Mackie TH-15A

[lowell]

This powered speaker is THUMPING every 10 seconds or so. It's a blump blump... 2 thuds in less than a second...every 10 seconds. The red OL light comes on every time this happens. I realize this is an SMPS and is most likely something cycling in the power supply. I usually lose hours and hours of time trying to troubleshoot these and don't fix it. I'm not good at this sort of repair. I know they are very dangerous and that a scope is not to be used or is to be used in a very cautious specific way. So I won't be using one until I review those procedures.

My question: Is this worth my time? Does anyone have an idea from my description on what the problem might be? Would you commit time to fixing this? And, how do you all go about this repair?

[The Dude]

Firstly, it's my experience that LOUD is very likely to send you a schematic. So, contact them, ask, and you shall receive. The power cycling is likely due to the amp trying to protect itself. Yes, it could be a power supply problem, but is more likely to be an amp problem. Check for shorted output transistors, DC on the output, rails coming up, etc., just like any other amp. IMO, it's certainly worth a look. It could be something as simple as a single shorted transistor. Or, it could be blown to smithereens. I would at least have a quick look.

[Jazz P Bass]

Lowell.
It's not worth it.
They're crap from the get go.

'Thump' is the sound the unit makes when it hits the bottom of the dumpster.

[lowell]

Ha. Rather conflicting responses. I could do some basic tests in regards to shorted output transistors. But when it comes to the smps and all it's protection circuitry I'll be lost.

[52 Bill]

Ha. Rather conflicting responses. I could do some basic tests in regards to shorted output transistors. But when it comes to the smps and all it's protection circuitry I'll be lost.

In general I would look to see if you can unplug the power supply from the power amp. If you can, then see if the power supply is cycling by itself.

Have you tested the output for dc voltage?

[lowell]

I unplugged the PA and the supply still cycles. The bipolar 29v supplies are there between cycles. It seems that the positive rail isn't there for as long as the negative rail. The negative rail is good for 5-10 seconds and then drops for a couple seconds. The positive rail is the opposite. I checked the diodes on the secondary and they read good. I'll need to check for DC on the output...but I don't hear DC hum on the output...just the THUMP THUMP. Maybe that's the DC which then goes away when the protection detects it? I'll report back.

[Enzo]

You unplug the PA and the SMPS cycles, so we have an SMPS problem.

Don't worry about what the thump thump is on the PA, it won;t matter until the power supply works.

yes, look for shorted rectifiers on the secondary side of the SMPS in the main rail supplies. But also check for shorted filter caps, or a shorted inductor. And difference in resistance across the two main rails?

[The Dude]

Sometimes it's just cheaper and easier.

Mackie 2042765 02 ASP PCB Kit for TH15A | Full Compass

[lowell]

So the rectifiers on the secondary are fine. I measured resistance of both main supplies and the positive rail seemed lower. So I replaced both of the 2200uf filters. Now the resistance reading SEEMS similar to the negative rail. However, the SMPS is still cycling. I cannot find the PWM IC on the circuit board anywhere. So a bit confused there. It's possible it's hiding under some glue on the capacitors. So other than rectifiers and capacitors on the secondary, what would be the next item to check? I am keeping the full replacement option on the back burner and treating this as a learning experience... for a bit.

[Jazz P Bass]

http://bmamps.com/Schematics/Mackie/...Schematics.pdf

If you cannot locate the optocoupler, try monitoring U104/ Pin 2. That is what the opto feeds.

http://www.onsemi.com/pub_link/Collateral/NCP1271-D.PDF

[lowell]

That's what I cannot locate - U104.

[lowell]

I found U103 LDR.

U103
pins
1 (anode) 195v - which then goes down to 150v for a second, then back to 195v
2 (cathode) 195v - which then goes down to 150v for a second, then back to 195v
3 (emitter) fluctuate 0mv-200mv all over the place
4 (collector) 3mv-28mv-700mv fluctuating

All readings from the rectifier negative (as ground)

Edited: 4:20pm

[Enzo]

I don't know where it is on that board, but U104 would be a tiny little 8 leg IC if I recall. And probably nestled in a little spot near the edge of the board. I may be thinking of something else though.

[olddawg]

Freeze is your friend. I always found it very helpful to have another unit to compare measurements to if possible. Btw, does the chipper quit oscillating when it cycles? As was stated before, the power supply may be shutting down because of protection. Figure out what it's sensing. Could be ESR on a cap that you will only identify through replacement or MAYBE tickling with freeze. There may be a bulletin as well. That's a pain when you aren't authorized.

[lowell]

Enzo. Ive looked and looked. I am at a loss. And yes I was looking for an opamp looking device. Got me at this point. I even thought maybe it had exploded and disappeared but dont even see a spot for it. I still havent cleaned all the glue off....but I dont see how or why itd be so close to the filters. Need to get some freeze spray.

I do have another one that has a noise issue but does work. I can check those readings and see. Thanks!