[time-nuts] HP 105B Problem?

Ed Palmer ed_palmer at sasktel.net
Sat Aug 8 20:55:17 UTC 2009


I just bought an HP 105B Oscillator.  It's doing something that may or 
may not be a problem.  I'm hoping that someone can help me out.

It's a newer unit that's based on the 10811 oscillator plus two 
interface boards to adapt the 10811 to the rest of the 105B circuitry.  
Does anyone have a soft copy of the manual for this configuration?  Mine 
has a serial # prefix of 2308.  I have the manual from Didier's web 
site, but it's for the older style with a serial # prefix of 808.  Or, 
does anyone have schematics & circuit descriptions for the 5061B Cesium 
Standard?  The more important of the two interface boards is part # 
05061-6165.  Pictures of the interior of the 5061B show a 10811-style 
oscillator with two interface boards in the same location that the 5061A 
had an older-105B style oscillator.  It looks like both units had 
similar upgrades.

My questions concern the performance of the 5 MHz output under different 
loading situations.  The older manual suggests that what I'm seeing 
isn't normal.

First, all circuit check functions are good (80 +-7) with nothing 
connected to the outputs.

My first question is whether its okay to use both the front and rear 
connections simultaneously.  The 1 MHz and 100 KHz outputs show the 
expected drop in level if you have two 50 ohm terminations.  The 5 MHz 
output drops from 76 (3V2 p-p) with a single termination to 60 (2V2 p-p) 
for the double termination which is fine since the spec is >1V rms i.e. 
 >2V8 p-p.  But this drop in level crashes the 1 MHz and 100 KHz signals 
completely.  The older manual includes a stability test with the 5 MHz 
output shorted and the test equipment connected to the 1 MHz output.  
This unit would fail that test completely.

Second, if  I attach a scope (1 Mohm impedance) via BNC cable to the 5 
MHz output, the circuit check level rises from 86 to over 100.  The 
scope shows a clean sine wave of about 4V2 p-p.  Even a 10 Mohm probe 
with a BNC adapter gives a circuit check reading of 96 and the same 4V2 
p-p signal.  Adding the 1 Mohm scope to the 10 Mohm probe raises the 
meter from 96 to over 100 and increases the amplitude to about 4V8 p-p.  
AC or DC coupling has no effect.  I checked for ground loops by plugging 
everything into the same power bar.  I ran the 105B on batteries.  I 
also tried a second scope.  No change.  I repeated the test with a 50 
ohm termination on the scope and the circuit check shows 76 with a 3V2 
p-p amplitude which is perfect.  But adding the 1 Mohm scope to the 
termination raised the meter to 82 with a 3V4 p-p amplitude.  The same 
tests on either the 1 MHz or 100 KHz outputs make more sense.  The high 
impedance connections don't affect the circuit-check reading and adding 
a terminator drops the level by about half as measured by either the 
meter or the oscilloscope.

In summary, should I be able to use both front & rear connectors 
simultaneously without disrupting the other outputs and does it make 
sense that high impedance connections cause the 5 MHz level to rise?

Sorry for the rather long message.  Thanks for any help anyone can provide.

Ed




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