Test Equipment Anonymous

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another of the SPRAGUE hermetically-sealed electrolytics (40uF/35-65V) blarffed its guts out inside the tube; that has been replaced by two stacked 22uF/63V "normal" electrolytics.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...

Another thing I've always missed on these PSUs is a indicator for the current-limit threshold; sadly, from the block diagram above, it appears that comparator and OR gate are inside IC101. *sad wah-wah-wahhhhhhh horn* . This appears to be custom silicon; it has the full unregulated 70VDC of C1 (9700uF, 75VDC WV) across it. That is a lot of energy density there... like arc-welding levels.




Since I couldn't have that, I decided to drop back & punt: I wanted to turn that switch-hole into a CC mode indicator, but as a consolation prize I did turn it into a fault indicator, warning of a blown fuse in the main DC (-) rail feeding the finals.




Now all the pieces are coming together; I have a battle-plan, and it's time to start making the bits that hold it all in place. First is a 3DP plate...




...followed by making eeny little LED panels from one of these cheap amber festoon-lighting blubs. This required first hot-air desoldering all the LEDs, then grinding the PCB paper-thin on the disc sander, then slotting between clusters of pads and reapplying just the LEDs I need...




...and then I was able to cut off with a diamond wheel these little LED panels that could stand up to having wires hand-soldered to them.




This gave me something I could pot into the mounting plate with epoxy along with the meter panel...




...and final installation of the completed meter panel could be done. I was not comfortable with any of the bodges I'd have to do to use the MOLEX plug at the current shunt connection on the meter, so I did solder & shrink-wrap right to the pins. Not my preferred solution, but the least of available sins. I did terminate the existing wiring to a JST-XH plug for the other connections.

To atone for the sins above, I did carefully label all the wires and the original meter PCB such that this mod can be undone. I will keep all the bits in a ziplock baggie with the units.




The next day's work was to repair the other LQ-532; this time it was the same capacitor as the first unit. Again, all caps reality-checked, and unit restored to operation. You can see the cap I replaced when I first got this one in the lower right.

Since I already knew where it needed to go, I added the mod for the F2 indicator; unused traces in the upper left corner of the PCB made a convenient place to add a 3K3 resistor, and an extra wire was added under the loom lacing for routing to the front panel area.




With this work done, I can take a break to compare the units side by side...




...and scope them out with an eye to noise factor; the modded unit hung aroung 17-20mV p-p, while the unmodded unit was 15-17mV p-p. That little deviation could just be variations in the units themselves, or even one just randomly picking up more common-mode noise.

I was quite pleased with these results, but during the course of this I noted the current meter on the modded unit sometimes displayed a few ticks when it shouldn’t, so while I was building the meter assembly for the 2nd LQ-532, I changed the routing of the fly wires in hopes it was picking up stray PWM noise from the displays. That seemed to work, so of course I had to do the same on the first unit. I also took this opportunity to swap the red and blue displays around per a conversation in the Discord; curse you Mounty.


mnem
Concluded in next post...

I do have to grudgingly agree with Mounty, it is definitely better with Blue for Volts and Red for Amps. Still undecided whether it was worth the extra assache.




I HAVE THE POWAHHHH!!!

Well, not all, but quite a bit of it in lots of different flavors and colors. I've set all of these PSUs up on a smart outlet strip like my Wall o' Scopes™; when I turn on the leftmost one, it turns on power to all of them. Quite pleased with this arrangement.

I have already been using the new setup, and the accuracy of these new meters is so much better. Almost a "poor man's SMU".

I acquired and created quite a bit of documentation on these; I'll collect it all below for posterity.

(ED: Well, I thought I would, but I still can't upload anything bigger than a MB or so that isn't a .jpg; and why in the hell is .pdf still not a supported filetype, FFS... )

Cheers, and keep the faith!

mnem
you can all blame Mounty for the following retaliatory action:


A dog walks in to a bar and orders a beer. The bartender pulls out a gun, shoots the dog in the foot, and yells, “We don’t serve dogs here. Scram!”

A year later, the dog comes back, and there’s a different bartender. The dog says, “I’m looking for the man who shot my paw.”

good post!

but you need to explain why volts are better in blue.

mnementh wrote: ↑Wed Jul 09, 2025 3:23 pm



I do have to grudgingly agree with Mounty, it is definitely better with Blue for Volts and Red for Amps. Still undecided whether it was worth the extra assache.

Very nice!
I do really appreciate your efforts in exchanging the colours. It looks (at least to me) now really good!

Well done, ol' Dwagon!

nixiefreqq wrote: ↑Wed Jul 09, 2025 5:55 pm good post!

but you need to explain why volts are better in blue.

No I don't.

mnem
green would actually have been my first choice.

oh...got it now.....red-green

Very nicely done! Looks OEM!

-Pat