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