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

Christopher Brown cbrown at woods.net
Sun Jun 23 17:27:46 EDT 2013

Do have have a shorting bar in place between guard and gnd?

One of the issues I tracked down on my worst unit was a leek from output
+ to guard.  Some of the tin had flaked off of the traces and formed a 6
ohm connection from + PS out to guard.  Besides guard being hot I saw
intermittent output.

Simply probing with a scope would return voltage to normal.

Simply placing a _dry_ fingertip across the traces/guard in front of the
guard would do the same.

I asked about the shorting bar, because connecting guard to gnd in my
case would pull the ps output _way down_.

Anyway, worth checking for voltage on guard.

On 6/23/13 12:27 PM, Orin Eman wrote:
> 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.
> Orin.
> 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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