[solved] Repairing L&R Tempo 400 Ultrasonic Cleaner PCB (With RE Schematic!) - Page 1
Forum section: electronics / repair

author: HotGlass
author_rank: Contributor
timestamp: February 11, 2025, 04:30:03 am
content: Hello all,I'm hoping for a little help here, but also posting a schematic that I couldn't find anywhere, and I had to spend the better part of a day reverse engineering from the circuit board. So at the very least, there's now at least an 'as built' schematic for this unit out there for people. This board is somewhat notoriously problematic, and seemingly had several revisions. What those were, and where my example falls in that, I have no idea. Even when running properly, it can run quite hot, particularly the 2k 10w resistor. The problem I am facing is that even though the ultrasonic element appears to be working, this same resistor is dissipating waaaay to much heat. I've heard it's normal for it to run at 450F but my example is hitting 600F and climbing after only about 2 minutes of operation. So my goal here is to figure out why.The machine is an L&R Tempo 400 ultrasonic watch cleaning machine. It's 80s vintage, but still a very competent and valuable machine. I'm restoring it for my watchmaking studio.What I've done so far:- Reverse engineer the board and draw the uploaded schematic- Replace the blown original 2k 10w resistor with two 8w series 1k resistors, for an increase of 6w dissipation. Still waaaay too hot.- Replace blown diode next to this resistor- Test the MJ1005 out of circuit on a low voltage transistor checker. It's not obviously shorted or anything.- Test all capacitors at full rated DC voltage for leakage, out of cuircuit- Measure all resistors (all within spec)- Applied 200VDC to the transducer element (no leakage current detected)I haven't hooked it up to scopes and checked out waveforms yet. Part of my problem is my lack of familiarity with this sort of circuit. I get what it does in general principle - uses a 555 to generate a high frequency signal which basically gets amplified by the MJ1005. There's transformer coupling and feedback going on, but that's about all I can decipher. I don't really know what waveforms are normal for driving a transducer like this, and what's going on with the various things that look like snubber networks. Obviously this circuit is crude for having so many high power resistors. It clearly wastes a lot of energy.Any help is greatly appreciated, and I'm happy to buy a pint for your troubles.NOTE: This is 60Hz 120VAC powered

author: watchmaker
author_rank: Unknown
timestamp: February 11, 2025, 01:35:17 pm
content: Your board differs from mine. Here is a post that may help; the cap array is what fails most often.https://www.eevblog.com/forum/chat/a-serious-(as-in-not-troll)-requestfor-help-wultrasonic-transducer/msg5465266/#msg5465266I think I have the schematics somewhere but am going out for the day. Will see if I can dig them up tonite.

author: HotGlass
author_rank: Contributor
timestamp: February 11, 2025, 10:42:28 pm
content: Wow, thank you for linking that thread! I'm going to read through it and take some measurements later tonight. I'm fairly sure all my capacitors are ok, I've tested them for leakage. I spoke to someone who works on these today and we discussed the possibility that the transducer head has drifted out of tune with the board, or the other way around. He proposed the possibility that I simply need to adjust the oscillator frequency slightly while checking for resistor temps and the cavitation action in the water. There's a sweet spot apparently.I'll post back when I learn more. Not enough is widely known about these things!

author: watchmaker
author_rank: Unknown
timestamp: February 12, 2025, 04:16:33 pm
content: I sent you a PM asking for your email address. We cannot add attachments to private messages. It is appreciated if you do not make this info public because it impacts the only two fulltime repair shops that service these in the US.Turns out all it lays out are the switches. But still, it is more than what is out there and the switches are important to understand.

author: HotGlass
author_rank: Contributor
timestamp: February 16, 2025, 05:22:17 am
content: UPDATE:I think I've fixed the problem! Basically a week ago I decided to a replacement output transistor just because, even though the old one was testing OK. To test it, I removed it from the board and plugged it into a small transistor tester. The tester correctly identified the type of transistor and the pinout and the readings didn't seem bad. And at the time, I didn't have another one to compare it to. Today I get a NTE-238 which is an equivalent to the MJ12005 originally on the board but easier to find at a decent price. I measured the new part before putting it in, and the only real difference was the hFE. 10 for the original part, 31 for the new part. Forward voltage was only about 30-40mv lower on the old part than the new one.I didn't have a lot of hope it would make any difference, but it made ALL the difference. The 2k 10W resistor (16w in my case) went from 600F and climbing after a minute or two to 350F and stable after 5+ minutes. The ultrasonic action seems to be less now, but still working. I think the old transistor was overdriving the head in retrospect. I was running it in a water bath with a small piece of aluminum foil. When you get good action, it fuzzies the edges of the foil. Prior to replacing the transistor, it was punching pin holes in the foil after just a couple minutes!! After replacing the transistor, the whole thing is making less audible noise, running cooler, and it still did a great job of cleaning a nasty Spidel watch strap with decades of crud in it without any mechanical agitation.I'm not sure what the failure mode of the original transistor is here, I mean... It still behaves as a transistor. I suspect it must be breaking down at the higher operating voltages of this circuit while somehow managing not to fail outright. I might hook it up to my bench supply and see if I can get it to misbehave, but I'm more keen to finish putting the Tempo back together now that I've finished servicing it in every other way.CORRECTIONS TO MY SCHEMATIC:I have HOT and NEUTRAL swapped on my drawing. Blue/White is the HOT side in reality. I think it's dumb fusing the neutral side, but whatever.I think my drawing is otherwise perfectly good enough to help anyone with this. Here's another important note. Apparently some of the older units out there omitted the 150 Ohm 25W resistor on the head. I've heard that resistor improves the reliability of the circuit significantly.Some modifications I might make: I'm thinking the 2K 10W resistor shouldn't even be on the board. I'm looking into ordering an aluminum 25W chassis mount resistor and mounting it to the rather substantial 5mm thick aluminum bottom plate. I think that would help the life of the diodes and caps on the board.Cheers for now!!

author: ME
author_rank: Frequent Contributor
timestamp: February 16, 2025, 10:44:54 am
content: I thought the transducers ran at 1kv+volts,mine runs at 1200volts, Also if i recall they also store energy like a capacitor?, I have had shocks even after being unpowered for 10 mins!.My homemade job is 250 watts and has a 15ltr tank and 6x transducers.

author: HotGlass
author_rank: Contributor
timestamp: February 17, 2025, 07:02:15 am
content: Yes, the voltages get quite high on the output side, and yes those transducers have enough capacitance to give you a small zap. And they seem to spontaneously build up a charge when disconnected from the circuit. Probably something similar to what happens in a quartz lighter is at work owing to the piezoelectric nature of these things.Also, marking this as . Thanks again.