HP 491C Microwave Amplifier
Problem
The unit would power on, but only the power indicator and cooling fan were working. The travelling wave tube (TWT) was not receiving any bias voltages or filament power. All the transformers received no power supply.
An inrush current limiting thermistor was missing and after replacement the filaments of all the vacuum tubes would warm up. All the high voltages required for the travelling wave tube were still not present.
Diagnosis
The high voltage transformer was not receiving power. This was traced to the delayed power up relay and filament protection relay having dirty connections with the sockets.
With this issue rectified, after the initial turn on delay, several relays would switch on and the high voltage transformer and voltage doubling rectifier would output approximately 1400V.
A pair of GE 8068 vacuum tubes are used to regulate the high voltage bias for the travelling wave tube, the glass envelope of one fell apart during working on the high voltage PCB.
The cap connection and seal was very fragile and appeared to have degraded, but without the tube getter showing signs of air getting in.
After replacing the pair of tubes with a new matched pair, the high voltage regulated supply worked again and the front panel cathode current meter showed some current was flowing in the TWT. Adjusting all the bias voltages according to the service manual and markings on the TWT, the cathode current rose to the rated power marking on the meter.
Solution
Replacing a missing thermistor restored power to the transformer supplying filament power.
Cleaning the terminals of the two socketed relays on the high voltage board restored the normal function of the turn on delay of the high voltage supply.
New 8068 tubes allowed the regulated high voltage supply to function again. During adjusting the high voltage to 994V needed by the TWT, another problem appeared. A 0A2A tube began to arc across the connecting wires inside the base of the tube. A short in a 12B4A tube was causing this. With both these tubes replaced the high voltage supply was working correctly and the voltage was possible to adjust as needed.
At this stage the TWT was providing the advertised 30dB of gain but the gain control was not varying the grid voltage, so external modulation and gain control was not functional.
Finally swapping a 6U8A which was serving as a differential amplifier for the gain and modulation control restored full function. Now the amplifier as a whole functions according to the manufacturer’s specifications.
If you are experiencing similar issues and would like to discuss or request a quote, email us at
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