SHS Mixer DC Protection Circuit

[tdlunsfo]

I am hoping someone smarter than me can help figure this circuit out. I have two identical mixers here both with the same problems. DC protection is activated but when relay is jumpered there is no DC on the output. All parts in the circuit test good. Anyone have experience with this type of circuit? I see the darlington pair acting as a switch but after that I got lost. Lastly, the relay tests good as well as I applied 12v to it and confirmed that the contacts close. Note: the middle line heading of the page goes to the 15v regulator.

[J M Fahey]

I am hoping someone smarter than me can help figure this circuit out. I have two identical mixers here both with the same problems. DC protection is activated but when relay is jumpered there is no DC on the output. All parts in the circuit test good. Anyone have experience with this type of circuit? I see the darlington pair acting as a switch but after that I got lost. Lastly, the relay tests good as well as I applied 12v to it and confirmed that the contacts close. Note: the middle line heading of the page goes to the 15v regulator. [ATTACH=CONFIG]46956[/ATTACH]

1) do 12/15V reach top of relay coil?
2) IF so, does shorting Q20 C-E activate the relay?

please answer these two first, so we can go on.

[tdlunsfo]

Well I looked at this further. This is a very similar schematic (I couldn't find the one I am working on) but with one exception - mine has a thermal cutout also feeding the relay. When I was taking the board and heat sink apart the wires broke off of thermal. After soldering those back, the mixer works fine. Now to dig into the other one. I still would like to understand how the circuit works though. Seems like it is is low pass filtering and monitoring for DC. I guess I could plug it into multisim and see.

[nevetslab]

Looking at the schematic you provided, and for the moment, disregarding Q17, if there is no DC level on the output, the input xstr Q18 is turned off, it’s collector is high, so the PNP xstr Q16 is turned off, and you have positive bias voltage at the base of Q19, which turns on, turning on Q20 and current flows thru the relay coil with Q20 in saturation.

If there is a positive DC offset (haven’t calculated what that threshold voltage is) Q17 is turned on, which turns Q18 on, pulling its’ emitter low, preventing Q19 and Q20 from turning on, preventing the relay from pulling in.

Now, looking at a negative voltage, something doesn’t seem right. What should happen is the same condition to prevent Q18 from turning on, preventing Q19 and Q20 from turning on. We want Q16 to be turned on in this case, but I don’t’ see how this will happen. Anyone else see what I’m missing here?

If Q17 is marked incorrectly, and you reverse the collector and emitter, then it would turn on if there is negative voltage on the output, and turn on Q16, inhibiting the relay from pulling in.

[J M Fahey]

As you suspect, the circuit is drawn wrong
And parts value choice is poor, yet circuit will work, sort of, just at different DC voltages than expected.
In any case, a gross output fault (say + or -20 or 30V at the output) *will* turn it on.

With positive voltages, you need some 0.7V at Q18 base as you correctly found, so you need about same voltage across 12k, so about +2V DC at speaker out.

Q17 is drawn wrong, emitter and collector are reversed, but it will still work (poorly) under the very limited voltage and current itīs handling.

Again, 0.7V at the (properly placed) Emitter will turn it on, in this case -0.7V so base is more positive than emitter.
It might easily need a quite higher voltage to turn on because of the misplacement and in any case a common base transistor has current gain of 1 , while Q10 has at least 50 or 100 but the demands are low so both will work.

The stacked cap ends up being some 220uF equivalent which together with the 22k resistor feeding it means that Q17 or Q18 will turn on after some 5 seconds of DC at the output, maybe half that if DC is gross.
Most speakers should stand that.