: 29
[Q] I can’t adjust to a smooth double hump response.
[A] Check that the system is being operated into a clean 50 Ohm load. Sweep and tune the line/load if
necessary.
Trips or other faults:
[Q] I am getting a lot of faults? What can I do?
[A] Work systematically, The fi rst step is to isolate the problem. Beam overloads, crowbar trips, grid current
overloads etc. can all be caused by faults in the IOT, the input cavity, the HV wiring harness, or the power
supplies. One technique is to sequentially remove components from the circuit. It is helpful to remove the
input circuit from the IOT and check for the fault without the IOT connected, then remove the high voltage
wires from the input circuit etc. To complete such operations consult with the transmitter manufacturer
[Q] What is the normal heater resistance?
[A] Check for the following values:
• The normal hot heater resistance is ~1 Ohm; less could indicate that the IOT has lost vacuum or that a
partial short exists within the IOT.
• The normal cold heater resistance is ~0.1 Ohm; more could indicate an open heater or poor contact in
the circuits.
• The resistance between the grid contact fi ngers and the grid bias supply lead should be ~5 Ohms; much
more or less indicates a fault in the wiring or the “video” fi lter components.
[Q] I am getting excessive negative grid current and crowbar trips. What is happening?
[A] First, to prolong the life of the IOT, it is important to properly manage the cathode current. It is good
practice to determine the nominal heater current, for best IOT life, for each particular combination of
equipment and operating power level. This is best done in the fi eld by measuring some important parameter
such as power output, linearity, sync compression; and for digital applications, shoulder levels or any other
signal degradation while the heater current to the IOT is reduced (see AB24 applications bulletin at the end
of the Trouble Shooting section). At some point, while lowering the heater current, there will be a noticeable
reduction in power output or increase in distortion. Safe operation may be performed at a heater current
slightly higher than that point at which performance appeared to deteriorate (see Figure 5 in Theory of
Operation Section). However, the operating power on the cathode should never be set lower than 70 Watts.
Second, if lowering the heater current doesn’t improve the situation, make the following tests:
• Check the resistance between the grid and the cathode. Normally the resistance on the IOT between
the grid and cathode, or heater contacts, is very high. Any value greater than 10k Ohms is acceptable.
Lower values (especially < 1k Ohm) indicate that the grid to cathode insulator has been contaminated
and so has surface leakage. This condition will result in negative grid current even if bias is applied
without the heater on. See the corrective action section below.
• A large capacitor is connected between cathode and grid within the input circuit. The capacitor is
protected by a 200v MOV. With 150 volts applied between grid and cathode, leakage could indicate
bad video fi lter components or grid to cathode insulation failure.
• Grid emission can occur if the surface of the grid becomes an emitter due to contamination by the
cathode. To avoid this failure ensure that the heater current is optimized according to the procedure
above. Also, the heater must never be operated above the current level noted on the data sheet unless
Revision D: February 2003 Page: 30
it is reaching the end of it’s life and the above adjustment procedure is followed. If grid emission is
occurring the grid current will go more negative with HV applied and generally also more negative with
drive.
• If the input circuit (removed from the IOT and with the open end supported away from ground) has
a fault to ground the 3 most likely offenders are the leads (check for carbon or dust on the insulation),
the high voltage blocker disk, or the 40 kV grid to ground capacitor. Note that low level leakage to
ground can show up as negative grid current.
[Q] Why do I have low emission and limited output power?
[A] This can indicate normal end of cathode life, which should not occur below 30 or 40k hours. If the IOT
has tripped off with a crowbar or beam overload it is possible that the cathode has been contaminated with
material from an arc (Arcing through the tube is always an indication that the protection circuitry failed. This
must be repaired to prevent further damage to the tube). In such a case the emission will usually recover over
a several minute period. If the arc was very bad (check for crowbar failure), a day may be required for
recovery. During the recover period don’t push the IOT hard or additional arcing may occur.
[Q] What is causing arcing in the output cavity?
[A] Output cavity arcing is usually indicative of improper tuning, output line reflfl ections, worn contact fifi ngers,
or cavity modes. The usual failure mode of the photocells is safe (resulting in false trips); so the fi rst step
should be to check or replace the cells. Most of the other faults will result in a swept output response that
is not a smooth double hump. If the response can not be corrected by normal adjustment of the tuning
controls and the line sweeps clean, output cavity maintenance is indicated.
[Q] How can unstable power output be corrected? The input level is stable.
[A] Unstable power output has often been caused by RF feedback into the transmitter power supplies or RF
leaking at the IOT or cavity contacts (fi nger stock damage). Move the DC supply umbilical cable while
watching the power supply and power output meters. Variation while moving the umbilical generally
indicates RF leakage of some type. Similarly, move or tap on the input circuit to see if any power variations
result. If so, check that all cables are properly connected. Check the heater, cathode and grid fi nger stock
contacts. Any broken contacts can cause RF leakage. Output cavity leakage can be caused by damaged
fi nger stock in the cavity doors.
[Q] On NTSC service, the picture is torn, streaky or has ghosts.
[A] A “torn” or “streaky” picture may indicate a “glitch”, which is a frequency and power dependent power
consuming mode in the IOT. Always check the drive to the IOT for the anomaly to ensure that the IOT is
the culprit. To determine if the IOT is glitching, look at a differentiated ramp test signal. A discontinuity
that moves with magnetic focus current is a glitch. The IOT focus current should be set to a level where
glitching does not occur. The focus current can safely be adjusted +/- 2 amps from the value specifi ed on
the IOT data sheet to see if the fl uctuation can be altered. If the IOT is in warranty (or otherwise) this
condition should be reported to the factory, because it is not normal. The heater current calibration should
also be checked.
[Q] I have had several crowbar tripoffs. The tube will not hold-off high voltage.
[A] First, the crowbar circuitry needs to be thoroughly checked and repaired. Arcing has occurred through the
tube. After you are sure that the protection circuitry is working correctly, tap the high voltage down to the
lowest value possible and apply high voltage to the tube. Applying voltage may have to be tried several times.
Once the tube can hold the high voltage let it stabilize for a period of time (1/2 hour). Check the ion pump
Revision D: February 2003 Page: 31
current. If there is more than normal current, leave the tube at this voltage until the current comes down.
Move up to the next higher voltage tap and repeat the previous step until the tube will hold off the full rated
voltage. Take your time. This process cannot be hurried.
Once the tube will hold off the rated high voltage, turn-on a small amount of RF. Increase the RF slowly. If
the system trips-off, back down to a lower value of RF and let the system stabilize. Slowly increase the RF to
rated power. This process should not be hurried - the best results come from patience.
If the tube still cannot hold off high voltage at any of the lower power supply taps or if the tube cannot run
with RF, contact Eimac Field Service.
[Q] I recently had a problem with one socket of my three socket IOT transmitter. When I took the failing
socket offl ine and increased the power in the remaining two sockets, they started having tripoffs.
What is happening?
[A] Any change in the transmitter’s operating parameters changes the tube operating environment once the tube
has reached an operating equilibrium. Changing output power causes this equilibrium to change. The
key is to make changes slowly, allowing the tube to reach new equilibrium slowly. The normal tendency of
immediately trying to bring your transmitter up to full power may actually take longer than using the slow
approach. Increase power in slow steps and allow at least an hour before increasing another increment. If
any tripoffs occur, slow the process down.
This gradual process should also be followed when it comes time to later reduce the power after your third
socket is brought back on-line.
If the power is being signifi cantly lowered (i.e., half power), the high voltage should also be reduced to lower
levels. For instance, if the high voltage is normally 34 kV and the power is reduced to half power, readjust
the high voltage to 27 kV. Then adjust the quiescent (idle) current to obtain the desired intermodulation
distortion values desired. Heater current should also be adjusted and monitored according to AB24,
included in operators manual.
If power is being increased signifi cantly, the high voltage, quiescent current and heater current should be
adjusted accordingly.
Taking corrective action:
• If trouble shooting indicates that repairs to the input/output circuits or other hardware is required contact the
factory because many fi eld replaceable parts and assemblies are available with installation/assembly instructions.
• If the IOT is causing crowbar trips, check the crowbar circuitry for proper operations and then operate the tube
at reduced voltage and power for a period of time. Then gradually increase it back to full power.
• If a small DC high voltage power supply (no more than 15 Joules of energy storage) is available, the IOT can be
checked / high voltage processed independently of other components as follows:
1. Remove the input circuit
2. Connect jumpers between all contact points on the IOT’s gun (grid, cathode, heater, ion-pump)
3. Connect the power supply to ground (clamp on the input circuit clamp ring) and the grid (clamp on
the raised seal ring surface on top of the gun). If possible connect the negative lead to the grid and the
positive to ground.
Revision D: February 2003 Page: 32
4. Slowly raise the supply voltage, resetting after trips, until 45 kV DC can be reached and held with no
trips for some length of time.
• If excessive negative grid current is causing trouble it is essential to determine the source of the current. First,
determine if the leakage is in the IOT or circuit components. If the circuit or wiring is at fault, take normal
corrective action. If the current “leakage” is in the IOT, fi rst check the calibration of heater power (an over
powered heater will lead to premature IOT failure). Then, determine if the grid-cathode insulator is at fault by
checking the resistance. ----Firing the crowbar will not pass current through the IOT-----
• If it is determined that true grid emission is at fault, every effort should be made to ensure that the heater power
is as low as it can be. Lower the heater voltage in .25 V increments until the signal can no longer be acceptably
corrected. Over a period of time the negative grid current should diminish (see AB24 applications bulletin).
• If the negative grid current was not noticed until trips occur frequently, the recovery process can be accelerated
by cleaning the grid of the contamination (see next page). This is accomplished by applying DC power to the
grid. The procedure listed below can be used, if required, but it should be noted that the requirement to “grid
scrub” is not normal and indicates some sort of fault that should be corrected. Once the grid has been cleaned
monitor the operation for a tendency toward negative grid current. If the IOT starts again toward negative
current implement heater current management as above.
NOTE: This procedure will cause the tube to become “dirty” (foreign material in the
tube). The tube will have diffi culty holding off high voltage at the end of the
scrub. Tap down to the lowest high voltage setting and bring up HV very
slowly.
Revision D: February 2003 Page: 33
Grid cleaning procedure: (used to remove contaminate from the grid surface)
• All the work here is to be performed without high voltage applied: shut it off and lock it out!
• With the system off and cool connect a power supply between the grid and cathode as shown below.
• Switch on the heater, and allow it to reach operating temperature.
• Set the scrub supply voltage at ~ 24 volts (a 24 volt battery pack can be used).
• Momentarily (~ 0.5 to 1 second) close S1.
• Note that the ion current may jump up.
• Allow the ion current to drop back to the original level (plus 1 μ amp max.).
• Adjust the voltage and repeat the procedure until the grid dissipation = 35 to 40 watts. (Grid dissipation = total
(E*I) loss minus the 5 * (I2*R) loss).
• Repeat closing S1 for longer intervals until it can be closed for 2 minutes w/o a gas burst.
• Return circuit to normal confi guration.
• Switch on heater and bias and confi rm reduction / elimination of negative grid current.
• Run at less than normal high voltage and at 50 to 80% power out for several hours.
• Set all back to normal.
Figure 11: Grid Scrub Schematic
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