[time-nuts] HP Z3805A GPSDO Oven Heater Voltages

John Stuart j.w.stuart at comcast.net
Thu Apr 24 13:15:44 EDT 2014

Thank you for your help on this.  It appears that either; the CPU has a bad
output pin, some unknown command has turned the heater off, or the firmware
is programmed that way.  The 10 MHz frequency had always been very 'stable',
and I didn't know there was a problem until I bought a 2nd HP Z3805A and
felt how warm the (50C) insulated double oven should feel.
I am trying to communicate with the seller in Hong Kong.  So far he hasn't
acknowledged that the CPU output pin can turn-off the heater.
By the way, your projects on    <http://risums.net/hjem/oz9mo/index.html>
http://risums.net/hjem/oz9mo/index.html  are very interesting and described
very clearly.  Nice work!

John, KM6QX


From: Jarl Risum [mailto:jarl.risum at gmail.com] 
Sent: Thursday, April 24, 2014 5:23 AM
To: John Stuart
Subject: Re: HP Z3805A GPSDO Oven Heater Voltages


Thanks for the feedback and congratulations with your result so far. 

Unfortunately I have no further information regarding the function of the
outer oven control signal from the processor board. You are probably right
in assuming that the signal might be used to prevent the outer oven from
overheating in case the thermistor went open, since the processor on the
main board do monitor the outer oven heater voltage through P2/9.
Speculating further: It might also be used by manufacturers to switch off
the outer oven heater circuit permanently in HP Z3805's using the 5 MHz MTI
OCXO if a suitable RS-232 command exist for this purpose.

Cheers from OZ9MO / Jarl 

in Denmark

Some of my Time-Nuttery:  http://risums.net/hjem/oz9mo/index.html

2014-04-24 3:13 GMT+02:00 John Stuart <j.w.stuart at comcast.net>:

Jarl, OZ9MO
Thank you for those two links!  They have the information I needed, and are
very well 'hidden' on the internet.
I have had partial success.  Per the work-around on www.realhamradio.com, I
pulled P2-8 on the power board up to +5V, and the outer heater came on and
is now being controlled by temperature controller on the power board; TP104
is modulating around 15.75 V.
However the heater on/off control signal from the main circuit board
(normally connected to P2-8) is still OFF at 0.0 V.  For some reason (maybe
firmware) the units CPU is not turning on this signal.  I am thinking about
permanently wiring +5V to P2-8 to keep the heater enabled.  
Why would the outer oven ever be turned OFF ??  Maybe to prevent
over-temperature damage if the NTC sensor fails?
When I bought this "HP Z3805A" from the Hong Kong seller a couple of years
ago, it was advertised as having been upgraded to the features (firmware?)
of the HP(Symmetricam) 58503A.  Does anyone else have this 'model' and does
the Double Oven XO keep the unit's top very warm, like it should?
John Stuart, KM6QX
Lafayette, CA


From: Jarl Risum [mailto:jarl.risum at gmail.com] 
Sent: Wednesday, April 23, 2014 2:22 AM
To: j.w.stuart at comcast.net
Subject: HP Z3805A GPSDO Oven Heater Voltages

Hi John

You will find a comprehensive description of the outer oven heater circuit


In my HP Z3805A the outer oven circuit is identical to the circuit in HP

A description of a Z3801A outer oven fault similar to the one you have can
be found here:


You will also find a suggestion for a simple remedy.

Good luck.

Cheers from OZ9MO / Jarl

in Denmark

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