Mackie CR4 amplifier no sound

[UserXP]

Re: Mackie CR4 amplifier no sound

Sorry, C39 1uF Lytics Cap connected in parallel with R92.

Oh, you meant that one. OK, got it. I will check that as well. I am also waiting for the input on those measurements.

[UserXP]

Re: Mackie CR4 amplifier no sound

Report back what results you get for all of the about items I asked about and then we can continue from there.

I did the measurements and some more/other observations above.
This looks like it's probably a power issue as the sound is superb when the unit is working, no hiss, no crackling or buzzing whatsoever.

[budm]

Re: Mackie CR4 amplifier no sound

Sorry, C39 1uF Lytics Cap connected in parallel with R92.

[UserXP]

Re: Mackie CR4 amplifier no sound

If C9 has bad leakage resistance it will affect the Mute pin Voltage, you can remove just for testing to see if the sound will stay on.
BTW, when test capacitance, ESR, etc., you need to remove the cap from circuit since it has other component connected to the cap.

C9? The small SMD capacitor above the U1 chip?

[budm]

Re: Mackie CR4 amplifier no sound

If C9 has bad leakage resistance it will affect the Mute pin Voltage, you can remove just for testing to see if the sound will stay on.
BTW, when test capacitance, ESR, etc., you need to remove the cap from circuit since it has other component connected to the cap.

[UserXP]

Re: Mackie CR4 amplifier no sound

On the working unit without phones, these are the values and all are quite constant:

Line A: +12.12V
R88: +12.12V and +2.9V.
Q1 Base: -1.93V.
C74 (negative lead): -1.93V.
R86: +12.12v and -1.93V.
R77: -11.89V and -1.93V.
Q2 (collector): +13.60-82V and does not go over 14V, let alone 15V.

[UserXP]

Re: Mackie CR4 amplifier no sound

And, momaka, you are right about C74. While the unit was playing, I placed another empty 16V100uF capacitor on the contacts of the soldered one - the unit immediately went silent for about 1,5 seconds while the capacitor was charging and then resumed. This, however, did not influence the voltage on Pin 5, it didn't go back to 14.1V, it just left off where it was. Once it goes over 15.1x volts the volume starts to fade or fluctuate. I guess we have to find what is leaking and adding those 0.01 increments. Any idas on that?
Hopefully, I provided all the measurements you suggested.

[UserXP]

Re: Mackie CR4 amplifier no sound

In circuit, C74 measures ~134uF. But so does the one in the working unit, it's not ~100uF.

[UserXP]

Re: Mackie CR4 amplifier no sound

Update: I switched off, waited a couple of minutes and turned it on again. Before that, I attached the probes on the ground and pin 5, so that when I turn on the amplifier I can watch the voltages immediately.
Upon power on, it displays +21V and after ~1.5 seconds it drops to +14.1V and then starts to slowly increase by 0.01, but the increment is not constant; at times it is steady, then it will increase more rapidly, then it will drop by a few 0.01 units. But it gradually increases overall. The amplifier then works until it reaches that threshold and then the volume starts to fluctuate and fade out. Does this new information shed some light on the possible cause? What should I check while it' still working?

[UserXP]

Re: Mackie CR4 amplifier no sound

I managed to "capture" the voltage on Pin5 when it started to fade. The threshold seems to be at around 15.25V. As it starts to fade, the voltage increases in increments of 0.01. Each additional 0.01V increment further fades the sound. As it gets close to ~15.45V the volume is very low and disapeears completely at around 15.5V.
I tried to measure the voltages on contacts you suggested and noticed that on certain contacts when I place the probe, the volume instantly returns and goes away again.
So, as I suspected, it worked for a minute or two and went silent again.
I measured the voltage on pin 5 when the amplifired first worked and it was around 14.1V. Then something obviously started to increase it. It really is like something is "leaking".

And I don't know what to do to reproduce the working again. The only common thing between those working moments I can think of is that the unit was left unpowered for a couple of days.

[UserXP]

Re: Mackie CR4 amplifier no sound

Momaka, thank you for the detailed instructions.

Ok, here it goes.

While the headphones are PLUGGED:

Line A voltage reads around 0.100mV (fluctuating).
R88 reads the same voltage of ~0.100mV on both its ends.
Q1 base reads -2.1V.
C74 reads -2.1V on its negative lead.
R86 reads +12.0V on one end and -2.10V on the other.
R77 reads -11.9V on one end and -2.10V on the other.
Q2 Collector reads +21V.

While headphones are UNPLUGGED:

Line A voltage reads around +12V.
R88 reads +12V on one end and +2.9V on the other.
Q1 base reads -2.1V.
C74 reads -2.1V on its negative lead.
R86 reads +12.0 on one end and -2.10V on the other.
R77 reads -11.9V on one end and-2.10V on the other.
Q2 collector reads +14V.

It appears that Q1 is fine or not? I didn't get exactly the same values like you suggested, but I'd say most of them are close. I didn't remove C74 because it measured close to what you expected, so it looks like its OK. But please have a say about this.

This is the first test after I resoldered back that R22 and tested the unit inside the speaker with all the cables attached. I don't get it. The readings now show that the amplifier will play. And it does. But I can't say what's different now because I haven't replaced any component. I have a feeling it's just waiting to start to fade out again and problem not solved. And that R22 still doesn't measure instantly at 10kOhms, it's stil fluctuating and giving uneven readings on switched probes.

[momaka]

Re: Mackie CR4 amplifier no sound

Blue is normal operation (sound on), red voltages are when the amplifier enters the mute/standby mode. I don't understand what switches those voltages, maybe you can.

He said that trace A comes from the headphone jack. In normal mode it is around 12V. When phones are plugged in, it drops to 0.250V and TDA switches to +20V and mutes.
Trace B is for the mute pin of TDA7265, again blue voltage is for the play mode, red is for the mute mode.
Trace C is what he called a "soft start". I suppose what he meant is that it mutes the amplifier on turning the speakers on as there really is a ~2 seconds silent delay from the moment you turn the power on until the sound starts (on the working unit, that is).

Going by that description, I think the way it works is this:

Upon power up, capacitor C74 (previously discharged) acts momentarily like a "short-circuit" while it is charging up. This pulls the voltage high on the Base of Q1. As such, Q1 is -conducting-. With this, regardless if headphones are plugged in or not, the collector of Q1 will be low (i.e. near 0V), which means the Base of Q2 will also be at near 0V. Because of that, Q2 will NOT be conducting and so VCC is left unchecked to supply whatever voltage (probably close to 19V as the schem suggest) to the Mute pin on the amp IC (pin 5)... which means the amp IC should be in standby.

Once C74 charges up, it essentially acts as if it's "disappeared" from the circuit, so now the voltage on the Base of Q1 is determined by the resistive divider formed by R86 and R77, which will come out to around -2.2V. This negative voltage makes Q1 to STOP conducting (and in fact makes it look as if Q1 is removed from the circuit - it will be high-impedance.) With that, now the voltage on the Base of Q2 will depend on if headphones are plugged in or not. If headphones are plugged in, Q2 base will be near 0V, so Q2 will not be conducting and again we are left with VCC to dump whatever voltage it is on the Mute pin of the amp IC.

Now once headphones are unplugged and C74 startup has been charged up (and thus Q1 still not conducting), then the voltage on the Base of Q2 will be pulled "high" through resistor R88 by that +12.99V line A. When the Base of Q2 is pulled high, Q2 will start conducting current. And with that, now you have a voltage divider formed (more or less) with resistors R91 and R92, dividing the VCC voltage according to their resistance... and hopefully in the range that is needed for the amp IC to come out of Mute/STBY and turn On. C39 is there to provide a "smooth" transition from STBY/Mute to On state.

So with this information, first measure your voltage on line A on the non-working unit when you have the headphones plugged in and when they are not plugged in. Do this test by actually plugging in headphones in the headphones jack, not just shorting the switches. Do you get close to that +13V mentioned by the Mackie guy when headphones are unplugged? If yes, continue below...

Now check the voltage on the Base of Q1 after the unit has been turned On for a few seconds. To avoid confusion, measure the voltage on the negative (-) lead of cap C74 relative to ground. Do you get around -2.2 or -2.3V after a few seconds? If NO, I suspect either C74 or Q1 may be faulty. Since C74 is an electrolytic cap with only two leads, try removing it and replace it with another cap. Note: you don't need to match the capacitance value of C74 exactly to the original. If you use a smaller capacitance, the soft-start will be shorter, and if you use a bigger cap, soft-start will be longer. The reason I suspect C74 is because electrolytic caps can get electrically leaky over time. If C74 is leaky, that could be the reason you're not seeing -2.2 to 2.3V on the base of Q1... or it could also be Q1 is faulty. However, if you do replace C74 with a new cap and the voltage on the base of Q1 is still not -2.2 to -2.3V, try removing the cap altogether and then power On the unit. This will remove the soft-start feature, but you should now get -2.2V to -2.3V on the base of Q1. (Also note that removing C74 may cause the speaker to "pop" very loudly during startup, so if you want, you can disconnect the speaker for this test.) If you still don't get -2.2V to -2.3V on the Base of Q1, then very likely Q1 is faulty. Before removing it, though, have one last check on the voltages on each side of R86 and R77 relative to ground. One side of R86 should have 13V going to it and one side of R77 should have about -13V going to it. The middle point where they meet should be -2.2V to -2.3V... but if it's not, then Q1 is definitely bad.

Report back what results you get for all of the above items I asked about and then we can continue from there.

[petehall347]

Re: Mackie CR4 amplifier no sound

q2 should be pulling the voltage down at the mute pin .
check voltages again at q2 .

[UserXP]

Re: Mackie CR4 amplifier no sound



He said that trace A comes from the headphone jack. In normal mode it is around 12V. When phones are plugged in, it drops to 0.250V and TDA switches to +20V and mutes.
Trace B is for the mute pin of TDA7265, again blue voltage is for the play mode, red is for the mute mode.
Trace C is what he called a "soft start". I suppose what he meant is that it mutes the amplifier on turning the speakers on as there really is a ~2 seconds silent delay from the moment you turn the power on until the sound starts (on the working unit, that is). He mentioned that "cutting the copper trace at line C will disable the soft start and enable the amplifier". Sounds to me like he suggested a "cut pin number 5" kind of a workaround, which is a no-no.

What are your thoughts on all this?

[UserXP]

Re: Mackie CR4 amplifier no sound

If you remember that image I posted a while ago sent by the Mackie guy, he marked blue and red voltage values. Blue is normal operation (sound on), red voltages are when the amplifier enters the mute/standby mode. I don't understand what switches those voltages, maybe you can.

I will test again, the difficult part is I have to dismantle the working unit every time I need to re-measure something.

[UserXP]

Re: Mackie CR4 amplifier no sound

An update regarding that R22 resistor.

On the working unit, this R22 instantly reads at a steady 10.02kOhm value no matter which way I connect the probes to its contacts, it simply remains consistent.

On the faulty unit, when I place the black probe on the contact connected to the the ground and also to the emitter of Q1, and the red probe on the other contact, I get a steday ~12kOhm reading. If I switch the probes I get around 8.4kOhm slowly increasing to around 9kOhm and then the increment becomes very slow but it's still there. All this is far different than on the working unit. And remember I desoldered and tested that R22 and it was within its specs ~10kOhm.

Does this indicate anything? Can a faulty transistor induce these results? I still haven't desoldered/replaced the transistor as I'd like to be sure I dont't have to deal with the unsoldering of that SMD unless we can conclude it is a definite cause.
I also tested Q1 and Q2 of the workng unit in the diode mode for NPN, both show equal results between themselves, and I get those same results between the Q1 and Q2 of the faulty unit, kind of makes me postpone the transistor replacement as all four of them show almost identical readings in diode test mode.

[petehall347]

Re: Mackie CR4 amplifier no sound

dont force it in any way or it will rip the pads off the board .been there done that . hot air is best if your back can take it unlike mine .

[momaka]

Re: Mackie CR4 amplifier no sound

I have a 30W soldering iron, but with a pointed tip.

That should work fine if you have a better tip for it or if you sand it into a chisel/bevel tip.

I can try to use the wick and slightly separate all legs from their pads, one leg at a time.

No, don't do that. You will most certainly destroy the transistor that way.
Those small SMD devices are not designed to flex their leads at all. If you try it, most likely you'll pull a lead out of that transistor, and then you would need to replace it for sure.

Leave the wick for cleaning through holes (or BGA work, if you ever do any of that.) For plain SMD work, wick has no place here.

I don't think I could leave the through-hole protruding from the board like that as it would probably hit the inner wall of the speaker housing as the board comes really close to it.

It also leaves the possibility of the transistor solder joints breaking away from the vibrations in the woofer cabinet... that is, unless you glue it down well to the PCB.

[UserXP]

Re: Mackie CR4 amplifier no sound

If you damage that transistor, you don't necessarily need to replace it with an SMD one. Through-hole will work fine too. Just may not look as pretty with all the leads soldered to the board, but who cares as long as it works as intended.

Thanks, the unsoldering tips sound really useful and I will try that. I have a 30W soldering iron, but with a pointed tip. I can try to use the wick and slightly separate all legs from their pads, one leg at a time.

Now, the through-hole transistor I bought will be for testing purposes only. If after soldering it in place the unit starts to operate properly, I'll know it was the faulty transistor and will order a SMD type. I don't think I could leave the through-hole protruding from the board like that as it would probably hit the inner wall of the speaker housing as the board comes really close to it. But, if its the transistor, at least that's something and I won't mind ordering the SMD version to make a full reapir.
I will post back with the results.

[momaka]

Re: Mackie CR4 amplifier no sound

I will attempt to desolder Q2 without damaging it, but it is so tiny i am afraid the soldering iron is going to melt it.

You won't melt it unless your iron is super-hot and you really keep the iron on that transistor for too long (i.e. more than 20-30 seconds at once.) By super-hot iron, typically those would be 40+ Watts irons that plug directly into the wall (i.e. no temperature regulation.) If it's a temperature-controlled station, things should be fine.

The way to remove those transistors with just a soldering iron...
Put flux on all leads on the SMD transistor and then add big solder blobs that stick up and above the case. Next, add more flux to the leads again. After this, melt a blob of solder on your soldering iron's tip that's big enough to cover the entire SMD transistor form the top... but just watch out not to make the blob too big and flood/bridge nearby SMD components too. Then put it over the transistor so the blob of solder on your iron contacts the solder blobs on the SMD transistor's leads. The transistor should either stick to the solder blob on your iron when you move it around a bit, or you may need to use tweezers to dislodge and pickup the transistor while you're heating it with the soldering iron (it just depends if the SMD components on the board were glued or not during manufacturing.)

It will also help if you use a fairly wide tip on your iron - so either chisel or bevel type. I suggest avoiding conical tips. If all you have is conical tips... if you have spare one (or preferably one that looks ugly and looks worn out), you can grind it into a chisel tip. Better yet, grind it into a "spoon" shape (i.e. concave like a cup.) This shape can hold a lot of solder and is what I use on my 30 Watt soldering iron for removing SOT-23 transistors all the time.

After you remove the SMD transistor, you may have to put flux on its leads and touch each one with the soldering iron to clear any excess solder or solder bridges. I do this while holding the transistor with tweezers.

My local stores do not have the SMD type of these amplifier transistors in stock.

If you damage that transistor, you don't necessarily need to replace it with an SMD one. Through-hole will work fine too. Just may not look as pretty with all the leads soldered to the board, but who cares as long as it works as intended.