A very good point!
I checked the output amplitude with a spectrum analyzer, a power meter
and a scope, the latter with a 50 ohm load to the input.
I re-checked the power supply connections and can confirm they are the
same as before.
I used three different power supplies for the oscillators and two for
the heaters, still no difference.
But...
To make it short, your comments helped to get me back on track. Both
beauties escaped unnecessary dismantling and are now working as they should.
Actually, the problem was caused by a long-term misunderstanding. I was
always wondering why 'HP used simple stranded wire' and not coaxial
cable on the 10 MHz output and EFC.
As long as I connected them on the bench, I replaced the 'missing'
ground connection with a short wired croc clamp between coax and ground.
Today I realized that the thin blue wire IS actually coax cable, so I
should have connected the coax shield to the BNC ground on my newly
built 2x 10811 enclosure. With the new wiring, the output ground had
just become much more inductive, up to a point where the nominal source
impedance of 50 ohms had increased to over 120 ohms, causing the
amplitude loss of some 3 dB and capturing noise.
Btw. the Sprague 6800 uF / 40 V from my R&S NGA power supply has indeed
died, but independently of the osc. problem.
Adrian
[email protected] schrieb:
Perhaps a silly question, but as I get the impression that both seem to
have failed simultaneously with the same fault I'm just wondering if you're
sure it isn't what you're checking them with that's developed a problem?
Regards
Nigel
GM8PZR
In a message dated 13/10/2012 13:52:46 GMT Daylight Time, [email protected]
writes:
Hi All,
both of my double oven 10811A's have been running flawlessly until now
when I noticed that the output power has dropped by about 3 dB (measures
only 4...4.5 dBm at 50 Ohm load), and the noise floor has gone up by
about 20 dB, while the frequency is still spot on.
I used them for various mesurements, so I can unfortunately not remember
what might have caused the failure. Only the 12V linear lab power supply
that fed the heaters had developed a completely dead filter cap, so the
feeding voltage was a 100 Hz sawtooth rather than DC. The heaters appear
to be still working though. I'm only using the internal heater. They
draw some 700 mA at power up and drop to around 150 mA each when warm.
So the oscillators are unlikely to be cooked inside. Something must have
happened to the 10 MHz output, possibly an ESD issue?
Before I start taking them apart, can anyone tell what has happened or
what to look for first?
Regards,
Adrian
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