[Test-Equipment] re: HP8640B audio oscillator thermistor

[Grant, Ken]

 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 ( PTC )  
 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
that it produces
 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 used a single CM type 7387 from point B to point A and two 7371s in
series from point A to     
 point C. Set R28 for about 200 Ohms. The oscillator should start right up.
Adjust R28 for 4.5
 Vp-p at test point 4.

  The 7387 is rated for 28V, 40 mA and the 7371 is rated for 12V, 40 mA. You
should be able to  
 substitute any incandescent lamps that will fit in the available space.

  The HP assembly is an obsolete part ( and totally unavailable, of course )
.

  Best of luck & 73.

  Ken, VE3FIT

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