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.
