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 <[email protected]> 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 >> >> [email protected] http://www.C-C-I.Com 281-331-2744 >> ========================================================================= >> >> _______________________________________________ >> volt-nuts mailing list -- [email protected] >> To unsubscribe, go to >> https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts >> and follow the instructions there. >> > _______________________________________________ > volt-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts > and follow the instructions there. > _______________________________________________ volt-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
