Hi

I'm trying to fix an amplifier that is built in to a speaker. When I play music I can hear it but the volume is very low. Any tips on what to check here?

I’ve found two u1620g on the circuit board. Does it sound like these might be blown?

 

Without more information it could be almost anything.
Give us a make and model - it may be possible to find some useful information on www.
Also some good photos of the board from different angles might help.
Do you have a multimeter?

 

Without more information it could be almost anything.
Give us a make and model - it may be possible to find some useful information on www.
Also some good photos of the board from different angles might help.
Do you have a multimeter?

I have multimeter and oscilloscope. The speakers are called Tapco Thump 15a. I see there is schematic online. I dont have the board in front of me right now, but I can snap some pic later if needed

 

First step would be to open the unit and do a thorough visual inspection.
Look and smell for any damaged or stressed components and pcb. Look for heat stress on resistors, discoloration on pcb, bulging or leaking capacitors.

 

It uses a switch mode supply which means a lot of exposed live mains inside. If you are not used to working on such equipment it is very dangerous to attempt repairs. Further, if the fault is in the power supply, they are very difficult to repair even with experience of such work and to try to do it by 'remote control' from here is unlikely to say the least.

If you want to proceed and if the fault is not in the power supply, as MrChips says the first step is a visual inspection.
I found the schematic but no board layout so good pictures will help a lot.

 

Thanks, I've already done a visual inspection (should have written that). Did not find any thing that looks wired.

I cant get a pic tonight but I'll fix that tomorrow. I found some pic on this link. And it looks like I might be right about the u1620g. I'll desolderer and check

http://www.fixya.com/support/p2153277-mackie_th_15a_active_dj_powered_thump_sp

 

Sometimes knowledge of the history of the unit can provide some clues of failure mode.

Are you the original owner or did you acquire this recently?
Did you acquire the unit as is, i.e. it was not working when you got it?
Same question, have you seen/heard the unit working? Did the unit fail on you while using it? If so, what were the conditions before and after failure?

This is not an interrogation but sometimes it might lead to some useful clues.

 

The next step would be to identify all the power supply voltages from the schematic and then take voltage measurements with a voltmeter.

+3.3V
+5V
+15V
-15V
+28V
-28V

We will side step +300V for now.

 

No problem I would not ask if I did not want you to ask back The previous owner said it just stopped working while using it. Don't know any more than that I'm afraid. I'm off to bed, but I'll check voltages to tomorrow. Thanks for your help so fare

 

Checked all the voltages. Everything checks out so I guess I can assume the power supply is ok and the amplifier board is the problem?

 

Ok, now the fun begins. You said you have an oscilloscope. That's great. How competent are you at trouble shooting using an oscilloscope?

Here are possible suspects that we want to identify.

1. Preamp stage is faulty.
2. Main amp is faulty.
3. Something else is preventing audio output, for example, mute.

Your main amp is TDA8924TH Class-D power amp wired in mono mode.

What is the make and model of your scope?
Are you using a x10 attenuation probe?

For simple starters, use the oscilloscope to probe the signal on L203 and L204, on the side connected to the TDA8924 output pins.
Do this with no input source.

Here is the schematic for your amp.


Here is the TDA8924 example for reference purposes.

 

Back up.
With your voltmeter, measure the voltages at both ends of R200 and R201 or at L106, L107.
This is the power supply feed to the amp (top left on your amp schematic).

 

After you've done that, measure the voltages at both ends of R204, Mode control circuitry with Q201.

 

Wow this is really helpfull R200 is +28 on both sides. R201 is -28 on both sides.

R204 is at 0V at both sides. Which leads me to belive the aplifier is off? But why?

(I messured R222 which is at 60 ohm.)

PS I'm not verry competent trouble shooting with a oscilloscope. And have not started that part yet since this seems to be the way to go.

 

I messured pin1 on conn10 also 0v

 

I messured mute on conn11 also 0V

 

That is very useful. Hopefully you're on the right track.

If R204 is at 0V then MODE is 0V and the main amp is off.

I see the circuit above the TDA8924 is TDA7294 which is also a 100W linear amp. What is that being used for? (side track)

(on track)
If pin-1 on conn 10 is at 0V then something is wrong with Q201 (unless if pin-1 needs to go negative for cutoff).
Can you unsolder Q201 or at least lift the drain lead off its solder pad?

 

Sidetrack: Looks to me like TDA8924 is for LF and TDA7294 is for HF

Ontrack: Yes I can desolder, but are you sure? When PIN-1 is 0 that means the gate of Q201 is 0 and open. So left side of R204 sould be 0 as it is. Or am I wrong here?

 

Ok, I now see where MUTE is generated. There is an opto-coupler off the power supply board.
Yes, conn-10 pin-1 has to go negative for the amp to be ON.

So something is wrong at the opto-coupler circuitry.

Be careful around that part of the circuit. You're looking at 300V.
It would appear that the opto-coupler is bad.
Measure the voltage across the emitter diode of the coupler, pins 1 and 2.
Measure the voltage across the photo-transistor, pins 3 and 4.

 

Hum 3-4 is 28v as expected. But 1&2 is 0.

D100 is 239 on the AC side