Home Cinema active Subwoofer malfunctioning

I think Part 1 isn't an issue - or have I misunderstood? When the LED is red, the voltage of ZD1 is 0. When it's green, it's 5.6V. Is this what we are discussing?

Part 2 seems to make though - something should come into play during those current jolts, including on power up, but doesn't.

On my print-out, I progressively marking which components have been tested, or changed, a process of elimination could lead us on the right track... Is it worth testing all of the resistors individually? It seems they can be tested easily whilst on the PCB, but I'm not sure what the likeliness of a resistor failing is, in comparison to transistors etc.

So if I understand correctly, I look at the E-B and E-C voltages of each transistor, while the amp is ON, and the 1.5 battery plugged in as indicated above (R18 lifted etc)? I better watch out I don't short circuit anything...!

KeepItSimpleStupid said:

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?

Click to expand...

In circuit, R3 has a resistance of 9.85k; according to the diagram, it should be 22k though? Is this meaningless, or revealing information?

OP said:

In circuit, R3 has a resistance of 9.85k; according to the diagram, it should be 22k though? Is this meaningless, or revealing information?

Click to expand...

Meaningless.

op said:

I think Part 1 isn't an issue - or have I misunderstood? When the LED is red, the voltage of ZD1 is 0. When it's green, it's 5.6V. Is this what we are discussing?

Click to expand...

At least it's changing, which is fine. So, when the voltage across ZD1 is zero is there sound? There should not be or it should be drasticly reduced.

If there is sound, then I would suspect Q3 being shorted. Meaning, you should be looking at the C-E voltage drop when you have the 5.6V present.

What is the C-E voltage of Q3 when the 5.6 V is present?
What is the B-E voltage of Q3 when the 5.6 V is present?

Basically what happens is that when the voltage drop across R18 is somewhat bigger than 0.6V, it should pull the junction of R34/Zd4 to ground. This should remove the 5.6 volts across ZD4. That's not happening either.

So, you could have more than one problem.

I do need this info:

With the LED green and the 1.5 V battery applied to the lifted R20 and speaker positive with the polarity that we discussed:

What is:
Q12, E-B
Q12, E-C

Q14, E-B
Q14, E-C

Q13, E-B
Q13, E-C

Let me have that info.

You did say that you said you may have shorted the speaker terminals. R15, R16, R18 and R19 protect against that. R15 or R16 could have changed value. Check these.

Once I have the info, I'll probably suggest decoupling the DC protect from the mute circuit. I don't understand at this point whay the battery does not make the 5.6 V disappear.

OK, I'll run those tests tomorrow, once I get R20 lifted etc again (It's been back in for the other tests). In the meantime, I checked the resistors, they seem fine (including R15,16,18 and 19). I also checked Q13,14 and 15 (as well as 12) the other day - unsoldered, tested, and soldered back on. They seem fine... The problem still seems to be there, strange.

I did find a strange thing though, "RJ2", which is not referenced anywhere on the plan: it's a tiny resistor, with 1 black line only on it, which is situated between R18 and Speaker + on the board, just below R5. RJ2 is directly connected to Speaker +. The resistance seems to be 0 though, and I find it strange not to be on the plan, why would they add something useless to the PCB? I'm sure it's not, but I don't get it (and not sure if it "works").

When you ask whether there is sound or not, during the tests, would you want me to physically plug the speaker back in? Or is checking the voltage with the multimeter between speaker + and - sufficient?

More tests to come tomorrow... Thanks for the help, this is proving to be more challenging than I expected!

OP said:

I did find a strange thing though, "RJ2", which is not referenced anywhere on the plan: it's a tiny resistor, with 1 black line only on it, which is situated between R18 and Speaker + on the board, just below R5. RJ2 is directly connected to Speaker +. The resistance seems to be 0 though, and I find it strange not to be on the plan, why would they add something useless to the PCB? I'm sure it's not, but I don't get it (and not sure if it "works").

Click to expand...

That little component confuses everybody. It's just a wire in a resistor body so that automatic insertion equipment can handle it. RJ, i.e. Resistor, Jumper. It's also called a 0 ohm resistor. It's not useless. It saves the cost of a double sided board.

OP said:

When you ask whether there is sound or not, during the tests, would you want me to physically plug the speaker back in? Or is checking the voltage with the multimeter between speaker + and - sufficient?

Click to expand...

Using the presence of voltage should be fine for all of the tests.

In any event. R18 is used to look at the current. When the IR drop is greater tha about 0.6V then it is applied through R20/C10. R21 and D3 drains the cap if the music swings the other way. This turns on Q13 and 14 which pulls CONT/MUTE low.
The R24/C12 adds a bit of a delay and probably adds a short power on delay.

When music is supposed to play. i.e. unmuted, then CONTtrol/MUTE(really not MUTE) is applied to Q18. Q18 turns on Q19 and the + amplifier rail is used to apply power to the R3/ZD5/C3 R6 Zener voltage regulator. This turns on Q3 and the differential amplifier (Q1,Q2,R4,R5) is supplied by the constant current source (5.6+0.6)/1500 amps.

So, that's what is supposed to happen.

Note that CONT and MUTE are apparently the same terminal. I don't like the switch in terminaology from what seems to be CONTROL and MUTE. The levels are also wrong because MUTE is active low, therefore the signal should have a line on top if it signifying NOT or a ~ signifying NOT or Active LOW. e.f. MUTE is ACTIVE when the voltage there is LOW.

ASIDE: C11 and R22 form what's called a Zobel network and it's for amplifier stability.

You could try 3V rather than 1.5V. I could look at the datasheet for Q12 (http://www.datasheetcatalog.org/datasheets/228/70942_DS.pdf) to get a better feel for the number.

At this point I suspect Q3.

Without that constant current source present, the amp will have a hard time amplifying anything.

ok, here come some test results.

- First thing to notice, when my home cinema amp is switched on, the Sub-OUT cable always seems to generate 3V, whether the music is on or off; it seems like a constant flow of current. Secondly, in relation to this, an important point to note : when the sub is on "AUTO" mode, ie RED LED if there is no audio input or GREEN if it detects an audio input, if the cable is not plugged it, it stays RED (no surprise so far). However, when the cable is plugged in, it used to stay RED until music was actually played, then it would switch green. But now, as soon as the audio source is plugged in (and the homecinema amp plugged in), it goes GREEN, music or no music. This could be due to the "jolt" somewhere?
N.B: this is with R20 lifted, but NOT connected to the battery.

Some voltages (I used 3V in the lift):

- Audio source cable NOT plugged, LED forced GREEN, battery connected:
* speaker -+ = 0.3V
* ZD1 = 0V

- Audio source cable plugged in, no music playing, battery connected
* speaker = 0.03V
* ZD1 = 0.008V

- Audio source cable plugged, + Music, 3V NOT connected:
* speaker = 0.08V
* ZD1 = 5.5V

What is the C-E voltage of Q3 when the 5.6 V is present?
What is the B-E voltage of Q3 when the 5.6 V is present?

Click to expand...

Q3 C-E = 31.5V
Q3 B-E = 31V

With the LED green and the 1.5 V battery applied to the lifted R20 and speaker positive with the polarity that we discussed:

What is:
Q12, E-B
Q12, E-C

Q14, E-B
Q14, E-C

Q13, E-B
Q13, E-C

Click to expand...

With 3V battery applied to R20 lift:
(speaker out = 0.3V)
Q12 E-B = 0.7V
Q12 E-C = 0.01V

Q14 E-B = 0.7V
Q14 E-C = 0.01V

Q13 E-B = 0.7V
Q13 E-C = 0.01V

WITHOUT 3V battery applied to R20 lift, but with R20 lifted:
(speaker out = 0.3V)
Q12 E-B = 0V
Q12 E-C = 5.5V

Q14 E-B = 0V
Q14 E-C = 5.8V

Q13 E-B = 0V
Q13 E-C = 5.8V

OK, a little confused, but not too much I think.

Your measurement at the speaker terminals really don't determine the presence of a signal if your measuring DC volts which I assume you are. Measuring DC there, checks to make sure there is no DC on the speaker terminals. No big deal.

Your Q3 test. E-C with 5.6 V present should be <0.6 V. So, I believe Q3 is bad. You can try testing it out of circuit. Replace and then check for a <0.6 V when 5.6 V is present across ZD1. Put R20 back together.

OP said:

However, when the cable is plugged in, it used to stay RED until music was actually played, then it would switch green. But now, as soon as the audio source is plugged in (and the homecinema amp plugged in), it goes GREEN, music or no music. This could be due to the "jolt" somewhere?

Click to expand...

This, however, is disturbing.

Everything is effectively DC coupled, so do a few things.

1. Remove both L & R input plugs.
2. Measure the DC voltage at the ends of the output (disconnected leads) from your
source. Tip to shield. If there is DC here, then your source has a problem..

3. Turn on the sub and the source, don't connect the inputs.
4. Measure the DC and AC voltage from the shield side of the output of your source to the shield side of the input of the sub.

I tested the following transistors, out of circuit, which behaved fine:

Q1, Q2, Q3, Q4, Q5, Q6

7, 8 and 9 soon to come. This is strange...

1. Remove both L & R input plugs.
2. Measure the DC voltage at the ends of the output (disconnected leads) from your
source. Tip to shield. If there is DC here, then your source has a problem..

Click to expand...

DC = 3.3V
AC = 0V.

So it looks like I bust something in the amp too... I have plugged a walkman into the sub, will use that as the source for now (no Voltage), Sub behaves normally with it, switches on automatically (in auto mode), when music is played... So the source is another problem to worry about another time, but it seems to be independent (although maybe caused by it or the testing?)

EDIT : By "behave normally", I mean the auto switch mode, but the jolt is still there...

3. Turn on the sub and the source, don't connect the inputs.
4. Measure the DC and AC voltage from the shield side of the output of your source to the shield side of the input of the sub.

Click to expand...

DC:
Shielded parts of sub input + source cable = 0V
Inner parts of sub input + source cable = 1.3V

AC
Shielded parts of sub input + source cable = 0V
Inner parts of sub input + source cable = 1.6V

Based on your in-circuit test (E-C > 0.6 with the 5.6 V present)
Q3 is bad for sure. This is a gain (Hfe) issue.

If you take your 3V battery in series with say a 330 ohm resistor, you'd have an ~1 mA source. If you applied this to B-E; B+, E- because it's an NPN transistor and put your voltmeter across B-E you'd have the "DIODE TESTER". You may or may not find a problem doing all of the combinations that you did with your ohmmeter.

Is the gain of the sub up high?

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The DC on the input to the sub WILL cause the LED behavior for sure.

===

If you disconnected one of your input cables and took the left over 4.7 uf cap and placed it in series with the center pin and made the shield connection the LED would probably behave fine.

Two problems make things harder.

Is the gain of the sub up high?

Click to expand...

Yes - the gain was on max (I'be turned it down now)

The DC on the input to the sub WILL cause the LED behavior for sure.

Click to expand...

No idea what I have done to the source, but well, good to know, thanks. Another problem for another day!


Unfortunately I haven't simplified my case, slight setback here. Measuring Q3 voltage, I shorted the pins, which fried Q10, Q11, and I suspect D2, or something else. A fair bit of smoke was noticed... Around that area. So, I replaced some parts:

Q10, Q11, Q3, D2, Q7 (precaution), Q8 and Q9. That has helped, but not fixed my short "issue". Now the amp switches onto a RED light fine (whithout blowing a fuse), but as soon as I switch it to ON, the fuse goes. So clearly I have something else bust in there. I have some spare Ds, ZDs and some transistors (Q16, Q1-2-3, Q12) - not sure about replacing more stuff randomnly, it's a bit like carpet bombing, not the most efficient way of solving a problem.

However - to prevent any short circuits from happening again during measurements, I did buy some some special clips, that grip onto the bare parts of transistors, capacitors etc - no idea what they are called, but should really help...

I'm going to start looking at the board again, and try to fix this setback...

Well I tested my "old" Q7, and that one was behaving strangely. The old Q3 seemed fine though? Suprising.

Is is worth testing the Ds and ZDs? I assume I'd have to remove from from the circuit in order to do so?

Sorry about the magic smoke. Will me a learning experience for sure.

I still suspect the old Q3 especially since the gain was was way up. The source problem is probably a coupling cap. You should have no DC coming out of the pre amp outputs.

Since you don't have a variable AC transformer, you need to make a jig that consists of a plug/socket and an incadesent lamp in series with the line. A 25 W to 40 W lamp to start. When there is a short, the worst that can happen in is the lamp will glow and the power will be limited to the lamp wattage.


I think you can do the tests below with the AMP in mute.

Check the voltage across across R11 and R12 and between E-C of Q7.

===

Lifting of R15 and R16 will tell you if there is a problem with Q8, Q9, Q10 and Q11 There shouldn't be because there is no magic smoke in mute.

With it out of mute and R15 and r16 lifted, you should look at the voltage across R4 and R5. they should be identical.

Check the voltage across across R11 and R12 and between E-C of Q7.

Click to expand...

0V in each case. Any combination of Q7 (ECB) gives me 0V in mute mode. This is with R20 lifted, I assume it has no influence here.

I'll try lifting R15 and R16...

So the idea would be to plug the sub (main power cord) in series with a 25W-40W lamp?

Lifting of R15 and R16 will tell you if there is a problem with Q8, Q9, Q10 and Q11 There shouldn't be because there is no magic smoke in mute.

With it out of mute and R15 and r16 lifted, you should look at the voltage across R4 and R5. they should be identical.

Click to expand...

With R15 and R16 lifted (and once again, R21 still lifted), the amp switches on (without shorting); R4 = R5 = 0V though...

You mean R20 lifted, not R21? I don't think it matters anymore, but you can leave it unconnected for now.

Yes a standard light bulb is a current dependent resistor.

My bad....R15, R16 acts as a fuse for the front end of the amplifier not the current stage.

Pick up a 9V battery clip, a 9V battery and a diode (1N4001 or so) and a 10 ohm 1W resistor. Put the 9V battery positive lead in series with the forward biased diode. Put two of your Ez-hooks at each end. If you have a red clip call it positive. Two 22 ohm 1/2 W in parallel will work to.

What I'm going to want you to be able to do is with R15 and R16 lifted, is to put 9V from the battery/diode combo, hereafter called battery (+) between R15/R11 and (-) between R12/R16 and to be able to turn on the amp.

Now look at the voltages across:
ZD1
E-C of Q3
R4
R5
E-C of Q7
R13
R14

and we'll go from there.

Good luck!

This is one heck of a way to learn.

You mean R20 lifted, not R21? I don't think it matters anymore, but you can leave it unconnected for now.

Click to expand...

Yes, sorry - R20 is lifted, no confusion there.

Pick up a 9V battery clip, a 9V battery and a diode (1N4001 or so) and a 10 ohm 1W resistor. Put the 9V battery positive lead in series with the forward biased diode.

Click to expand...

- I have a 10 ohm resistor (a spare R15), I assume it'll handle 1W, although I have no idea how to tell. Only one way to find out!
- I have a spare 1N4148 - good enough I assume?

Let me track down a 9V batt, some wire, connections and get building...

Put two of your Ez-hooks at each end

Click to expand...

Good to know that's what they are called, useful bits of kit!