M-Audio BX8 Studio Monitor Repair

[gorgamin]

Hey guys,

I've got a single M-Audio BX8 Studio monitor that I'm repairing that has given up the magic smoke. The other one in the pair is still fine.
There is no schematic for it, but I have managed to find a similar schematic (attached). Its identical apart from the opamps and the voltage regulation transistors.

It is basically a plate amplifier with two amps in one, one for the woofer (TDA7294) and one for the tweeter (TDA2052) on one pcb.

I'm tearing my hair out as I can't seem to find the issue.

1. Its initial symptoms were as following:
When turned on it would be completely silent, but there would be noise if you turned up the volume.

2. After basic troubleshooting, I determined that the transformer is delivering its promised voltage, the bridge rectifier is still ok, filter caps were ok and most of the components look ok. I checked the pcb tracks for heating and only saw the D667 and B647 transistors had slight heating discoloration on the pcb.

3. I first thought it was the main chips - TDA7294 and TDA2052 as well as the D667 and B647 transistors that were bust. So replaced them.

Now the monitor makes a thump on the subwoofer and a high pitched whine when turned on and makes my computer monitor display wobble even though its an lcd. I immediately turned it off.

4. Next I ended up replacing the filter caps anyway, and replaced the bridge rectifier as well. I also replaced all diodes on the pcb.
I checked all components for the precise replacement.

5. It still makes the thump and high pitched whine.

6. I replaced the opamps on the board making sure I get the exact model.

7. Still the thump and whine. Does not output anything.

8. It ended up killing my Focusrite Scarlett 2i4 interface (won't turn on anymore) I think it leaked something back to the TS jacks and killed the USB host controller or perhaps the DSP too. There's a burnt smell inside it, but hopefully its only a surfacemount diode/cap/opamp that popped(lets hope) I've tested the monitors on another audio source and they do the same thing, so it isn't the interface. Getting the interface fixed in the meantime.

9. I've tested each track to make sure there are no breaks, tested each passive component for values, all seem fine. What can the issue be?


Is there anything I'm missing.  What should I check next? How would you have troubleshooted it? What did I do wrong?

Please help.

[DrGeoff]

Since you don't appear to have any output from either speaker (it is a 2-way active monitor) then it is likely to be either PSU or input stage.

Check both the VDD +/- rails (18V) and the VCC +/- rails (32V), as the amp chips look like they are using different rails.
Also check the +/-15V rails.
If all OK, apply a signal at the input and trace it through the balanced input to the volume pot (VR1).
Then trace through to the point where it gets split to the LP/HP filters (IC1-B/7).
Then check that the signal is present at the filter outputs. You may need to sweep your input signal if it is a sine wave in order to check this.

[DrGeoff]

Are these the monitors?


Looks pretty horrible. Bad solder joints would be common.

[gorgamin]

Thank you for the reply.

I haven't checked the inputs! What if I did all that and it was something as stupid as the inputs shorting or a bad solder joint. Yes that's the one in the video. Although the pcb tracks underneath look a little different.

Yes the pcb is horrible. Thin tracks and pads that lift if you just look at your iron, and substandard construction with lots of hot glue everywhere. The caps are also of questionable quality. For what M-Audio charged for these they could really build it better. Their after sales technical support is also substandard as they refuse to release any schematics even after the entire industry has nearly abandoned Class AB amplifier chips for Class D.

I'm going to check the inputs and the components in the input path and get back to you.

[gorgamin]

Inputs are fine...

Following the circuit through and seems like one of the 101 caps C48 is dead. C47 seems ok. Gonna replace, and see what happens.

[gorgamin]

C48 replaced, testing 100pF now

C4 should be 22uF, was showing 17uF, cheap noname cap, replaced anyway with panasonic brand.

Still going through the components... making my way to IC1-B/7.

Don't have a function generator so will just use a sweeping sine wave mp3 on a cheap old mobile phone.

Question, would connecting my scope to IC1-B/7 be ok? Wouldn't it hurt my scope in any way?

[DrGeoff]

C48 replaced, testing 100pF now

C4 should be 22uF, was showing 17uF, cheap noname cap, replaced anyway with panasonic brand.

Still going through the components... making my way to IC1-B/7.

Don't have a function generator so will just use a sweeping sine wave mp3 on a cheap old mobile phone.

Question, would connecting my scope to IC1-B/7 be ok? Wouldn't it hurt my scope in any way?

Nope, it's an op-amp output. Won't swing more than the power rails of +/-15V.
It feeds the two filters that split the signal into HF and LF parts.

Have you checked for signal on the volume pot?
I'm assuming that you are using a balanced input and you have checked your cable first.

[DrGeoff]

Looks a bit ugly. What's the V/div on IC1A-1?
Also make sure your unbalanced cable has the XLR pin 3 shorted to pin 1 (gnd) when you are feeding signal into pin 2.

[The Soulman]

On the first video everything looked ok at r7?
The other videos looked a bit messy? Where was the probe common connected? IC1a is in front of r7,
so if the signal is good at r7 we can assume IC1a is ok.
You really want to isolate the problem, to do so you could remove the two power amp input signal coupling
caps (c16 and c27), before you do you could measure the voltage across it shouldn't be more than a couple of millivolts dc.
A pop when switching on suggests a larger dc offset between the various stages..
A 4680 op-amp in this circuit could be replaced by an 4556,4558,4560,tl072,ne5532,njm2114d,opa2604,tle2072..
almost any dual op-amp you might have.

[Anks]

This screams missing connection to me. You need to check your soldering and all through holes and that the parts you have used are good. something is wrong that wasn't when you started as far as I can tell unless a capacitor as gone short but that would be evident by measuring. What dose the input to the main amplifiers look like.

[Anks]

Try and measure them Dc coupled so we can see if they have a bias.