at the prospect of servicing your Z3801A oven oscillator? In
this article, Steve Smith, G8LMX (Nottingham, UK) shares his
experience in exploring the inner details of the HP 10811 dual
oven used in the Z3801A GPS receiver.
After many years of using a Sulzer precision oscillator as my
primary frequency reference, I decided it was time to update.
At this time the obvious choice based on accuracy versus cost
was the Z3801A. These appeared in abundance on Ebay, and on
the fourth attempt I secured one. After waiting two weeks it
finally arrived. I built it into a 19" rack chassis, designed
and built a regulated power supply with four sealed lead acid
batteries, put my antenna on the roof, and switched it on.
After a short while the GPS Lock LED lit. "Is that it?"
I thought. After checking the 10 MHz output several times, I
resigned myself to the fact I would have no more fun. There
was no point in using the VLF receiver tuned to the local frequency
reference station as I did with the Sulzer every month, as any
long-term error would most likely be their fault anyway.
Graham Baxter, the program author of GPSCon and also a friend
of mine had purchased a Z3801A some weeks before and had started
writing a program to log the long-term performance of his receiver.
GPSCon at this time was in the early stages of development,
but already showing promise.
From the start, GPSCon showed me that all was not well with
my Z8301A. GPS lock light or not, the EFC count changed erratically,
sometimes more than 1000 counts per day! I felt quite disappointed.
Then I realized this could be far more interesting than using
a paper chart recorder on the Sulzer for two days every month.
With GPSCon I could see changes and trends in real time, while
the performance when locked to GPS far outstripped my Sulzer,
it should be better. So I set to work. After many days of adjusting
my antenna, power supply, receiver positioning and even maintaining
the room at a constant temperature, nothing I did had any affect.
It was time to find out the cause by the only way I knew how....
swap pieces with a known good one. Now where could I find a
good receiver, and someone willing to take his apart?
Graham did the honorable thing and after a day of juggling we
proved conclusively the OCXO was the culprit. Graham's oscillator
had been very well behaved since day one, and in my receiver
after a few hours of plotting with GPSCon it did exceptionally
well. There was only one thing left to do: disassemble my OCXO.
The Outer Oven.
removal of the OCXO is straightforward. Disconnect all leads
from the rear of the receiver and remove the top cover. Carefully
pull the 10 MHz and EFC SMB connectors off the motherboard followed
by the outer and inner oven connectors. Remove the four Torx
screws holding the oven to the chassis bottom, try not to bump
it around as this could induce stresses in the crystal mount.
At this point I created a very clear, clean well-lit area on
the bench, in effect a mini clean room. Remove the screws around
the periphery of the base and carefully remove the base cover.
is the outer oven you can see the orange oven heater film, there
are two of these heaters in series with a total resistance of
around 18 ohms. The wires from the outside world to the inner
oven are wound round the outer oven on foam rubber tape, this
is most likely done to attenuate thermal shocks traveling to
the inner oven. The tape joint in the red wire is just that!
Now remove the outer oven from the casing. I did this by just
pulling gently on the lead-out wires and it all slid out easily.
Keep note of insulation position and oven orientation.
Now comes a tricky part. After removing several strips of yellow
polyester tape, unwind the lead-out wires from the oven. The
foam is stuck in places so care is required. Try not to contaminate
the ovens, the inner oven in particular. Oils from the skin
or bench will evaporate at oven temperatures adding extra thermal
anomalies; you could make it worse! Very carefully lift the
heater tape from the bottom of the outer oven. If you do this
slowly the adhesive remains mostly on the tape. The outer oven
has screws around the bottom just like the oven outer casing.
Remove them. As you get closer to the inner oven things get
more and more stuck together so slow down! The tapes and glues
in my oven still came apart, it just took longer for them to
separate as I pulled. Once the bottom of the outer oven has
been removed the inner oven insulation can be seen.
my oven this was a soft, sticky, black foam rubber material
held in place by yet more yellow polyester tape. Pulling out
this sticky lump was a bit of a worry, but with time and persistence
it eventually came out.
In the photo you can see the outer oven thermistor within the
oven can. There were four thin strips of black foam rubber taped
to the insulation. I assume these were used as packing. After
removal of the insulation the lead-out wires need unwinding,
again being careful not to tear the foam backing. You should
now be down to the basic HP 10811 oscillator and inner oven.
Make a note of the connections and remove the wires and the
small card discs from the pins.
At this point I cleaned the oscillator unit with isopropyl alcohol
to remove the glue residue as the next part involved getting
it a bit hot!
The Inner Oven.
With the inner oven nice and clean its best to sit back and
admire it while you contemplate the next part. Before attacking
the oven with a hot stick (soldering iron), remove the screw
from the frequency adjusting hole. The oven can is soldered
together with the base lip about 4-mm into the can itself. The
method of removing the base will vary depending on the equipment
available to you. As it turned out it came apart nicely. I used
a micro-flame blow-lamp (blowtorch) (gasp I hear) and a desoldering
station with a suction pump.
Although the desoldering station iron is 100 watts, it alone
could not get the solder flowing sufficiently. Using the micro-flame
blowtorch I heated the solder until quite fluid and using the
desoldering iron and pump I sucked the solder away trying all
the time not to overheat the oven. I decided that if it got
too hot to hold at the top of the oven it was probably time
to rest and let it cool down some. After three such rests I
had sufficient solder removed to attempt to lift the lid. I
used a modeling knife blade to crack the remaining solder just
by gently pushing the blade between the oven can and the lid
lip. Then with a small sharp screwdriver, I slowly pried the
lid, bit by bit, until it just popped out. Don't pull!
Under the lid is a thin, easily damaged, flexible ribbon.
This ribbon terminates with pins that push into a socket on
one of the inner oven circuit boards, a potential cause of problems
I suspect. This board is the oven controller with the thermistor
bridge, error amp, heater control, current limit and 10 volt
regulator, although in my oven this turned out to be a 5 volt
regulator. On the sides of the oven are the two heater transistors.
The gold pin in the corner of the board is a ground post.
Four screws hold the assembly together. After removal of these,
the top cover can be removed and a base cover can also be slid
out gently, noting orientation of parts at all times. The two
white wires go to the thermistor mounted in the crystal housing;
these wrap around the middle board.
With the covers removed, the base of the crystal and the glass
piston trimmer can be seen on the top side of the assembly.
was quite surprised to see marks on the adjusting slot of the
trimmer as the oven assembly appeared not to have been apart
The crystal has three wires spot-welded to its pins. The wires
are stranded and did not look well attached so again care is
required. Unsoldering the wires from the PCB allows the oscillator
board to hinge on a flexible connection allowing measurement,
or replacement of parts.
This board contains the oscillator, varicap and the amplitude
adjusting pot. The third board contains the ALC circuit, a voltage
reference and an emitter follower. After basic checks no obvious
fault could be found, but the exercise had paved the way for
later investigation. I removed the crystal, which is seen here
next to a UK penny.
crystal is held in the die-cast assembly with a nut and locking
washer. On my oven this was not very tight!
Inner Oscillator Major Parts
At this point I have to say that my journey into the depths
of the 10811A oven was more out of curiosity than any belief
that I could improve its performance. It has, however, given
me a tiny bit more understanding of the black art of oven design.
The oven was reassembled and put back into service with no apparent
deterioration or improvement in performance. In reality it seems
to be improving on its own. If it continues to improve I estimate
it will be a good one in about 5 years!
I only tack soldered the inner oven because on my next trip
inside I intend to bring out some wires to monitor interesting
signals. For example, if I plot the 6.4 volt regulated varicap
reference I may find a correlation with my wandering EFC plot.
The 5-volt regulator for the thermistor bridge is another potential
candidate. I'm currently building a PIC based A to D interface
to a computer serial port. This will allow plotting and comparison
with GPSCon data plots.
Since Bill, K8CU, has started the Web-Plot page, a selection
of receivers has been available for comparison. In one respect
I should just leave my receiver alone and be content; it is
not the worst!
I hope my contribution will spark interest among receiver owners
to solve the problem of varying performance levels in the receivers.
2002 Steve Smith
determined the cause of his oscillator problem. He adjusted
the crystal oven temperature to match the crystal temperature
turning point. Full details are