[time-nuts] question about HP 5601a harmonic generator

[Richard (Rick) Karlquist]

I extensively studied the 5061 harmonic generator when I
was designing the harmonic generator in the 5071A.
We are now going on 30 years since that work.  The diode
had some HP part number.  Even if you knew this part
number, you would need to have the Part Information Report
microfiche that gave the vendor part number.  The
chances of this diode still being available are very slim.
So you are not going to get an exact replacement, except
by cannibalizing another 5061 harmonic generator assembly,
assuming it doesn't also have a bad diode.  As far as
replacing it with a different diode, that too would be
questionable.  SRD's have magic unspecified parameters
when it comes to what they do in a circuit.  The 5061
generator in particular has extremely high efficiency
due to its unique design.  It was designed by a Korean
professor named Soon Choi (IIRC) who spent a summer
at HP.  He left no known documentation as to how the
design works, or how to specify the SRD.

After a long period of study, I concluded that I was
never going to match the efficiency in any simple
circuit.  The diode mounted directly in the waveguide
is the magic.  Fortunately, by that time, we could use
a phase locked DRO and get around this problem.  Len
Cutler in his usual style criticized me for "taking the
easy way out", but he eventually came around to my
way of thinking.

I will give you a little hope.  The harmonic generator
consists of a bunch of silver plated parts that are
supposed to fit together and achieve electrical conductivity.
After many years, the silver and tarnish and upset the
electrical conductivity.  You might try disassembling
the diode mount and cleaning up everything.  After
you reassemble it, you might have good news.

I will say that I have never heard of an SRD failing,
and as you say, it looks good at DC.  What possible
failure mechanism could selectively make the diode
stop generating harmonics but other wise be OK at DC?
Somehow let the pixie dust escape without letting the
smoke escape.  Seems unlikely.

Also check the drive circuitry.  SRD's need tricky components
to make them "step" correctly.  Again, poor grounding due
to corrosion could spoil the harmonic generation.

Rick Karlquist N6RK
Member of the 5071A design team

On 12/24/2017 3:54 AM, Kejia Lee wrote:
> Dear All,
> 
> Happy Christmas!
> 
> May I know if anyone knows the part number or the specification of the step
> recovery diode in the harmonic
> generator for the old HP 5601A cesium frequency standard?
> 
> I recently received this toy. The cesium tube seems to be OK, and I had
> located the problem being
> no 9GHz from the waveguide. I checked the diode, the DC properties seems to
> be OK, but just
> can not generate frequency comb any more. I really hope I can repair it as
> a personal new-year gift.
> 
> I will be really appreciated, if you had any suggestions.
> 
> Best regards,
> 
> K.
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