Leeds & Northrup 9878 - Page 1
Forum section: electronics / general

author: depmco
author_rank: Newbie
timestamp: February 11, 2025, 12:04:00 am
content: In my stash of obsolete and ancient test equipment, i ran across a Leeds & Northrup 9878 "Guarded DC Constant Voltage Supply." According to the nameplate, it's supposed to provide 2 volts at 12ma or so to emulate a single lead-acid cell. I got this thing a long time ago (30 years?) and it's been setting around ever since. Anyway, I pulled it out and fired it up. For some reason, it puts out more like 6 volts, even with a load current of 12 ma. I opened it up and there's not much to see. A cool transformer with widely split windings to minimize coupling and a pcb with two potted modules. Has anyone explored one of these? If so, does anyone know what's in the potted modules? Schematics?I'm well aware that modern electronics could probably provide better performance, though that transformer is pretty cool. Still, I'd like to get the old thing working as well as it ever did.(BTW, in my looking for info on this unit, which info doesn't seem to succumb to Google, I did find someone trying to sell one of these on the 'bay. He also shows a DVM on the output voltageindicating around 6 volts, so maybe I'm missing something or there is a common failure mode.)Any pointers?-- Greg

author: trobbins
author_rank: Frequent Contributor
timestamp: February 11, 2025, 01:00:13 am
content: Greg, it doesn't look good for youIf some info doesn't show up then you may be the first to try and interrogate and reverse engineer. Even if a tear down of the potted modules makes them impractical to resuscitate, it would allow yourself and others to clone better circuitry to get the most out of the transformer. The description in the L&N 1972 General Cat does look promising for specs, and any clone would have the option to also emulate a weston cell.Vintage detectors seem to be the other 'module' that manufacturers liked to dissuade repair attempts on, given I've reverse engineered two models from ESI.

author: Vgkid
author_rank: Super Contributor
timestamp: February 11, 2025, 03:02:48 am
content: Have you tried putting a load on it. If I recall correctly I remember seeing one on Ebay before that was outputting 6 V.

author: depmco
author_rank: Newbie
timestamp: February 11, 2025, 05:57:41 am
content: As I mentioned in my opening post this thing is putting about 6 volts even with the specified 12ma load current.

author: Poroit
author_rank: Frequent Contributor
timestamp: February 11, 2025, 06:12:14 am
content: G'day Greg,Referring to the one on Ebay.They are not connecting the multimeter to the correct terminals.The 2V should be between Red and Black terminals.A link is installed between Red and Gray for 11mA.

author: depmco
author_rank: Newbie
timestamp: February 12, 2025, 12:01:08 am
content: Perhaps beating a dead horse here, but I though someone might be interested. I'm attaching a few pics of the innards of this instrument:The first is the nameplate.The next shows the transformer I described, mounted on long plastic (nylon?) standoffs. It puts out about 90 VAC with a center tap.The next shows the PCB. The potted modules are the gray and green blocks. Besides those, the only remaining components are the two power supply diodes (full wave rectification), a capacitor, and a couple of precision resistors.The next shows the connection of the instrument to a dmm.The final shows the dmm measurement. Note that the leads are connected "backwards" hence the negative measurement. Sorry for the glare.