[volt-nuts] Fluke 731B parts xref

WB6BNQ wb6bnq at cox.net
Thu Nov 22 02:04:38 UTC 2012


A serious question !  Out of curiosity, why are you changing all these diodes as
it is unlikely that they would all fail at the same time.  Do you know for
certain that they and the other parts are bad ?

The only output on the 731B that has current capability is the 10 volt output.
All the other voltages are delveloped via resistive dividers and will not handle
any loading.  I.E., they were made to drive a very high impedance, like a Fluke
845 null meter.  Generally, 1.xxx voltage outputs are useless these days unless
you have some "Standard Cells" laying around.

The TD8253 diodes can be replaced with 1n4148 or 1n4448 (preferred) diodes.

For the 1N967B, it is available and here is a web address for that:


As for the FET current limiter, that is going to be a little more tricky.  No
idea about a replacement at this point.  You could just use a resistor of the
right value to limit the current through the zener.  That particular stage is
just a "pre" regulator to provide roughly 17 volts to the real action further
down stream.  Yes, having a more stable 17 volts is better.  The purpose of CR10
is to force a fixed amount of current through CR11 irrespective of input voltage
variations; within reason of course.  A number of parameters would have to known
to find a suitable replacement.  But, again, this is not a real critical circuit.

In the case of the Bourns' pot, any modern replacement part that may be fitted is
going to run more than $30.  What makes you think the pot is bad ? ? ?

Good luck with the project,


Christopher Brown wrote:

> Just picked up a 731B that turned out to be in a bit less than ideal shape.
> It has been broken, repaired and then broken again.
> I need to replace a number of bits, and am having trouble xrefing a few.
> The UV pot, listed as 3509S9-502, may be bournes 3509S-9-502 but cannot
> find the part or a data sheet.
> Switch,toggle MSTL206N (locking on/off switch)
> On the power and adjust board
> CR5-CR9, diode, Si,hi-speed switch TD8253
> CR10, diode,FET,current regulator E505
> CR11 diode, zener, 18v 1N967B seems to be discontinued.  Is fairchild
> 1N5248B a suitable replacement.
> Anyway, if anyone can point me at suitable replacements or sources it
> would be a great help.
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