On other instrumentation -equipment I've encountered in 50 year of Field Engineering, a FAN fault and the fan is spinning, can mean the control circuitry has failed --and the fan can be in an always on condition, or partially on.. or air flow impediment.
A $12 fan many times is less expensive than a set of finals..
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Years ago, I had a customer in a hospital.. They kept blowing one of a pair of 4CX5000's in push pull in an Xray generator.. after about the 4th set, I asked to set in for an entire day to see if I could be there if the problem occured and maybe gain insight to a permanent fix.
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7am I was there and so was the early morning X-ray Tech... We had coffee while the 1st patient was being brought down.? When the patient arrived? the Xray tech placed his newspaper on top of the X-ray generator - a 6 foot cabinet and went to position the patient. After positioning the patient, he returned and took an exposure or two.....A 4CX5000 finament is 10 volts at 51 amps.. That's? a Kilowatt just to heat the filaments...the fan had circuity to vary speed as the generator mostly sits in standby.
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Well, guess what? He laid the morning? newspaper on top of the cabinet right over the chimneys to those 4CX5000's....impeding air flow.. then the morning patients would line up on gurneys and production would start.?
Had I not been there I would of never known as after the circuit breakers would pop, he return to reading the newspaper..and no air restriction,? it usually took 20-30 exposures to blow a tube in a "rush hour" situation.? Since they were in push pull, it was both sides that would blow, but only 1 tube would be lost.
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The design of a 4CX5000 X-ray Generator, much like any Ceramic Tetrode Amplifier, the chassis of the tubes were pressurized by a blower fan and the air, escaped by running past the fins of the 4CX5000 and directed by the glass chimneys out the top.
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The fan was about 20-30 inches from the class chimney holes on the top of the cabinet.. So the fan circuitry was working fine..
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The cause a 20 cent newspaper. I don't even know what a 4CX5000 cost back then.. maybe $2000 they were Emiac.After that they ran over a year without a hick-up.
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Larry Macionski