[Test-Equipment] RE:HP audio oscillator thermistor

[Craig C. Heaton]

 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

Hi Ken,

I broke out the dremel tool & cut-off wheel yesterday then removed some of
the epoxy. I used a file to smooth the surface and added a drop of water on
the surface. Using a small flash light and magnifying glass, I could see two
small bulbs in series that are used between points B and C. They were still
intact. The interesting discovery was the bulb used for the circuit between
points A and B. The glass had been removed from the bulb before the filament
and the other two bulbs were imbedded in the epoxy!

HP must of manufactured several versions of this audio osc board. R28 is a
fixed resistor on my board and varies slightly from the manual downloaded
from Agilent. I'll have to take a closer look and see if there are shiny
solder joints near what I think is R28. Maybe someone has made a mod on the
board?

The AUD LEVEL and AM-FM ADJ pots are still there, with notations on the
board. Nothing at all for a OSC LEVEL ADJ.

Still hoping for an entire board replacement, does anyone here on this list
know the number of revisions of the audio board? The HP8640B is a 3050A
series.


Ken, thanks for the information on bulb replacement. I'll give it a try.

73's
wd8kdg
Craig