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.

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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.


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!!

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)

writes:

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.

--
Steven D. Swift, , http://www.novatech-instr.com
NOVATECH INSTRUMENTS, INC. P.O. Box 55997
206.301.8986, fax 206.363.4367 Seattle, Washington 98155 USA