Lambda LQ-532 CV/CC Bench PSUs - Repair & Upgrade
Posted: Wed Jul 09, 2025 3:21 pm
So... for a while now, I've been down to just one one of my big ol' LAMBDAs on the bench; the one on the right lost its mind several months ago, so I just used the one on the left. Then a few weeks ago, it went dain-bramaged too.
So, once I'd finished with my Marantz CD changer refurbishment, I cleared off the bench and brought one of the 12kg beasts down for a peek inside.
As expected based on what I found & fixed when I first bought it approx a decade ago, another of the SPRAGUE hermetically-sealed electrolytics (40uF/35-65V) blarffed its guts out inside the tube; that has been replaced by two stacked 22uF/63V "normal" electrolytics. This restored the PSU to normal operation, tho I did "reality-check" every cap in the thing. It is of that age, and there's only a handful aside the great big chode right in the middle of the box.
While I was in here, I decided to investigate the possibility of a mod that had occupied my mind while dozing off to sleep many times: the LED display only shows V or A and is toggled by a switch. That has always been a bit of a pain; it really complicates everyday use of the unit and 3 digits really isn't adequate resolution. I've had several of these dual-display panel meters in a storage bin for a few years, originally bought for some other homebrew PSU projects. Obviously, the over/under arrangement doesn't lend itself to this project, so a little revision is necessary.
A little hot-air rework freed the Ammeter display, while a quick bit of 3DP yielded a snap-on collar to hold it where it needs to be next to the voltmeter. Time for a little soldering therapy!
That is more than enough solder for one last joint... I refuse to cut another length!

Mmmm... yummy reclaimed transformer wire... soldered all nice & tidy. I love the smell of roasted varnish in the morning! Smells like victory!!!

aaaand... doyt! I figured the ammeter would just be a bit dimmer and purplish... but noooo... they had to use a proper instrument-grade optical filter lens, because LAMBDA; this means the narrow-wavelength blue LED light doesn't get through at all.
So I ordered 2 sheets of grey smoked acrylic (approx 20x what I needed) because cheapest and quickest way to get it was, again, to buy it off Bezos' Online Crack Shop.

While I waited on that to arrive, I spent some time poring over the UM/SM and schematics for this thing; here I've made my own with the meter circuitry deleted. I wanted to find spots to measure voltage and current from the original design, such that loads connected at either front panel or rear programming terminals would be measured properly.
Unfortunately, I was unable to adapt the internal current shunt to this meter module; it is designed around a shunt in the (-) rail, while the one in the LAMBDA is in the (+) rail, so I had to insert the meter's current shunt at the front panel only. I was able to repurpose the built-in +5V power supply and +VSENSE wiring, however.
This pinout reflects that research, and is what I'm using to patch in the new meters.
mnem
Continued in next post...