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Home Cinema active Subwoofer malfunctioning

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Great news, I replaced Q10 and Q11 which were indeed open, and now... I'm back to square 1! At least it partially works again, and the transformer is fine, thankfully... So, back to the original problem, how should I go about finding the problem between Q3 and the CONT terminal (if your suspicion is correct)?

This is a fun learning experience, but I am very grateful for hidden fuses !

Oops! There is a fuse. You might get lucky. Placing a 60 W incadesent lamp in series with the mains can lessen damage when powering back up.

Check Q8, Q9, Q10, and Q11 for shorts with an ohmmeter. Does your meter have a diode test mode? Do you know how to use it and what it does?

I suspect your thumping issue is between Q3 and the CONT terminal. Right now, that's a secondary problem.
 
It seems that I cannot test the transistors without removing them from the PCB, is that correct? I tried testing Q19, had some funny results, then unsoldered, tested it again, and seemed to be fine. What would the best way be to go about this, start testing all of the transistors individually (removing them)? Resistors can be tested on the PCB, that's easy enough, but those little bipolar transistors seem to be more of an issue?

Any quicker way of trying them out? Or maybe I should look for something else, maybe the Zener diodes? I will be honing my soldering skills anyway...!

Thanks again for the advice - this is interesting stuff.
 
I do want to to look at the voltage across R17, a 220 ohm resistor just to check if there are any problems with the amplifier repair.

I'd like you look at the voltage across ZD1 under 2 conditions:

1. As the unit is switched on
2. At steady state

The diode test of a meter basically used a constant current with enough voltage to turn on a diode junction. What you measure is the voltage drop or in the case of semiconductors, a diode drop which is about 0.6 V for silicon. Generally, this mode is also used for a beeping continuity test.

I'll print the schematic and take a better look at it.
 
In circuit tests are just that. In circuit. Way back in the days of vacuum tubes, it was common to do resistance measurements. With solid state stuff, you never know what transistor will be on and how on it will be. Thus, the diode test mode was invented to try to cope with this.

Devices such as the "Huntron tracker" allow one to look at a "signiture" which is effectively an I-V trace in circuit. When you have something known to compare against, it's a big help.

An ohmmeter can determine whether or not a cap is bad, but cannot determine if it's good. A capacitance meter can tell you if the capacitance is wrong, but will not tell you if the ESR is out to lunch. I've even had a multimeter (ohm test) tell me that "bad" fuse was good. Another example, an ohmmeter can tell you if a transistor is shorted, but may not be able to tell you if the gains are drastically different. I have an in circuit transistor checker which uses the principle of inversion to tell you that the transistor is grossly working. In circuit capacitance meters generally use a low oscillator level that won't turn on a semiconductor junction. Fortunately, most, not all, of the time we are looking for gross failures.
 
I do want to to look at the voltage across R17, a 220 ohm resistor just to check if there are any problems with the amplifier repair.

I'd like you look at the voltage across ZD1 under 2 conditions:

1. As the unit is switched on
2. At steady state

The diode test of a meter basically used a constant current with enough voltage to turn on a diode junction. What you measure is the voltage drop or in the case of semiconductors, a diode drop which is about 0.6 V for silicon. Generally, this mode is also used for a beeping continuity test.

I'll print the schematic and take a better look at it.

Hi,

I carefully took the measurements and found the following so far:

R17 = 2.17V (sub switched on); 0 in stand-by mode (LED1 = red)
ZD1 = 5.49V in both cases. When I switch the sub on, the voltage goes to 2.x and then 5.5V in a short time frame (less than a second). I do not have a hold function, but I can't seem to notice a jolt of any sort. When LED1 = red, or when the switch is off, the readings show 0V (no surprise). Steady state, I assume, is when it's ON, but with no signal going through it, after a few seconds (after a jolt);

In response to your last post, the LED1 works fine (on the other PCB - there is only one in the whole system anyway): RED = OFF / stand-by (the sub has a 3 positions switch, off, auto, and on, the auto switching the sub on when it detects a signal, and putting it in stand-by after 10-15min without a signal), and GREEN = ON

I hope my problem is due to a gross failure, some of those rarer diagnostics sound rather challenging !
 
OK, good. The time constant of R3C3 is short, so it's not responsible for a power up delay.

Make sure the amp thinks it's muted. e.g. remove the signal input and check for the voltage across ZD1. It should not be 5.6 V.

Now, go back to the point where you have 5.6 V across ZD1 and measure the voltage across the E-C junction of Q3. This is to basically check Q3 to see if it's shorted.
 
When I run those tests, their is no audio input connected to the sub in anyway - just purely the power cable (the subwoofer speaker is also disconnected, but I'm making sure the red and black cables (speaker out) are fully insulated...). Regardless of what I seem to be doing, ZD1 voltage is close to 5.5V when the LED is green (forced on), and 0V when the LED is red (auto and OFF modes) - am I doing this wrong?

Q3 E-C junction = 0.56V (LED green), 0V (LED red)

OK, good. The time constant of R3C3 is short, so it's not responsible for a power up delay.

Make sure the amp thinks it's muted. e.g. remove the signal input and check for the voltage across ZD1. It should not be 5.6 V.

Now, go back to the point where you have 5.6 V across ZD1 and measure the voltage across the E-C junction of Q3. This is to basically check Q3 to see if it's shorted.
 
That's good too. That's the bahavior I expect.

You can try to look at the voltage across R24 on power up. Blanket replace C12 and C10. Particularly C12. Q12 may be missing some protections, so check Q12 out of circuit.

Another thing, you can try is to lift R20 at R18 and take a 1.5V battery and apply 1.5 V to the lifted side of R20(+battery) and speaker + (-battery). The output should mute.

Suspect components are Q12, Q13, Q14, C10, C12, R24, R23, D4, D3, R21 with the higher probablility going for C12, Q12 and C10 in that order.

If you can force the circuit to turn on with the battery trick, then you can check for diode drops at the transistors and diodes. The power on delay/mute looks like it comes from R24 and C12.
 
Good stuff. With the power on (green LED, no source connected, as usual), I get the following readings:

R34 = 7V
R24 = 0V (couldn't get anything)
RX1 = 0V (not sure it's measured the same way, but I tried)
R23 = 0V

Blanket replacing = replacement with new parts, just to be sure, right?

Let me unsolder Q13 and report back with test results...

EDIT = Unsoldering Q12, at first, not Q13!

That's good too. That's the bahavior I expect.

You can try to look at the voltage across R24 on power up. Blanket replace C12 and C10. Particularly C12. Q12 may be missing some protections, so check Q12 out of circuit.

Another thing, you can try is to lift R20 at R18 and take a 1.5V battery and apply 1.5 V to the lifted side of R20(+battery) and speaker + (-battery). The output should mute.

Suspect components are Q12, Q13, Q14, C10, C12, R24, R23, D4, D3, R21 with the higher probablility going for C12, Q12 and C10 in that order.

If you can force the circuit to turn on with the battery trick, then you can check for diode drops at the transistors and diodes. The power on delay/mute looks like it comes from R24 and C12.
 
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Following those instructions Testing semiconductors with analog and digital multimeters (Testing Bipolar Transistors section), Q12 seems to be behaving fine, although I was a little surprised to find that the base wasn't in the middle of the transistor this time : with the text facing, the order seems to be as follows E C B. That caught me off my guard but apart from that, seems fine as far as I can tell.
 
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Once in circuit, Q12 behaves as expected, except for the following:

- Red on Base + Black on C = 0.93 (should be 1 I believe)
- Black on Base + Red on E = 1.2 (should also be 1 I believe?)

It's in circuit, so it might be meaningless (it is to me), but there goes anyway...

Following those instructions Testing semiconductors with analog and digital multimeters (Testing Bipolar Transistors section), Q12 seems to be behaving fine, although I was a little surprised to find that the base wasn't in the middle of the transistor this time : with the text facing, the order seems to be as follows E C B. That caught me off my guard but apart from that, seems fine as far as I can tell.
 
C12 is likely shorted and may measure shorted both in and out of circuit without power.

Blanket replace. Is just replace them.

Drops across transistor junctions are not as absolute as diodes and it depends on a lot of things. The test you did is really inconclusive, but leaning toward good.
 
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Great stuff, OK, thanks. Will get a couple of new caps tomorrow and throw those on to replace C10 and C12.

Another thing, you can try is to lift R20 at R18 and take a 1.5V battery and apply 1.5 V to the lifted side of R20(+battery) and speaker + (-battery). The output should mute.

I'm taking a look at R18 and R20 but I'm not sure I understand what you mean though - do I leave them on the PCB and connect a battery to R20 and the speaker +?

C12 is likely shorted and may measure shorted both in and out of circuit without power.

Blanket replace. Is just replace them.
 
"Lift" is a term for unsolering one side. So unsolder the side of R20 that connects to the Emitter of Q10 and the resistor R18.

To the lifted lead attach the positive side of a 1.5 V battery. The negative side of the 1.5V battery would connect to speaker positive.

Power up the amp in this condition.

If the circuit is working, the amp should mute. i.e. no sound or low volume.
If the circuit is defective, the AMP will pass sound.
 
Let's make sure I got this right.

Did you say that you ALWAYS get 5.6 V across ZD1 no matter if the mute light is ON or OFF? A no signal should generate a MUTE and signal should unmute it and the voltage across ZD1 should change.

I think were still in the right ball park.
 
No no, I get 5.6V when it's ON, and 0 when it's OFF. I was expected a jolt, or some change, but that's all I can get from it.



Let's make sure I got this right.

Did you say that you ALWAYS get 5.6 V across ZD1 no matter if the mute light is ON or OFF? A no signal should generate a MUTE and signal should unmute it and the voltage across ZD1 should change.

I think were still in the right ball park.
 
OK, so, I changed C10 and C12; still no change. I then carried out that battery test, with a voltmeter connected to the speaker terminals, and there were no fluctations, no change, with or without the battery. I made sure the source was pumping some music, and the sub input was around 3V roughly, constant. I didn't do the test with the speaker connected, I assume the voltmeter reading amounts to the same thing?

Does that mean you've found the defective part of the circuit?

"Lift" is a term for unsolering one side. So unsolder the side of R20 that connects to the Emitter of Q10 and the resistor R18.

To the lifted lead attach the positive side of a 1.5 V battery. The negative side of the 1.5V battery would connect to speaker positive.

Power up the amp in this condition.

If the circuit is working, the amp should mute. i.e. no sound or low volume.
If the circuit is defective, the AMP will pass sound.
 
Right now I don't have the schematic in front of me (on phone),
but the lack of the 5.6 V signal not dropping coincident with no
Audio is the problem.

Part 1 is the lack of muting with no signal in, but the LED indicates that it is
supposed to be muting.

Part. 2 is that the circuitry at the ouput basiclly should come into play
When there is excess DC on the speaker. It may come into play for a brief
Period on power up.

Part 3 is that they are interrelated. Either should be able to mute the output.

The battery basiclly should force the system to think it's in current limit. It does't
In this mode,, you should be able to check to see if the transistors are functoning.
Shorts are common transistor failures.

Meanwhile, you can look at the E-B and E-C voltages with the battery in place and
The LED indicating UNMUTE.

When I get home, I'll try to figure out how to disconnect the DC PROTECT side.

Make sense?
 
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