Lambda LQ-532 CV/CC Bench PSUs - Repair & Upgrade

[mnementh]

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another of the SPRAGUE hermetically-sealed electrolytics (40uF/35-65V) blarffed its guts out inside the tube; that has been replaced by two stacked 22uF/63V "normal" electrolytics.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

[mnementh]

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...

[mnementh]

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.”

[nixiefreqq]

good post!

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

[BU508A]

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!

[mnementh]

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.

[nixiefreqq]

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

red green .jpg

[Cubdriver]

Very nicely done! Looks OEM!

-Pat