[volt-nuts] Fluke 731B battery & charge circuit

Orin Eman orin.eman at gmail.com
Sun Jun 23 16:27:22 EDT 2013

Well, an E505 and some 1n967Bs showed up yesterday.  I breadboarded them
with a 180 ohm resistor and a bench power supply that goes to 30V.

The 1N967Bs ranged from 16.8V to 18.1V when driven with the E505.  Voltage
across the resistor was about 0.195, so about 1.1mA.

I chose the 18.1V zener and started the fun and games of installing them.
Someone had soldered the mains connectors.  Hard to see what was going on
under the heat shrink.  Hard to apply enough heat to get them off without
lifting pads from the board.  I'm going to have to do some repair here -
find some new pins/connectors.  It also looks like a previous battery has
leaked and there is some corrosion in places.

The part that had been used to replace the E505 in the past is marked
2789.  Anyone recognize that?

This was a disaster.  Although fine on battery, and even with the battery
disconnected, on AC, output voltage is millivolts(!) low.  So I connect up
my 3455A to the pre-supply output and the output is now correct.  Poked
around with the 'scope and same problem, as soon as I connect it, the
problem goes away.  I figure something is oscillating and connecting
anything damps it.  I can see the problem on the 10V output with the
scope.  2mV jumps up and down.  Not a square wave, but perhaps 25% duty
cycle at 120Hz.  I did eventually manage to catch a similar waveform at the
emitter of the pass transistor.  It would vary in amplitude - up to 40mV
peak, dropping if I put my hand near the probe.

After a couple of hours tinkering (try a zener that was 17.5V, capacitor
across the zener etc.), I gave up and put the '2789' and original zener/2
1N4148s back in.  The 10V output is back to normal, but the 1.018 setting
is reading 1.0V.  A careful inspection found that the grey wire had broken
off the switch - a relatively easy fix.

What next?  Perhaps try the 18.1V zener - at least I can replace that from
the top of the board.  I don't feel like taking everything apart again to
swap the '2789' out again.


On Thu, Jun 20, 2013 at 1:49 PM, Bob Smither <smither at c-c-i.com> wrote:

> On 06/17/2013 02:29 AM, Christopher Brown wrote:
> <snip>
> > That jfet and the shielded transformer are hard to get.  Everything else
> > on the power supply board is nothing special.  All of the specially
> > selected stuff is on the daughter board.
> <snip>
> I needed the current diode to repair a 731B as well.  Try EBay.com.
>  Search for
> "j505 current" and you will find them.
> --
> =========================================================================
> Bob Smither, PhD                                   Circuit Concepts, Inc.
> "There is no worse tyranny than to force a man to pay for what
>  he does not want merely because you think it would be good for him."
>  -- Robert Heinlein
> Smither at C-C-I.Com            http://www.C-C-I.Com            281-331-2744
> =========================================================================
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