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TD Control System


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Dear Forum,

I have encountered a TD Control problem that bothering me for the past 3 weeks and still not resolved. We have a test cell for testing both -15 Bare and QEC engines couple to a Water Brake for power adsorption. All these while have been working fine until recently. Consecutively 4 QEC engines failed test as a result of the same problems:

1. Engine unable to run to take off TIT of 1077°C when Throttle Lever at 90 degrees
2. Unable to perform Normal Limiting Check as Relay unable to activate.

The strange part is the above checks will fail only when the QEC engines were tested with Raven Solid State TD Amp (P/N: 23052609A). As part of the troubleshooting, we used the oldBendix TD Amp (P/N: 5502636-4) and all were working perfectly. Extensive troubleshooting has been performed for all QEC engines with a quick summary as follows:

No. 1 QEC Engine – Unable to achieve take off TIT of 1077°C. TIT starts to trim down at 1060°C

Components changed include Raven TD Amp (replaced twice), QEC Relay box, J3 Lead between TD Amp to TD Valve, TD Valve, Thermocouple Cable, Thermocouple Terminal Block, Coordinator & complete Thermocouple set.
Verified QEC wiring and no fault found

No. 2 QEC Engine – Unable to achieve take off TIT of 1077°C. TIT starts to trim down at 1063°C

As FCU outlet pressure was observed to be too high, it was replaced and same problem persisted. This is the only engine passed test after replacing the TD Valve

No. 3 QEC Engine – Unable to achieve take off TIT of 1077°C. Actual TIT observed at 1082°C and unable to trim down

Components changed include Raven TD Amp (3 times), TD Valve, J3 Lead and Thermocouple set. Also carried out Null orifice adjustment on TDV to bring down the scale by 1.5 and onsite verification of the potentiometer for the Coordinator with no fault found.

No. 4 QEC Engine – Unable to achieve take off TIT of 1077°C. TIT starts to trim down at 1067°C

Components change include Raven TD Amp (changed twice). Also adjusted the Null Orifice on TDV by increasing 2 major scale. Carried out adjustment on BIAS & SLOPE setting on TD Amp by increasing 2 and 8 clicks respectively. TIT is able to achieve but failed Normal Limiting Check as Relay unable to cut in.
At this juncture, we decided that it could be problems with the power supplies or grounding as the Solid State Raven TD Amp may be sensitive to power drop or noise interference. 2 dedicated external Power supplies (one for 115 VAC, 400 Hz & the other 28 VDC) were connected direct to the J4 connection to the TD Amp. A dedicated Ground Cable was connected from the TD Amp all the way to the building ground.

With all the above efforts, TD Control system still failed to function as desired. I am totally running out of ideal. Can anyone shedding some light what I can do further to get out of this difficult situation?

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  • 1 month later...

Over several years I have found this problem on most of the solid state amps. In my test cell, I have a Bendix amp that I use for

troubleshooting TD system problems. I have no idea what causes the solid states to do this. My test cell generally runs off

the base 400hz generators and i have a large rectifier for the dc supply

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Solid state or "switching" power supplies for AC voltages may be the problem. Something in the realm of harmonics, which seem to now plague even residential electrical systems. Grab a ground power unit (motor generator) from the flight line and try that as your source for AC ad DC power.


Just a hunch.

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Just to give an update, we are far from solving the problem and continued to fail QEC engines testing from the Test Cell only with the Raven TD Amplifier. Further action and troubleshooting on the past weeks on the following yielded not better result:

-Run engine with Ground Power Unit (GPU) from Flight Line for the AC supply. Problem persisted

-Verify TD Control System using another TD Cal Set. No fault found

-Sent both Bendix and Reven TD Amplifiers to the Components shop to verify the operation using a master test set. Both passed test.

In the interim, we released engine tested with Bendix Amplifier and pending operator to feedback on ground run result after the QEC engine is fitted to the aircraft with the Raven Amplifier.  

In my opinion, it must be a test cell related Power supply problem. We have so far focus very much on the AC Power supply. Can anyone share will the DC supply could contribute to this problem? If pjrv99 has any recommendation. Please feel free to enlighten me as the pressure is building up badly on my end.

Edited by TC2
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TC2, I wish I could give you a positive answer and shout out "EUREKA! We have it!" But alas we are going thru the

same thing right now. My guys are changing TD valve as a 2nd to last resort, as even my good amp is not performing

to spec. DC voltage I am happy with at 26.6v,and both hobart and /60 GPU ouputting 115v 400hz.


The only other suggestion i can make at this time is to swap over your indicator and amplifier leads on the T-block

and see what happens. There should not be more than 6°C difference at take off. We have found several thermocouple

harnesses bad in this way over the last year or so

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pjvr99, we did the swapping of the Indicator and amplifier leads before as what you suggested with no effect to the control problem. In fact, we have 2 indicators on our test console, one reading the TIT and the other reading the temperature to the TD Amp. Both reading the same with difference less than 3°C. Only thing that I am concerned is when we did the TD Cal.  we observed that the Fixed Phase voltage of 90 V for the Bendix Amp but only 80 V for the Reven. I am not too sure this will affect the motor function on the TD Valve but again the limits under the test procedures is acceptable as long as it is over 75 V. What we intend to do during the next engine test is to measure the output voltage to all the coils (Motor and Generator) of the TD Valve via the jumper connection for the J3 lead that we fabricated and compare the difference between the Bendix and Reven Amp as I am also thinking that if the voltage is too low, movement of the plunger inside the TD Valve may be affected due to sticky gears and bushings.  


Edited by TC2
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