Fwd: [Premium-Rx] HP8640B

Fri Mar 14 12:18:55 EST 2003

Here's a rather long thread that I just dug out of my archives that discusses the problem in more detail.  Note, to return the unit to functional operation you shouldn't have to clean out the grease - rather, use the oven-bake procedure (@ low-temperature!!).  It's MUCH simpler.  Both Dick Benson (W1QG in the correspondance below) and I have used it to success.

- Jeff, WA6AHL

*********************************************

Greetings,
    I have 2 hp8640b cavity oscillators that exhibit temperature dependent erratic behavior predominantly at the low frequency end of the range.

    The frequency stability is poor, and oscillation will stop
    on occasion. Cranking the tuning knob vigorously seems to
    improve things ... for while. The documentation on these
    oscillators is spartan at best since the label says
    "factory service only".

    Has anyone ventured into the guts of these and had the
    patient recover ??

Thanks in advance for any information.

Dick, w1qg

Sender: jjohnson at hpl.hp.com
Date: Wed, 24 Mar 1999 13:35:02 -0800
From: "Jim JJ Johnson (HPL)" <jjohnson at hpl.hp.com>
Organization: Hewlett Packard Laboratories
To: RAB1 at compuserve.com
Subject: HP8640 problem

Hi Dick,
    Your HP8640 problem interested me as I have one myself.  As
luck would have it, I know a guy who was the production engineer
on the product and he still has a few neurons that fire on the old 8640 occasionally, so here is some of the dialog that took place between me, a colleague here at HP Labs, and the guy who worked on the 8640.  The sequence of emails is bottom to top.  I have some more detailed info if you're going to inside the cavity.  Good luck!

Jim W6SC
jjohnson at hpl.hp.com

--------------------------------------------------------------------
  Hi JJ,

     Yes, I use to work on the 8640 a long, long, long, time ago in a galaxy far, far, away.  Rick is right, the cavity oscillator has a non-contact slug inside that moves back and forth to change the frequency.  If I recall right, you can think of the cavity as a shorted transmission line or wave guide and you are changing the electrical length.  When the unit went through environmental test, it was found that a DC electrical path between the slug and the sleeve would kill the oscillator.  This showed up in humidity testing when the water vapor provided the path to short the slug to the sleeve.  Silicon grease was added to the inside of the oscillator to correct this problem.  The slug traveled on Teflon pads and never needed to be lubricated.  Unfortunately, the line forgot this every once and a while, and thought the silicon grease was meant to be a lubricant.  So as you can imagine, if a little grease was good, more was better.  What happens is that the grease will build up on both ends as the slug moves back and forth.  At one end, the grease is not in the cavity and has no effect on the operation.  At the other end, the grease is inside the cavity and will effect the stability of the oscillator.  We saw this on the line as the oscillator was not meeting its retuning specification.  After this many years, who knows what the grease is doing.  My recommendation is to carefully open the oscillator and clean the grease out and replace it with a SMALL amount of new grease.  This might help the oscillator.

     Hope this helps.  Let me know what else I can do.  Give me a call, I might remember more over the next few days.

     Regards,
     Ray

________ Reply Separator ___________________________
Subject: [Fwd: RE: [Fwd: [TEST-EQUIPMENT] hp8640  cavity
             oscillator
Author:  jjohnson-at-hpl (jjohnson at hpl.hp.com)
Date:    3/23/99 1:05 PM

Hi Ray,
Are you familiar with a problem like the guy describes
below Karlquist's reply?

Thanks,
Jim Johnson
jjohnson at hpl.hp.com

-------- Original Message --------
Subject: RE: [Fwd: [TEST-EQUIPMENT] hp8640  cavity oscillators] Date:
Mon, 22 Mar 1999 10:09:43 -0800
From: "Karlquist, Richard" <karlquis at exch.hpl.hp.com> To: "'Jim JJ
Johnson (HPL)'" <jjohnson at hpl.hp.com>

When I was at Santa Rosa a few weeks ago, someone mentioned
how the line technicians hated working on 8640's, especially the
cavity oscillator. I am pretty sure it uses a non-contacting plunger per HP Journal article that I read last month) and hence the "obvious" cause of the problem (corroded contacts) would seem
to be ruled out.  However, it might still have some secondary
contact resistance issues.  The current scheme of things regarding 8640's is to buy several and mix/match parts to get one working one.  Of course, this guy has 2 with the same problem.  He should probably swap with someone with an 8640 with other problems. Of course, we would need to clarify that when he says "oscillation will stop", he means just that. Not merely that no RF appears at the output connector of the whole instrument.
Also, we would need to verify that "frequency stability is poor" is really poor frequency stability, not PM type noise modulation.  Only the oscillator can create FM, but things down stream can create PM.
If he really is working on just the cavity oscillator (not calling the whole instrument a cavity oscillator) then maybe he means what he seems to be saying.

In any event, I haven't looked inside one, but I have looked at the diagram of the oscillator in the HPJ article.

Rick

-----Original Message-----
From: Jim JJ Johnson (HPL) [mailto:jjohnson at hpl.hp.com]
Sent: Monday, March 22, 1999 9:27 AM
To: Rick Karlquist
Subject: [Fwd: [TEST-EQUIPMENT] hp8640 cavity oscillators]

Rick,
  Ever looked at the insides of one of these?
JJ

$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$

Dick Benson wrote:

> Hi Jim
>
>     Thanks **very** much for the info ... it has sparked some ideas.
>
> You're welcome - hope it is all fruitful.

> >> My recommendation is to carefully open the oscillator
> >>  and clean the grease out and replace it with a SMALL amount of
> >>     new grease.  This might help the oscillator.
>
> It is sure worth a try !

I've been thinking about this - I'm in Morgan Hill so it seems that we could get away with NO grease in this cavity, as we are not in a high humidity environment anyway.  If we don't need the
patch, why not leave it out as it causes problems down the road?

>
> >>  I have some more detailed info if you're going to inside the cavity.
> This would be most helpful.   I am currently in Boston, but will
> return to Saratoga late next week. Any hints and kinks would be
> most appreciated.

There was  a special tool they used for opening the cavity, but
nobody has those anymore.  Instead, you can unscrew a collar
that is at one end of the cavity (carefully).  Ray says to be sure not to open up a cone-shaped piece inside the cavity as it contains varactor diodes that are only held on with conductive epoxy.  If you accidently bump one of those, good luck getting it back on there (they used alignment fixtures to put them on).  He says there is a fine tuning rudder-shaped fin inside the cavity - be careful not to damage it.  Sounds like if you're careful and only disassemble it far enough to clean the grease out, you should be OK.  >From your description, it certainly sounds like this could be the culprit. Talk to you when you get back to the Bay Area.
73,
Jim W6SC

>
>
> Thanks very much,
>
> Dick, w1qg

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