[time-nuts] HP E1938A schematics.

[Richard (Rick) Karlquist]

Regarding the E1938A schematic:

I will try to answer your questions when I have time.
Watch this reflector.

The story on the voltage reference is:

The original part I chose turned out to be too noisy.
I selected a different, quieter part for the final
design.  I don't remember the MPN's of each reference.
If you are curious, I would advise looking at the
actual PCB and reading the number off the IC package.

Now that 20 years have passed, that IC may or may not
still be available in the original package or in any
package, and in any event, there are now probably better
references available.

Although not references per se, the LT3042/LT3045/LT3045-1
are extremely low noise voltage regulators.  Actually,
if there are any time nuts who haven't yet heard of these,
the data sheets should be considered required reading.
Recently, ADI introduced a family of negative regulators
that are basically exact mirror images of the positive
regulators, right down to the same pin out.  The LT3093
is a negative version of the LT3042 and the LT3094 is
a negative version of the LT3045-1.  ADI has done an
outstanding job of supporting the chips with reference
designs and PC board layout tips along with evaluation
boards.

Rick N6RK


On 7/7/2019 10:50 AM, Rob040 . wrote:
> Hi,
> 
> I've made some new schematics of the HP E1938 oscillator/controller module. I derived them from the HP schematics that I found on Brooks' site.
> The aim was to have more clear versions for service/repair. They should reflect the situation as they left the factory, what isn't on the PCB isn't present in the drawings (not sure about the content of the oscillator 'puck'). It's the version with the 20P connector on the component side.
> I made a One DAC and Two DAC version and made a separate sheet for the power supply, otherwise I wouldn't fit on one sheet (format is A2).
> Please feel free to use it, you can find the package attached to this mail.
> 
> Because I didn't open the oscillator module, I'm not sure about the following things:
> 
> - In the voltage reference section: C65 (100nF) is in parallel with a 0 ohm resistor (R74), so I expect C65 is not present on the PCB. Is "No load" in this case the same as "Not mounted"?
> 
> - In the input of the amplifier section: R28 (0 ohm) is in parallel with potmeter (R77) of 50 ohm. No clue which of both is present.
> 
> - The capacitance mention on the label of the 'puck', where is it placed in the circuit? Is it one of the C2/C3 which has a fixed value of 100pF in the schematic? Or is it in parallel with CR1 (the BB512)? Or is it like I drew in the oscillator part (bridge section)?
> 
> - Type of U5 (Vref IC) is still unknown, anyone knows which type it is?
> 
> Best regards,
> 
> Rob Tromp
> 
> 
> 
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