[Test-Equipment] re: HP 8640B audio oscillator thermistor

[Ken & Marlene]

Hi Craig,

 I went through the same problem. I managed to 'unpot' enough of RT1 in my
'8640B to recognize the innards.Those are shown as thermistors on the
schematic, but in fact they are plain old Chicago Miniature light bulbs!
They're used as positive temperature coefficient thermistors. Only one bulb
survived and I couldn't make out it's type number so the rest was detective
work.

 The circuit is called a Wein Bridge Oscillator and it's claim to fame is a
stable sine-wave output. I found a good description of this circuit at
www.sound.westhost.com/project22.htm. As the author points out, you will
notice a momentary "bounce" in the output amplitude when changing frequency.

 I haven't been able to find my restoration notes, but it appears that I
used a CM type 7371 from point B to point A and a single 7371 from A to C.
This should let the oscillator start up. Then adjust R28 for 4.5 Vp-p at
test point 4.

 HP used a string of three lamps, but I don't think that we'll ever know
what type they were. The assembly is an obsolete ( totally unavailable, of
course ) part.

 Best of luck & 73.

Ken, VE3FIT