[time-nuts] "The Penultimate HP5065 A15"

[timeok at timeok.it]

   It has been a while since there has been talk of a redesign of the A15 and it is certainly the right way to technologically update the HP5065A that deserves better electronics.

   A better C field generator is necessary and the LM399 remains a good solution. It would be interesting to add barometric pressure compensation. Two types of setting could be used to adjust the C field, one with fixed high stability resistors resistors that can be switched for the coarse and a potentiometer for fine tuning in order to mitigate its low temperature stability.

   Excellent solution of two separate power supplies for the lamp and the rest of the electronics but I have doubts about the LM317. Since we already have the LM399 available for the CField, why not use this reference voltage already present on the board to drive two low noise regulators?

   I hope to be of contribution to this project.

   Luciano

   Luciano P. S. Paramithiotti
   timeok at timeok.it
   www.timeok.it

   Da "time-nuts" time-nuts-bounces at lists.febo.com
   A time-nuts at lists.febo.com
   Cc
   Data Fri, 31 Jul 2020 21:31:13 +0000
   Oggetto [time-nuts] "The Penultimate HP5065 A15"
   The A15 board in one of my 5065s is in bad shape, and I have started
   to look at designing a plug-compatible replacement board.

   The main reason I dont just repair/replace the A15 is that
   I want to find out how much instability the PSU contributes.

   The outline idea currently is:

   LM399 self-biased voltage reference
   (See: Linear app-note 42, fig 72)

   The ultimate board would use LTZ1000, but I have not quite
   convinced myself yet, even if it would be cute to have a
   HP5065 which delivered both precise frequency and voltage :-)

   Op-amp based C-field driver

   Vishay SMR1DZ resistors for VREF/C-field stability

   C-field polarity switch. An experiment. If nothing else
   I will be able to measure the residual magnetic field.

   Optional adjustable C-field. Optional because the pot may
   degrade the C-field stability.

   Two +20V on-board LM317-style linear regulators, one for
   the lamp, one for the rest. Split for noise reasons and
   to be able to play with the lamp voltage/power.

   One or two pre-regulator current measurement shunts.

   DC/DC-brick switchmode -20V supply with brutal filtering.

   The downside of the two linear +20V regulators is that even with a
   heatsink, they will probably get hot-ish if the internal DC bus is
   too much over 24VDC. For this reason, and because I may simply run
   out of PCB space, I may leave the bridge rectifier out, so it will
   only works with EXT-DC.

   I'll post kicad schematic&pcb once I get further.

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