Mackie CR4 amplifier no sound

[UserXP]

Re: Mackie CR4 amplifier no sound

The failed capacitor data:

Brand: Cap-top
Series: SK
Rating: 50V1uF 105°C
Size (hight x diameter): 11mm x 5mm

Could you suggest a suitable replacement, preferably but not necessarily with a black sleeve and white lettering and a "x" shape of the vent?

[UserXP]

Re: Mackie CR4 amplifier no sound

I think you solved the problem. It really is C39 that was the cause of the problem. The amplifier is still playing fine, the voltage on pin 5 hovers steadily between +14.11V and +14.25V. The Mackie guy said that these voltages are normal to vary; on the working unit it also varies, it's just slightly lower and below 14V, probably due to variations in material.
Momaka, Budm and Pete - thank you so much, guys! You did it! From bad transistors to a bad capacitor, you persevered 7 pages with me and the remote diagnostics from somebody with little to no knowledge in electronics such as myself.
I am looking forward to your input and suggestions on which caps to use.
And who knows how many CR4s this thread will save from being thrown into junk, thanks to you.

[UserXP]

Re: Mackie CR4 amplifier no sound

It is definitively hovering right. Now it dropped to 14.10V. It's still playing without stuttering the volume, this is great! Please, advise which replacement capacitor should I go for? Rubycon, Panasonic, any Japanese and an adequate series.
If the problem gets solved, and it definitively is promising, we should compose a troubleshooting guide for these monitor speakers for repair as no complete schematics are available. Just for future reference.

[UserXP]

Re: Mackie CR4 amplifier no sound

It is still playing, now at +14.2V. The whole board is now warm. I guess the temperature might be influencing the capacitors. They are some "Cap-Top" brand, all of them. The small ones are especially prone to drying due to heat. C39 is the closest to the heatsink. I guess the working unit has a better batch of caps in this area of the PCB as they've been fine for 5 years now.

[UserXP]

Re: Mackie CR4 amplifier no sound

Still hovering at +14V. This has been the longest play mode so far.
Without C39, I noticed low crackling when switching to headphones and measuring.

Speaking of which, here are the transistor voltages:

Without Headphones:
Q1 B = +2.10V
Q1 E = +0.00V
Q1 C = +2.93V

Q2 B = +2.93V
Q2 E = +2.35V
Q2 C = +14.00V

With headphones:
Q1 B = +130mV
Q1 E = +0.00V
Q1 C = +160mV

Q2 B = +200mV
Q2 E = +0.00V
Q2 C = +21.00V

[UserXP]

Re: Mackie CR4 amplifier no sound

lowest voltage goes to -

Ok.
Well, it's just reached +14.06V but very, very steady. Nothing dramatic like before. Still playing, though.

Does this capacitor need to be 1uF? I have Panasonic 50V2.2uF I think FR series at my disposal. Will it be fine to use it, or should I go for the original 1uF? If yes, what brand/series would you recommend for a cap of this function and this close to the heatsink?

[petehall347]

Re: Mackie CR4 amplifier no sound

lowest voltage goes to -

[UserXP]

Re: Mackie CR4 amplifier no sound

Guys, I think you are on to something here! I've just connected the amplifier to the speaker's housing and it's working! With C39 out of the circuit, I get almost steady +13.89V on pin 5. It doesn't even reach +14V, let alone approach the +15V mute threshold. It's been playing for almost half an hour with no change.

What I don't understand is that one pad of C39 measures +20V and the other (the one on pin 5 lead) measures +13.90V. What kind of a capacitor is this, how can it have two positive supply rails?

[budm]

Re: Mackie CR4 amplifier no sound

If that C39 has leakage resistance, it will affect the Voltage of the divider circuit Voltage, just power up the unit and see if the pin 5 will be stable or not without the cap.
At this point I would get a couple 1uF for this unit and the working unit since more likely the one in working unit will fail later on as well

[UserXP]

Re: Mackie CR4 amplifier no sound

One additional observation: now that C39 is out of the circuit, R22 measures perfectly! It instantly shows 9.997 or 10.00 kohm no matter how I place the probes, like on the working unit. I am looking forward to testing the unit tomorrow in the full set-up. But, this is the first time I've seen that R22 measures like it's supposed to. I really hope this will impact the voltage in a positive way.
I have desoldered the leg of C39 that is in contact with Q2's collector and TDA's pin 5.

Any thoughts on this?

[UserXP]

Re: Mackie CR4 amplifier no sound

I will disassemble the workng unit tomorrow to attach its cables to the non-working unit and test the behaviour with the C39 unsoldered.
To me, the ESR data looks normal for a capacitor with those ratings.

[UserXP]

Re: Mackie CR4 amplifier no sound

I unsoldered one leg of capacitor C39, labelled as 50V1uF. The ESR meter displays:

965nF
ESR=1.5ohm
Vloss= 0.4%

[petehall347]

Re: Mackie CR4 amplifier no sound

Q2 is controlled by its base .

[UserXP]

Re: Mackie CR4 amplifier no sound

OK, one silly question - since Q2's collector, which sends the different voltages to TDA's Pin 5, is connected to that pin through a jumper, would it be possible to replace that jumper with a small resistor that will always keep the voltage going through it like 2 volts less and thus make it work?
So, if in play mode Q2 sends 14-15V to pin 5, the resistor could lower it to maybe 13V.
And in mute mode, instead of those 21V, it could reduce it to like 19V which would certainly make the amplifier mute (by what I witnessed yesterday when it comes to fading sound, I'd say it would mute fully at 15.5V, 16V or 17V for sure).

Again, as this does sound like a workaround and it's just there as a question, I would like to fix the cause of the issue and not improvise anything, it's just a question ( ). Of course, the current leak might continue and still reach those 14-15V even with that resistor, I am just curious and making a wild guess here.

[UserXP]

Re: Mackie CR4 amplifier no sound

Guys, thanks so much for the input and for perseverance. Although I haven't undesrstood half of what you wrote about ( ), I do see what I am to do. OK, I will desolder one contact of C39 and test again to see what happens. I will also measure again the voltages on both transistors' three contacts.
Momaka, sorry, I must have misunderstood your prvious message with the instructions on what to measure, you were referring to Q2 only while explaining the entire mechanism, I didn't realize I was to measure its base also so I only measured that of Q1. But, seeing that Q2's base is connected to one end of R88, the voltage is probably the same (+2.9V in play mode and +0.100mV in mute mode) - sorry about that, this time I will measure all three legs on both transistors with headphones both plugged and unplugged.
I pray that it is a cap's fault as it would be the easiest part to replace.

[momaka]

Re: Mackie CR4 amplifier no sound

i would have added wires to Q2 C and E and shorted them when it went back into mute . then i saw that 10v difference . it should be hard on .. maybe i not looking at it right and the resistors are throwing me off .

You know, that actually brings a good question:
Should Q2 be "hard on" (i.e. operating in Saturation) in the above circuit?

The more I look at it, the more I think not. Perhaps a Mackie designer can answer why R91 is connected between ground and Emitter of Q2 instead of pin 5 of amp IC and Collector of Q2 (with Q2 Emitter tied directly to ground.) In that arrangement, Q2 can easily be used as a "switching device". Whereas right now, I think it looks more like a current amplifier... and the problem with this is that even small mV changes in the Base voltage of Q2 can induce large changes in the current flow through its Emitter. And while B-E voltage will always be about a diode drop apart, C-E doesn't have to be. In fact, C-E can be any voltage if Q2 is operating in its Active region... which is what I think may be happening here too.

So if C39 is determined to be OK... or at least the amplifier still acting goofy and muting by itself without C39, then I think there is also a possibility that we may be looking at a marginal design in the mute circuit.

**EDIT**
I looked at the circuit again, and indeed Q2 is used more like an amplifier than a switch so that it can drive different voltages on pin 5 in order to go between STBY/Mute/Play.
But IMHO, this still looks like an awfully complex way to do that.

[petehall347]

Re: Mackie CR4 amplifier no sound

whoops i meant C to E .
anyway if Q2 base was monitored and it stays the same then it should be staying out of mute .
i would have added wires to Q2 C and E and shorted them when it went back into mute . then i saw that 10v difference . it should be hard on .. maybe i not looking at it right and the resistors are throwing me off .

[momaka]

Re: Mackie CR4 amplifier no sound

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

Never measure caps in circuit - it won't give you accurate results.
In the case above, the cap is over 20% of nominal capacitance, which means the cap could be bad. But it may not be. We simply can't know, because the cap is in circuit, and that extra capacitance could be caused by something else. So if you have to measure electrolytic caps in the future, just desolder them and then test. I know it's an extra step, but it gives accurate results. Otherwise, you just have to ASSUME that C74 is OK. And when you assume, that's when you might miss an issue somewhere.

With that said, it does look like C74 is OK, since you do get around -2.1 to -2.3V on the Base of Q1. Really any voltage below 0.4V should keep Q1 off / not conducting, so I think the soft-start circuit with Q1 is working normally... and moreover, you verified that when you added another 100 uF cap in parallel with C74, which immediately muted the speaker for around 1.5 seconds again.

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.

Well, I can't say with 100% confidence - I believe I also asked for the voltage at the Base of Q2 (which would also be the voltage at the Collector of Q1)... but I think we can give Q1 a pass for now, because it looks like the voltage at the Collector of Q2 switches between Mute and Play mode normally... so don't worry about measuring the Base of Q2 / Collector of Q1 right now.

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?

Yes, this info does help, actually.

Since the voltage on the Mute pin (and also Collector of Q2) is slowly increasing, it looks like there is a leakage current / drift from somewhere. It may be Q2, but it may also be C39 as budm suggested.

Overall, I concur with budm - remove C39 and see if the voltage creeps up again and whether or not it goes over the threshold. While at it, also check capacitance and ESR on C39 (out of circuit) and report back what you get.

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.

No, I don't think it's a power issue, unless you see both VCC voltage and the +/-12V rails on the faulty unit to be somewhat far off from the working unit... in which case, the small voltage differences could be what's causing this issue, and that means Mackie designer(s) didn't factor in circuit voltage drift over time (and due to mfg. tolerances) when they designed the STB/Mute/Play circuit. So if that's the case, we may be able to correct the circuit by modifying R92 or R91 resistance values. But try the test with C39 first as budm suggested (i.e. remove from circuit), and report back if voltage is still drifting up. Then we can go from there.

looking at the schematic it looks like there should be 10v B to E on Q2 .. seems a bit much to me .

I don't think that's possible. Z1 should clamp the voltage to ~3.9V max.
On the other hand, maybe Z1 could also be leaky, causing Q2 Base voltage to slowly decrease over time and make Q2 conduct less, thus Q2 Collector voltage increasing. IDK, just throwing that out there as a possibility too, though I think it's less likely. I think the main suspects here are still C39 and Q2.

[petehall347]

Re: Mackie CR4 amplifier no sound

looking at the schematic it looks like there should be 10v B to E on Q2 .. seems a bit much to me .

[budm]

Re: Mackie CR4 amplifier no sound

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

You can just lift one leg off the board and see if the unit will function.