[time-nuts] Broken Ovenaire OSC 85-50

[J. L. Trantham]

What a wealth of new thought.  Let me see if I can remember all the
questions asked in prior posts.

First, the unit is Option 004 for the HP5316A and B (and perhaps others)
frequency counter and some information about it can be found in the service
manual here:  http://cp.literature.agilent.com/litweb/pdf/05316-90011.pdf

Second, mentioned in my original posts and probably not emphasized enough
recently is the fact that there is an Oven Controller Board attached to the
oven that is connected to the Red wire, the Black wire and the Red/White
wire.  I disconnected the Red wire from the Oven Controller Board and saw no
significant change in the voltage on the Red wire.

Third, the unit does adjust frequency with both the COARSE and FINE
adjustments (Cap and Green wire) and does come spot on frequency as compared
to my Thunderbolt.  It is quite stable as observed overnight on a scope with
the appearance of being with in 1 Hz after an overnight.  It may well
improve with longer 'warm up'.  Sorry, no phase noise measurements but I am
working on adding that to my repertoire.

As shown in the manual, there is an external board that connects to the
85-50 with three wires supplying regulated +5VDC, ground, and a connection
for the 10MHz out with a 100 ohm resistor to ground on that connection on
the external board (which I don't think I have mentioned before).

Forth, I did make one error in component value that is now corrected on the
most recent schematic and that is the 8.2K resistor connected to the base of
the oscillator transistor I originally miss read as a 1.2K.  I suspect that
Bruce's original analysis used 1.2K as the value of this resistor.
Component values were obtained by using a magnifying glass to read the
values or color codes then verified by using an LCR meter (4274A or
SmartTweezer) or Fluke 8050A DMM.  I wish I had a 'should be' schematic
(anyone with a connection to OAC that could help with this?).  All I could
do was to 'derive' the schematic you have.  The pot measured 4.7K.  I do not
know what it 'should be'.

I did obtain the DC and RF voltages in stages as I progressed through the
repair effort and my original schematic did not include all these
measurements.

The DC voltages were measured with the 8050A and the RF voltages with a TEK
485 and a 10:1 probe.  The only component value I found 'out of range' was
the .01 uF cap on the 330 ohm resistor in the emitter circuit of the output
transistor on the Output Board and replacing it provided only 20%
improvement in amplitude.

All measurements were made with the oscillator oscillating and before any
effort at repair.  I did not go back and remeasure DC or RF voltages after
replacing the 470 ohm resistor with a 47 ohm resistor.

I must admit, I overlooked the low voltage at the bottom of the 4.7K pot.
Perhaps I made an error in measurement but it does serve it's function.
Perhaps there is a leaky .01 uF bypass cap on the Green wire on the
Oscillator Board that I missed but I do not see how that might cause a low
output amplitude.  The 100K resistor measured correct and that should limit
current there to the 15 uA range.

Fifth, unfortunately, I do not have any 'spare' 10 MHz crystals and I did
not think of substituting a different crystal.

Sixth, I was wondering about the AGC issue and the only thought I had was
that the 510 ohm resistor dropping the voltage to the Oscillator Board
somehow might serve that function.  However, raising that voltage to 5 volts
made no difference in output amplitude.

Lastly, the only part of the 85-50 that is not shown is the Oven Controller
Board.  It seemed to be functioning normally based on current measurements
as the oven warmed up and the fact that the unit would come on frequency and
was stable.  It would be quite the challenge to disassemble that unit
without risk of destroying the oven.  As I noted above, the Red, the Black,
and the Red/White wire connect to the Oven Controller Board but the Green
and Yellow wires do not.

My hope was to create a resource for anyone else looking for help in fixing
this unit and it appears that there is quite the wealth of information here.
If anyone has a site and would like to have the schematics and pictures I
have, I would be happy to upload them.

Thanks again for all the information.

Joe




-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of WarrenS
Sent: Saturday, July 04, 2009 5:06 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Broken Ovenaire OSC 85-50



Here is a new observation about low Vcc.
It is not likely that the Osc Vcc voltage and the Varicap freq control
voltage is coming from the same 510 Ohm resistor without a regulator.
Something is missing on the schematic, because now any Osc current change
would cause the freq to change. And there is the missing 3 plus ma that can
not be accounted for in your voltage measurements. The Osc and pot circuits
as shown are drawing about 1.5 ma, and there seems to be almost 5ma going
thru the 510 ohm. My guess is that this is from a 2.50 volt precision shunt
regulator on the Vcc, in the Osc oven, but not shown on the schematic. That
should be pretty easy to find. If so, make sure your 47 ohm fix does not
cause the 2.490 volts to loose its regulation when the circuit is put back
in the oven. The increase temp will cause the osc current to increase.

If  the EFC pot is really 10K ohm end to end, that would account for the
other strange voltage readings. 

Also if the Freq as small as it is,  is still correct,  that would eliminate
most of the bad xtral possibilities. 

ws