[time-nuts] HP 105B Modification

[Bruce Griffiths]

Ed

The attached circuit is for a JFET input transformer feedback amplifier 
with a relatively high input impedance at 10MHz along with relatively 
low noise and distortion.
A Minicircuits T9-1 should suffice for the transformer.
The major limitation when using these transformeres is that dc 
magnetisation of the core causes distortion.
A differential version using a transformer with centre tapped secondary 
and primary will be less sensitive to this in that only dc current 
mismatch in the 2 halves of each winding contributes to core magnetisation.

A negative supply is used to ensure the JFET is biased correctly.
Other arrangements such as using a string of capacitively bypassed 
diodes (or a Vbe multiplier) in the source circuit are also possible.
Its important for stability that the gain bandwidth of the FET is 
somewhat greater than the ft of the pnp transistor.

Bruce

Ed Palmer wrote:
> Bruce Griffiths wrote:
>> Ed
>>
>> Given that the divider load current, if one substitutes a 5V CMOS 
>> flipflop for the 7474, only increases to about 5mA pp, using a 74HC74 
>> is probably prudent given the PCB layout used, in that it may avoid 
>> EMI issues that may be a problem with faster logic. However it may 
>> then be prudent to attenuate the output of the divide by 2 fliflop to 
>> avoid overdriving the following emitter coupled pair.
>>
> ...By compensating for the different output voltages of the 7474 vs. 
> the 74HC74.  Good point.  Which means more hacking on a perfectly good 
> circuit board.  I hope I can leave this alone and duplicate John 
> Ackermann's 'loose coupling' connection on the input of the 7474.  It 
> should be relatively easy to duplicate the circuit on a breadboard for 
> testing.
>
> Thanks for the ideas, Bruce.
>
> Ed
>
>> Bruce
>>
>> Ed Palmer wrote:
>>> The schematic is available here:  
>>> http://www.murgatroid.com/t_and_m/hp-00105-6100-figs.pdf .  It's 
>>> also available with circuit description & waveforms here:  
>>> http://www.kennethkuhn.com/hpmuseum/scans/hp5065a_part3.pdf on pages 
>>> 93-101 (pdf pages - not document pages).  However, this copy doesn't 
>>> show the schematic for the power supply board.  I also see that 
>>> document page 8-54 is missing which might be the problem.
>>>
>>> Ed
>>>
>>> Bruce Griffiths wrote:
>>>> If you send me the schematic for the circuit in which its used I 
>>>> can answer that question better.
>>>>
>>>> Bruce
>>>>
>>>> Ed Palmer wrote:
>>>>> Perhaps, but given what it's replacing and what it's driving, is 
>>>>> the difference significant?
>>>>>
>>>>> Ed
>>>>>
>>>>> Bruce Griffiths wrote:
>>>>>> Ed Palmer wrote:
>>>>>>> Yes, I have the same configuration.  The part number for the 
>>>>>>> divider board is 05061-6165.  The 10811 + daughter boards are 
>>>>>>> used in the 105B, 5061B Cesium, and the 5065A Rubidium.  The 
>>>>>>> circuit loading problem is exactly the kind of issue I was 
>>>>>>> expecting.  Thanks for the warning.  I could probably resolve it 
>>>>>>> by replacing the existing 7474 with a 74HCT74, but I'd like to 
>>>>>>> minimize the amount of hacking I inflict upon the unit.
>>>>>>>
>>>>>>> Ed
>>>>>>>
>>>>>> A 74AHCT74 would be a superior replacement.
>>>>>>
>>>>>> Bruce
>
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