[NLRS] 8478b thermister head

[Mike]

Hi Jerry, Barry, all, I think this is on the right track.  I just put a 
DC block in line with it and then the attenuator and it works fine.

Does anyone have any idea if this is something I'm likely to be able to 
correct myself or is it really magic inside there and I'm best to leave 
it alone???  ;)

Thanks
Mike
KB0ozn

Dr. Gerald Johnson wrote:

> 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