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I now have two of the HP 651b Test Oscillators that are unstable at on
the lowest frequency range. From 5Hz to 90Hz they will oscillate for a
moment, stop, and start up again. Waveforms from each unit are
distorted. They both do fine on the higher ranges.

Before I go in and start fiddling around, is someone out there familiar
with this particular problem? The service manual says to check
frequency alignment, which I will do, but I wanted to check first since
they're both afflicted with the same malady.

 

[Tweetldee]

I now have two of the HP 651b Test Oscillators that are unstable at on
the lowest frequency range. From 5Hz to 90Hz they will oscillate for a
moment, stop, and start up again. Waveforms from each unit are
distorted. They both do fine on the higher ranges.

Before I go in and start fiddling around, is someone out there familiar
with this particular problem? The service manual says to check
frequency alignment, which I will do, but I wanted to check first since
they're both afflicted with the same malady.

Frequency alignment is the first thing you should do before opening the
case. It will tell you a lot about the instrument.

It's been many years since I was inside one of these generators, but as I
remember, it is basically a solid-state RC Wien-bridge oscillator. If the
RC network in the bridge is not balanced properly, the distortion will be
high, and frequency alignment will be off. There is a level control in the
main oscillator circuit too... but I don't remember if it's adjustable. All
will exhibit problems similar to yours if something is wrong in that area.
Things to look for:
Check all electrolytic capacitors in the unit, especially in the area of the
oscillator
Make sure the power supply is good.. voltages close to spec, very low
ripple/noise
Check the values of the capacitors and resistors in the bridge, especially
on the lowest range
Check the values of resistors in the oscillator
Transistor gain may have changed

--
Dave M
MasonDG44 at comcast dot net (Just subsitute the appropriate characters in
the address)

Never take a laxative and a sleeping pill at the same time!!

 

[Steven Swift]

The 651B requires following their cal procedure to the letter to stop
this motorboating. Even the slightest change in stray capacitance
will cause a problem. The manual even shows how to make a shield to
let you do the final cal with the case on.

Don't fiddle around. It took me hours to get back to working after
touching the interactive controls.

I have a djvu file of the manual: http://novatech-instr.com/Fun/hp652a.djvu
(Sent to me by another kind usenetter).
(The 652A is the same except for a level expander circuit)

 

[W1xyz]

The 651B requires following their cal procedure to the letter to stop

this motorboating. Even the slightest change in stray capacitance

Hi and thanks to both who responded with helpful info. I've studied the
oscillator circuit from the djvu 652a file and the rest of the manual, and I
have an original service manual on the way too. Next week I'll start on the
cal procedure per the specs. Sure seems touchy though.

Thanks to all.
Bob
w1xyz