[time-nuts] HP 10811A failure

[WarrenS]

Sounds like another source of this confusion is that there is more than one 
version of the HP10811 inner heater circuit.
Where as most 10811's start loosing performance at around 15 volts on their 
inner oven.
I have one 10811, that was taken out of a dual oven version, that maintains 
full temperature regulation with under 10 volts on its inner oven circuit.

When I opened them up, the main difference I saw was:
On the unit that needs the higher heater voltage, the circuit is as shown in 
the manual's schematic,
U2 = 10 V and R17 = 10K. R17 is used to reduce the bridge voltage to ~  5V.

On the unit that works at lower heater voltages, R17 = 0 Ohms, and U2 has a 
5V output.
Both circuits give the same 5 volts across the bridge, and both where set to 
operate at approximately the same temperature and both had similar factory 
temperature trim resistors in them.

BTW the 10811's outer oven will work fine with under 10 Volts on it. I drive 
mine from a simple home built linear temperature controller.

ws

******************
On 10/14/2012 1:19 PM 8:30 AM, Richard (Rick) Karlquist wrote:

>> 12V for the oven because inside the outer oven lives a 10811-60158 ( see
>> http://www.realhamradio.com/GPS-oven-journey.htm ) that, as by the specs
>> sheet, is specified 12 to 30 V DC, 11 W max. at turn on (mine draws some
>> 9 W), and Steady state power drops to approximately 2 W at 25°C in still
>> air at 20 V (mine draws some 1.9 W at 12 V without powering the outer
>> oven).

>This is surprising to me.  Can you give us a citation to this spec?
>AFAIK, all 10811 ovens are the same, and the ones I have looked at
>sort of work at 15V, but they don't really work properly on 12V.
>
>One source of confusion is the case of the 5334A counter.  The power
>supply IMHO is poorly designed and the voltage sags down to 12V
>during 10811 warm up.  (All 10811's and 10544's are guaranteed not
>to draw more current than a 47 ohm resistor).  It turns out that
>you can count on the 10811 oven to function sufficiently at 12V to
>turn on the heater transistors and get the oven warmed up.  After
>the oven warms up, the current drops back and the 5334A power supply
>voltage gets back up over 20V.  This is different than saying
>that it is OK to just connect a 10811 heater to a constant 12V supply.
>
...