DC detection circuit failure on Subwoofer

[eccerr0r]

Q12 and Q13 though are the same part number, but don't need to be matched looking at the circuit configuration. You could try putting Q12 into Q13 slot and leave Q12 open (or use another cheap transistor here) to see if it works but I think Q13 slot may be a transistor killer in the long term, though the spec sheets seem to indicate the KTC2874 is a bit more robust than the typical 2SC945, 2N3904, 2N2222, etc.-like transistors which would not like being put in that kind of circuit.

So make sure you use an authentic KTC2874 if you buy one, if the KTC2874 datasheet is trustable to begin with (translation: DO NOT SUBSTITUTE IF DATA SHEET IS REAL!). Position Q12 is a bit more safe for substitution, you can use a cheap 2sc945 or something like that here instead when you borrow the KTC2874 for Q13.

[budm]

Vebo of the original Q13 Transistor is 25V so what ever you use make that spec is also matched the original, 2n2222 has Vebo of around 6V, you can see the E-B junction of Q13 is exposed to reverse bias of 10V or more.

[Durenth]

Hi!

First off all I'll answer Budm question:
"I think you need to report the Voltage readings of all the Base Voltages (with ref to circuit ground) of the Transistors when Q13 is in place."

Here are the voltages with Q13 removed. They're all matching the service manual references (indicated in pink):


Here are the voltages when I put Q13 back in place:


If I got you well Budm, you think Q13 is bad but the problem is a leak from base to collector?

[Durenth]

eccerr0r, if I substitute Q13 with Q12, it works fine.
But then I don't have the DC protection in the audio signal anymore.

So find myself with the problem of the replacement for Q12.
It's kind of hard to find a C2874, I only found an Ebay shop in Great Britain that have some and they're quite expensive..

I understand you say a 2sc945 will do but don't you guys think the 2SC2878, that is exactly the same as C2874 (see my post above with datasheet) except for the Collector Power Dissipation that is 400mW instead of 625 mW is also ok?

Then I'm curious about the "Q13 slot may be a transistor killer in the long term". If you have time could you tell me why? Maybe In can improve the circuit if I get to understand the issue..

Bye!

[Durenth]

I think I should add that the "audio signal form speaker" is AC +/-18V
It might be important though the RC (R27/C25) circuit on the base of Q12 filters it if I understood the circuit correctly...

[budm]

Q12 is for detecting + DC offset of the amplifier output and shutdown the relay by killing the bias for Q11, when Amp output has - DC offset, Q11 will be bias off due to reverse bias.
So right now you have problem with leaky B-C junction of Q13, D18 is clamping the negative Voltage to - 0.5V because it is forward bias by that -4.5V Voltage source.

[Durenth]

Originally Posted by budm

Vebo of the original Q13 Transistor is 25V so what ever you use make that spec is also matched the original, 2n2222 has Vebo of around 6V, you can see the E-B junction of Q13 is exposed to reverse bias of 10V or more.

Ok, thanks! So 2SC2878 is ok: it has a Vebo of 25V

[budm]

2SC28278 is fine, you are not dissipating that much power in that section of the circuit.
They could have put 4.7V zener to clamp the maximum reverse Voltage on E-B junction to -5V so you do not need to get such high Vebo spec.
Q13 is used to turn off the relay when AC power is removed, C13 negative power supply filer will go away fast when AC power is removed then Q13 will be bias on by the 12V supply (which drained slowly) via R34..
So basically, the speaker relay must:
1) Delay turned on when AC is first applied to the unit so the speaker will not pop due to amplifier need settling time for the AMP DC off set to be at 0V..
2) Protect the speaker when it detects + or - Voltage offset of the amplifier that will damage the speaker.
3) Turn off the real fast when AC power is removed.