Hi Roman,

see below....

Roman Black schrieb:
>
> Jochen Feldhaar wrote:
>
> > I operate a spectrum analyzer model HP 853A with a 8558B RF unit, which
> > has a strange problem around the first LO, a YIG oscillator.
>
> > When switching on the unit, the noise floor on the screen is 20 dB above
> > the usual value, from 0 thru 700 MHz, then declining to normal at 900
> > MHz and higher. The effect will vanish if I tune to a CF of 1100 MHz or
> > higher. It will appear only if the unit has been in daily use for my
> > usual 8-10 hours, for several weeks continuously.
>
> A common failure mode of semiconductors (especially
> in small signal transistors), is a heat related fault
> where after sustained heating they produce a lot of
> noise. Get the unit hot with a blanket and see if the
> fault occurs quicker, then try freezer spray on the
> individual components in that area.

It is immediately after switching on - and the memory effect needs weeks
of daily operation to make it occur, and it will even vanish after 2
months of not using the unit..... seems not like a transistor gone
bad....

Apart from that you are right.
PNP's seem to have a higher inner resistance, so your observation is
absolutely valid!

Greets Jochen
>
> I've replaced countless small noisy transistors in
> amps and older TVs, mainly appliances used for long hot
> hours. And for some reason (lower Vcb ?) they are mainly
> PNP small signal types. If your freezer spray points
> to a component also try resoldering it first, you can
> get noisy solder joints in hot equipment. And if you
> trace it to a large expensive module, maybe heatsinking
> or fan cooling it might save you some bucks? You do have
> adequate cooling for the appliance now? ;o)
> -Roman
>
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