[NLRS] 8478b thermister head

[Dr.Gerald Johnson]

Could be the coupling capacitor in the thermistor head is shorted and 
when you connect the attenuator you put in a DC ground that's not 
present in the other low power sources. Could be the coupling pin in the 
attenuator is a bit long or off center and pushing the thermistor head 
center pin away from the circuitry. That's not good.

There are four thermistors in the 8748 thermistor head, two for reference 
(ambient temperature) and two with RF applied. They are sensed in 
series but the RF is applied in parallel. They are compared at DC or 10 
KHz depending on the meter model. They are sensitive to humidity and 
storing in a basement or barn will wreck them. Sometimes they can be 
resurrected using a tiny screwdriver inside. There are suggestions for that 
in the HP manual. An that's posted somewhere on line these days.

73, Jerry, K0CQ
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Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.