[time-nuts] HP Z3805A GPSDO Oven Heater Voltages

John Stuart j.w.stuart at comcast.net
Thu Apr 24 01:13:35 UTC 2014

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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