[Test-Equipment] HP 10811A - 5335A

[Mike Feher]

Talking about the 10811A series and its predecessors, I have found that the
thermal fuse in these often goes bad (opens up). In every case, and I must
have repaired about 10 at least, the oscillator and oven was just fine, it
was only the fuse that opened up. It is no surprise considering that a lot
of these ovens are left on even when the equipment is shut off. In my
opinion, someone real clever thought up the design of these fuses, however,
they can fail even if the marked temperature is not exceeded. These fuses
work on the principle of two conductors being in contact with a small spring
wanting to push them apart. You would think the spring would succeed, but,
the cylinder is filled with wax and allowed to cool while the connection is
still intact. The wax composition is changed to set opening temperature.
Once the temperature is exceeded the wax becomes liquid and the spring
separates the contacts. Of course this shuts the oven off solidifying the
wax again, but, in the open position. I think years of being in the high
temperature environment allows the wax to become malleable enough for the
spring to break the contact. So, if you have one of these oscillators, and
it does not work, take it apart, remove (and check) the fuse, and, either
replace it with a new one or put a piece of buss wire in its place. The fuse
typically is the size of a 1/2 watt, or slightly smaller resistor, with a
metal cylinder shell, that plugs into two wire sockets one on each end of
the fuse. Regards - Mike



Mike B. Feher, N4FS
89 Arnold Blvd.
Howell NJ, 07731
(732) 901-9193


----- Original Message -----
From: "Bob Parnass, AJ9S" <parnass@megsinet.net>
To: <test-equipment@mailman.qth.net>
Sent: Sunday, December 15, 2002 6:07 AM
Subject: Re: [Test-Equipment] RE: HP 5335A Counter Need Help!


> Option 010 for the HP 5335A counter consists of:
>
> A15 oven oscillator assembly, p/n 10811A (qty 1)
> 6-32 x 5/16 screw, p/n 2360-0115 (qty 2)
> fiber washer, p/n 3050-0005 (qty 2)
>
> There is a separate operating and service manual
> for the 10811A/B quartz crystal oscillator.
> The manual p/n is 10811-90002