Mackie TH-15A

[Enzo]

You know I am thinking tiny surface mount IC, not a big old 8 pin DIP like the old 4558s. It could easily be under a glob of glue next to a cap. It could be next to the filters because why not, it has to be somewhere, they don't need to make the board bigger just for space around the caps. And just in case, did you look on the underside of the board? Sometimes larger size parts up top and little surface mount goodies on the underside.

Call Mackie and ask if they can send you the board layout drawings. The schematics we have here don't include them.

[olddawg]

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

That should allow you to swap boards to see where the problem is if the SMPS is separate from the PA. I seem to remember Mackie Service repairing units for a flat rate. Might be an easy out. Call them.

[Jazz P Bass]

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)

Odd readings on the Emitter & Collector of U103.

The Emitter should be at PS Ground.
So how is there a signal there.
The Collector, which ties to U104 FeedBack Pin #2, should sit at 1.2Vdc or so.
The FB pin operates by being 'pulled' low.

http://www.promelec.ru/pdf/ne572.pdf

See note 1 & 2 above.

Check the working unit to verify this.

[lowell]

What is the NE572 for? I don't see it on the schematic.

Ok here are updated readings, not sure if I had the pins right on that last post.

On the working unit:

U103

4: collector 3-400mv
3 emitter 0v
1/2: 13v pulsating

Broken unit:

4: collector 0-3.5v jumps around
3: emitter 0v
2: cathode 0v-195v jumps around
1: anode "same"

It seems like it tries to settle around 12 or 13v like the working unit, for a second, then goes to zero and/or +195v

[lowell]

Also, I cannot find any of the components in the box around Q106. ? Trying to see where the 195v is coming from.

Enzo, found the PWM. As you guessed, under the pcb as an SMD. D103 is there as well.

[Jazz P Bass]

What is the NE572 for? I don't see it on the schematic.

Sorry about that.

That 572 is used in another unit, another thread. I got confused. (it's a compressor ic)

What I meant to say was NCP1271.
It' sthe housekeeping IC.

It starts the SMPS.

[asfi]

It's on the underside of the PCB.

And to be honest, whenever I run into a Thump with a bad SMPS, I board-swap the sucker. I admire your fortitude.

[lowell]

So the dotted box around Q106 and other components: non-existent. I see where these parts go on the under side, but they were never installed. So, I've pulled the switch-mode transformer and tested the secondary resistances and both schottky diodes. No issues. Also went through the feedback circuitry and all resistors are good, I even pulled ones that read odd to double check. I am awaiting an ESR meter to be more efficient with testing electrolytics. So I have not tested C21 next to the TL431 shunt regulator. Not sure how to test that regulator, and don't have one to sub. But I could take the working one out of the other unit.

So.... the startup voltage for the PWM hits 15v for a quick second and then goes low while the thing cycles. The startup resistor and Run DC diode test ok.

I ordered the Blue ESR Meter and the Blue Ring Meter from Anatek. I will test all electrolytics and the switch mode transformer when they arrive.

Am I on the right track? Should I just replace the dang PWM and see if that works? Feel as though I've tested most everything else....MOST.

Finally, there is a 65khz and 100khz option for the 1271 PWM. Which one is correct?

[lowell]

Contacted Mackie and Full Compass and both say that only the full assembled kits are sold. So, if the power transformer in here is bad... I guess I'm left with the full replacement option. I guess it's better than nothing.

Still got some more tests to run once the new meters come in.... oh, and finally got an isolation transformer so I can scope on the primary side.

Can't beat this https://www.jameco.com/webapp/wcs/st...gDrillDownView

[lowell]

So a little theory here...

The opto coupler in the feedback loop operates as so: when the B+ is good, the led lights, turning the Collector/Emitter junction on, lowering the FB pin voltage on the PWM. If the B+ is low or being pulled down by a short, the opto LED won't glow and the Collector Emitter junction will raise the voltage on the FB pin....turning off or slowing the PWM pulse.

Correct? Just takin a shot at understanding this.

[Jazz P Bass]

that is how it should work. yes.

[lowell]

Thanks!

Ok, soooooo.... after my feedback loop testing of components, replacing one resistor, and leaving the heatsink mounted devices touching the mica pads, but not screwed down, the thing isn't cycling! I have +/- 30v rails. I'm monitoring the devices' heat and they don't seem to be getting hot. When I touch the TDA8924 high voltage rail pins I hear it through speaker. However, no input signal is passing through the amp. I checked for the +/-15v rails and only see the -15v rail on CON11. No +15v. Oddly enough, this voltage is not on the working unit either. Very odd.

EDIT: Ok so now the other unit isn't working either haha. But, I found R111 is heating up very hot and I can smell it. So at least I have a clue.

[lowell]

Ok so I disconnected the power amp and the preamp from the SMPS. I definitely got a lowish reading on the +15v rail compared to the -15v rail. I ended up finding that Q104 in the +15v regulator supply was bad. Replaced it, and both 15v zeners cause I broke em getting at the transistor. I then, after this repair, had NO VOLTAGES anywhere. This perplexed me, so I checked connections and specifically grounds for both voltage rails. They weren't grounded. It seems that both supply grounds go through the preamp board all the way to the input jack for grounding. I found that the ground wire had broken off the XLR board. Reconnected it and the supplies all look good! Touched the XLR jack with my finder and got sound! This is the first time I haven't been beaten by an SMPS. Rather rewarding if I may say so myself haha. So it's all good. Now onto the next one, we'll see if it's any easier.

[The Dude]

So you have successfully stopped the Thump from thumping.

[lowell]

Checking temperatures on things with my infrared therm gun. Had the unit on with the speakers connected for an hour. One of the Schottky diodes is getting to about 55deg C. The other one is 40deg C. Is this of concern?