[R-390] update: HP8640B RF Sig Gen Leakage

[Bob Camp]

Hi,

Hey, good job!!!

This is the same sort of thing they used to have to do back at Motorola on
the same gear fresh from HP. The bottom line seems to be that most people
don't check really low levels with this kind of gear.

    Take Care!

        Bob Camp
        KB8TQ


----- Original Message -----
From: "Randy Zelick" <zelickr@biohazard.pdx.edu>
To: <r-390@mailman.qth.net>
Sent: Monday, April 14, 2003 1:48 AM
Subject: [R-390] update: HP8640B RF Sig Gen Leakage


> Hello again,
>
> Thanks to all who made suggestions to remedy the generator leakage
> problem. It looks like the leak is plugged!
>
> Here is the tale....
>
> I made a sniffer out of some double-shielded coax and a miniature antenna
> at the end. Then I tested the generator at 440 MHz where the problem was
> the worst, using a very sensitive 2-way radio as the detector.
>
> There was RF floating around the case at different points, but the worst
> leak by some margin was coming out along the shaft of the freq calibration
> knob. This is the tiny knob just to the right of the frequency display. If
> you turn it, the "uncal" light comes on. Next to that is a little hole for
> frequency calibration.
>
> So I popped the hood to look for problems. This 8640 is quite a nice one,
> just a year out of cal with the stickers still over case screws. Clean as
> can be inside. So rough use and/or corrosion was not a likely issue.
>
> I took the cover off the frequency counter section which is also where the
> calibration control shaft enters. Inside there is a small spring leaf that
> grounds the shaft, which became immediately suspect. It was already clean,
> but I cleaned it more and added a little stabilant. Still RF leaking out.
>
> Then I re-surfaced the edges of the casting which forms the cover of the
> counter section and tightened all the screws down. Still leaking.
>
> Finally, I made my own shaft grouding gizmo from a piece of shim bronze. I
> captured it with the screw that faces front and holds the two halves of
> the counter case at their seam just right of the display. The other end I
> notched so it would rub against the shaft. Unlike the HP grounding shim,
> mine is mounted on the outside of the counter case. No leak!!
>
> I sniffed all around the 8640 and discovered that in addition the top
> cover must be very well secured, otherwise more minor RF leaks present in
> the interior get out.  These leaks seem to be coming from several sources,
> which I did not track down. But with the outer covers in place, I realized
> satisfactory behavior.
>
> The bottom line? I am a bit perplexed. Here is a generator of the highest
> quality and certainly appears as though attention was paid to every detail
> of construction. Solid castings held with many screws, gold plated circuit
> boards, best coaxial interconnects, and so forth. My specimen is extremely
> clean, unmodified, unabused and yet *barely* can keep RF from leaking out.
> Only with every little detail of grounding perfect does it not radiate. No
> overdesign there!
>
> I might speculate that the counter is the problem. The 8640 started life
> with a slide-rule frequency display (the A model) and likely was
> retrofitted with the digital counter to make the B model. Perhaps it was
> just too early in the days of RF counter design to seal all the RF leaks.
> Just for fun I ran the generator with the cast lid completely off the
> counter module. GOBS of RF pour out. C'mon HP!
>
> As I final note I want to plug the perverse obsession I have of saving
> little mechanical parts from cannibalized equipment. There is an old
> military transit case in the corner of my shop where this variety of
> schmutz accumulates. I'm sure it will be the first thing my kids pitch in
> the dumpster when I go to the big test bench in the sky, but it took but a
> few minutes of rooting through the six inches of levers, pawls, shafts,
> couplings, washers and (very) etc to come up with the magic grounding
> shim.
>
> So there you have it.
>
> Cheers,
>
> =Randy=
>
> --
> R. Zelick email: zelickr@pdx.edu
> Department of Biology voice: 503-725-3086
> Portland State University fax:   503-725-3888
>
> mailing:
> P.O. Box 751
> Portland, OR 97207
>
> shipping:
> 1719 SW 10th Ave, Room 246
> Portland, OR 97201
>
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