Suggestions from Successful Users
received a note from Mitchell Janoff "I wanted to thank
you for posting the information on the HP Z3801A on your web
site. I recently purchased one of these units from a gentleman
in Korea who was also selling on Ebay. I might have been more
cautious if I had read your web site before my purchase. It
turns out I was pretty lucky. My unit is factory wired for 110v
AC (uses a standard PC type power cord) operation and also has
a standard 9 pin RS232 input. Since it also came with the antenna,
setting it up was a snap.
I did need to get a null modem for the connection to the computer,
but otherwise I didn't have any problems. Tom Van Baak gave
me the UTC diag. Instruction and the reboot tip. Thanks again
for providing a valuable resource.
information from Chuck Zabilski, WB6MOB, on modifying the receiver
to use RS232:
just modified a Z3801A GPS receiver and I discovered a way to
get it to interface at the RS-232 level and not have to resort
to RS-422. The main board has a set of uninstalled headers marked
RS-232 and RS-422 respectively near the DB-25 connector. These
are arranged as 3 rows of 8 connections on 0.100 inch spacing.
By installing 3 rows of 8 male headers I am able to select between
the RS-422 and RS-232.
In terms of the jumpers and headers, I could only verify that
the 1st 5 actually connected anywhere, but I went ahead and
installed all 8 headers time 3 rows. The only other thing required
is to flip the board over and remove the five 0 Ohm jumpers
which preselected the RS-422 interface. Once these surface mount
jumpers (resistors) are removed, the newly installed headers
select RS-422 or RS-232."
addition, Chuck WB6MOB modified his receiver to have a self
contained power supply:
"Also the Volgen SPN75 power supply fits (barely) within
the GPS receiver (towards the front). I installed an IEC 320
AC power connector in place of the DC connector and the receiver
is self contained for AC in one package."
K8CU notes: This is a commercial
switching power supply: Volgen SPN75-48S
from Digikey part number
note from Ken, W6GHV
: He has identified an economical switching
supply that fits inside the Z3801A case. Jameco has a 48 V @
1.35 Amp switching power supply for $29.95, as their part number
201953. The power supply mating connectors are their part numbers
104432 and 104731 (page 79 of their printed catalog). This is
an open frame switcher.
WB6MOB also offered the following on selecting UTC time:
"There has been some comment about how to switch the Z3801A
from reporting GPs time to UTC time. A document from Symmetricom
discusses an undocumented command :diag:GPs:utc 1 to switch
from GPS to UTC (:diag:GPs:utc 0 will switch from UTC to GPS).
Although issuing this command this will subsequently report
that the time mode is UTC, the receiver needs to be reset or
rebooted for the change to take effect.
Newer firmware supports a :syst:pon command older firmware does
not. Rather than unplug or power down the receiver, the self
test command *tst? will perform a reboot of the receiver."
K8CU notes: I tested this on my
receiver, and it appears to work. A reboot of the receiver requires
that GPs acquisition be established again. This takes the receiver
a minute or so to complete.
received a note from Dennis Polito, W6DEN:
"I wanted to thank you for the fine article regarding this
GPS receiver. The information that you compiled made the conversion
and start up process relatively easy. I recently purchased one
from Hi-tech Cafe and I was able to get it up and running last
night. I have a couple of questions for you.
The 10 MHz output as measured on my HP 5345A is a bit high,
10.00000131 to be precise. I compared the output to a HP 5061A
Cesium standard and it is indeed off frequency. Satstat reports
FFOM as a value of 1. If and when this value reaches 0 will
the 10 MHz output then be correct? How long does it normally
take for FFOM to achieve 0? I have had the unit on for about
3 hours as the longest power on cycle. I ran the survey and
the receiver is now running in hold mode.
Answer - In the manual, HP states that
"the receiver typically reaches stable state 24 hours after
power-up, and it will learn best if its experiences no holdover
in the first 24 hours....thus, it is recommended that the receiver
is always kept locked to GPS during the first 24 hours."
When FFOM (Frequency Figure of Merit) reaches zero, the 10 MHz
output frequency characteristics should meet accuracy specifications
of < 1 x 10-9, one day average.
Feedback: Dennis later reported to me that another Z3801A was
tested and had the same apparent frequency offset. He then found
that the synthesizer in his cesium standard was the source of
My antenna has approximately 70' of RG-58 feed line, should
there be a feed line delay factored in? If so, do you know what
the delay factor is for this feed line? I get excellent signal
strength numbers on all six satellites.
Answer - Since
I use my receiver only for frequency purposes, and not for timing,
I did not enter my specific feed line numbers into Satstat.
That is why the Satstat photo above shows
an antenna delay of zero ns. The delay factor is related to
the specific velocity factor of your coax. If the cable you
use has no additional signal delay time caused by the cable
dielectric material (a velocity factor of 100%), the delay factor
would be the time it takes light to travel the length of the
coax. Light travels almost exactly one nanosecond per foot.
RG/58 with a polyethylene dielectric (not foam) has a velocity
factor of 66%. The specific calculations:
of Light = 186,284 miles per second.
(feet per mile) X 186,284 = 983,579,520 feet per second
X (10 -9 ) = .98357952 foot per nanosecond
x 66% = .6491624832 (speed through RG/58)
(one over x function) = 1.5404463842 or 1.54 ns per foot delay
for your length of cable.....70 feet of RG/58 has a time delay
of 107.8 nanoseconds.
HP manual gives some examples of other cables and various lengths.
The Satstat clock reports a +13 second error. The query "show
accumulated leap" reports +13 seconds. Can this value be
reset to 0?
Answer - You are set up to view GPS time.
Setting the time to reflect UTC time (see note by WB6MOB above)
will correct this apparent error. Since
GPS time is "perfect", it is off from UTC by the number
of leap seconds that have been added to UTC since GPS time began
in January of 1980. As of today, there have been 13 leap seconds
added to UTC to compensate for variations in the earth's rotational
speed. More information on this can be seen on the U.S.
Naval Observatory web site.
Ward, W5LF reports:
I have two of these fine devices and the Ebay ads say they run
on 32 to 48 vdc. I can tell you plainly that neither of mine
will even start without 48v or more on them, with or without
adding 7-8000pf at 70 of electrolytics across the dc input (which
didn't work for me). Mine will start at 48 vdc but won't run
at less than 46.7 volts or so.
notes:The Z3801A is specified by HP (depending upon model) as
either a nominal minus 54 or plus 27 volt device. The best bet
is to keep the voltage at the nominal value specified by HP.
This way any out of specification internal DC to DC converters
are satisfied, and the units will perform normally. Reports
from other users confirm this.
had an email exchange a few weeks ago regarding a power supply
problem with the Z3801A. I will copy portions of it here:
I have just had a very strange and distressing thing happen,
and I'd like to ask you if you have any thoughts. I have two
Z3801s, bought on eBay some weeks ago. I checked them out with
an antenna and computer when I got them, and they seemed to
work flawlessly. I put them aside 'til tonight, when a power
supply I ordered was available. This is a serious HP multiple-output
supply, 0-50V, .8amps.
I connected it to one unit and found the supply voltage jumping
around, a flaw I attributed to the switchers inside the Z3801.
Putting a 1000uF cap across the leads fixed that, and the voltage
settled down to a perfectly steady 48.00V and .560 amps. My
intention was to wait 'til the oven warmed up and see how much
lower the steady-state current would become. Although I had
no antenna or computer connected to the Z3801, it seemed to
power up OK. So far, so good. However a few minutes later, the
unit died! No lights, zero power drain. Power supply still reading
Not having a clue, I figured maybe something went wrong inside
the Z3801, so I connected the second one. It powered up, the
current read .559 amps, and all was well. Until, a couple of
minutes later, it, too, died! Now they're both dead. I have
no schematic. I see no fuses. And I can't imagine what I could
have done to kill them. OTOH, the coincidence of both dying
minutes after power was applied is too much to believe.
So, and I hope you'll forgive me for the unsolicited inquiry,
have you ever heard of anything like this? I have the manual
for the unit, but no PS or other schematic, and I'm reluctant
to rip it apart 'til I've at least asked someone familiar with
My response was this: Okay,
from my standpoint, your power supply is the culprit. Do NOT
use it anymore until you are certain what is going on. The good
news is that there are fuses inside the unit, and they are in
the primary circuit. Go to my web-site, and scroll down to the
part of the page that shows the Z3801A power supply. I describe
two fuses in the primary circuit. These are different values,
but look like little resistors. They are called "Pico" fuses.
Look in the lower left hand side of the photograph to see them.
An ohm-meter check on them will reveal what their condition
is. There is another fuse described in the circuit, but it is
used in the outer oven controller.
Hopefully, all that has happened is that your power supply has
taken out the primary fuse(s). I suggest building up another
power supply (transformer-diode bridge-big cap) and control
the voltage with a variable variac on the transformer primary.
Then test your receivers with this.
"Ripping it apart" as you describe is no big deal. All you need
is a Torx size 10 driver, and simply remove the top cover. Your
DC to DC converter is visible, and is right on top. You can
check the fuses without removing anything inside the box. Try
this, and let me know what you come up with.
interesting return email response:
Hi again! Thought you'd be interested in the resolution of this
1: There's nothing wrong with the power supply
2: There isn't and wasn't anything defective with either Z3801.
Rather, the power supply is TOO GOOD, and there's a subtle design
defect in the Z3801. Key is the power supply rating: .8A maximum
current, and the fact that the Z3801 uses switching supplies.
A switcher characteristic is one of "negative resistance" on
the input. To provide a constant power output, as the input
voltage decreases, the input current increases. It turns out
that with a 48V supply, the Z3801 takes ~.56 amps on turn-on.
However, after a few minutes, the current drain sharply increases,
presumably because the oven supply is switched on. With a normal
high-current supply (or CO battery), this extra few hundred
MA wouldn't be noticed. But with a lab supply set to .8A maxiumm,
when the oven turns on, the supply is called on momentarily
to exceed its output rating. It goes into current limiting,
and the output voltage starts to drop.
Enter the Z3801 switcher: As soon as it sees the supply voltage
drop, it will try to take more current, causing the voltage
to drop even faster. This negative feedback cycle drains the
PS output capacitor (and the cap that I added for good measure)
fast enough to exceed the energy rating of the picofuse which
protects the capacitor by blowing out!
figuring out this scenario on the way home, I tried a very simple
experiment. I INCREASED the PS voltage from 48.00 to 50.50V,its
maximum rating. At this voltage, the maximum current (briefly)
seems to be about 780mA, and it eventually equilibrates at around
440mA. The "subtle design defect" is the fact that there is
no current limiter (other than the fuse) on the input of the
switcher. Of course it could be argued that if one uses a heavier
supply it's not necessary, but there could certainly be an occasion
where primary power is interrupted with a big capacitor remaining
across the Z3801 input.
Finally, the most astonishing thing about this adventure is
that it only took me a couple of minutes to find a big reel
of 3A Picofuses in the stockroom!