The insulation turns brown with age even in the absence
of smoke.

Rick N6RK

"Fred McKenzie" wrote in message
...
Ovens
very rarely run away. It is far more likely the fuse will fail
or its socket will corrode (can't solder it in because the solder
would melt the fuse). If the oven does run away, the heater
transistors will open up and serve as fuses

Rick-

I've seen photos of these ovens on E-Bay, that had been stained by smoke
coming
out of the adjustment hole. I'd rather have some kind of protection.

I believe the oven uses proportional control, so the transistors' maximum
dissipation would occur when the heating element is half on. In a
"runaway"
mode, the transistors would be switched on with maximum current but nearly
zero
voltage. Also, one transistor failure mode is a short-circuit.

With regard to John Miles' comment about the thermal fuse being to far
from the
oven's heating element to be effective, perhaps that is true. However,
the
earlier thermal fuse was rated at 108 degrees C, and it occasionally would
open
in an oven that was apparently operating correctly in the range of 80 to
84
degrees C. The newer fuse is rated at 115 degrees C. I suspect the
problem is
that it is opening due to a combination of time and temperature, not
temperature alone.

I've been running one of the new parts for about two years without a
hitch.
The frequency has not been adjusted since about two years ago, and it
still
takes 15 or 20 seconds to drift one Hz against a 10 MHz rubidium
oscillator.

That HP 5334B is one nice counter!

73, Fred, K4DII

In article ,
Fred McKenzie wrote:

Rick-

I've seen photos of these ovens on E-Bay, that had been stained by smoke coming
out of the adjustment hole. I'd rather have some kind of protection.

I believe the oven uses proportional control, so the transistors' maximum
dissipation would occur when the heating element is half on. In a "runaway"
mode, the transistors would be switched on with maximum current but nearly zero
voltage. Also, one transistor failure mode is a short-circuit.

Seems like a self-resetting "Polyswitch" overcurrent limiter might be
a workable alternative. You'd probably want to pick one whose "will
pass" current is somewhat above the highest amount of current that the
oven would draw when it's quite cold. If the transistor shorts, and
the oven draws more current than that, the polyswitch would heat up
enough to go high-Z and chop off the current. Cutting power for a
minute or so would reset it.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

In the very early days of newsgroups (musta been in the early late 1980's --
back when I had a Compuadd 286-25) there was a post put together from an HP
Engineer and an HP Sales Guy -- to the extent that you could order a tuna
sandwich, 8566A, but mayonaise OPT-010030 was an additional $0.30, if you
wanted the sandwich sliced, that was OPT-010031 for which you had to pay
$0.05. Of course, you could have the sandwich diagonally sliced, and on and
on.

If you filled out your registration cards you could get updates on the menu,
but once tuna fish sandwiches were discontinued spare parts could not be
reordered.


"Rick Karlquist N6RK" wrote in message
news:rKXHc.53423$MB3.51741@attbi_s04...
I was the project manager of the 5334B frequency counter.
It had an option to have a 10811 timebase. The standard
timebase, which I inherited from the 5334A design was
embarassingly bad, barely able to do 10 PPM. Unfortunately,
you cannot retrofit a 10811 to a 5334, because you need an
extra PC board. This board is required to be able to put
the 10811 on its side, because there isn't enough height for
it.

The gray market in used HP/Agilent test equipment is
really hurting Agilent in some products. My old division no
longer exists. I now work at Agilent Labs.

Rick N6RK

project manager for the
"John Walton" wrote in message
...
One of the great things which the telecom bust did was to make the
equipment
you designed at HP available to us experimenters -- I use my HP3586C
Receiver's ovenized oscillator to drive my HP5334 frequency counter

The runaway oven problem isn't caused by too much transistor
current, rather it is caused by the transistor staying on too long.
Thus a current based system wouldn't work. Also, in the case
of the 5334A, the power supply is barely able to supply minimum
current for warmup, so the oscillator couldn't draw more than
normal warmup current even if it tried. In the 5334B, I improved
the power supply so that it warms up the oscillator in half the
time or less compared to the 34A.

Rick N6RK


"Dave Platt" wrote in message
...
In article ,
Fred McKenzie wrote:

Rick-

I've seen photos of these ovens on E-Bay, that had been stained by smoke
coming
out of the adjustment hole. I'd rather have some kind of protection.

I believe the oven uses proportional control, so the transistors' maximum
dissipation would occur when the heating element is half on. In a
"runaway"
mode, the transistors would be switched on with maximum current but
nearly zero
voltage. Also, one transistor failure mode is a short-circuit.

Seems like a self-resetting "Polyswitch" overcurrent limiter might be
a workable alternative. You'd probably want to pick one whose "will
pass" current is somewhat above the highest amount of current that the
oven would draw when it's quite cold. If the transistor shorts, and
the oven draws more current than that, the polyswitch would heat up
enough to go high-Z and chop off the current. Cutting power for a
minute or so would reset it.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Hi All,

I had thought the output transistors are the heaters; Q7 and Q8.

There's a couple low wattage resistors in series with transistors for some protection too.

Regards,

Bill Hall
N6TKC