speaker protection board help

[Prime73]

Hello folks,

Got off ebay a speaker protection board kit ( here is a link for these who interested: http://www.aliexpress.com/item/Stereo-Speaker-Protection-Board-DIY-Kit/1505413451.html

Assembled it as per instructions and schematics, but the relay doesn't work when I feed the board with a signal from my amplifier. Here is the schematic for the kit:

I'm trying to understand how this schematic works so I can troubleshoot it. I have access to an analog scope (and basic understanding how to use it). When I feed a signal to point A (B) ( see the above schematic) I don't see anything on Q1's base and nothing at point C. I don't understand how "C2, C3 and Q1, Q2" (C4, C5 and Q3, Q4) part work. I would really appreciate if someone could spend some time and help me out.

thanks.

[SeanB]

Q1, Q2 ( along with Q3,4) form a simple level detector. If the DC signal input is above ( or below) around 1.2V for long enough ( the 2 capacitors form a low pass filter to remove most of the audio signal from the input leaving only the DC component) the transistors turn on. Q1 is a simple common emitter turned on for positive signals over 1V2 typically, and Q2 is a common base so needs a slightly higher negative input to get a similar collector current.

At point C you will have a voltage of around 1V2 ( 2 base emitter voltage drops of Q7 and Q8) and when one of Q1-Q4 turns on long enough you will see the voltage drop down below this and the relay will turn off. Q5 Q6 are a basic astable multivibrator, and the diode forces the LED on to show power applied and the relay is on, with it starting to flash when the relay turns off.

To test you apply around 3V DC to any of the inputs ( either polarity to test both halves) and this should turn the relay off.

[Prime73]

thanks for the explanation, makes sense now. Tried 3V on point A and B. I see about 600mV on bases for Q1 and Q3 however nothing at point C. Replaced Q1 and Q2 with new ones (just to make sure and tested the old ones - they seem ok). Still nothing at point C. What else should I look at?

[Rerouter]

point C should be at about 1.4V with nothing connected to the inputs, voltage has to be coming out of the LM317 in order for the output to work,

on 20V for the V+ it takes about 3 seconds to start functioning,
at 11V which seems to be close to the minimum you want it takes about 7 seconds to start functioning,

so make sure the LM317 is throwing out a decent voltage,

[Prime73]

thanks again. replaced Q7 (it appears to be a faulty one) and all seems to be working now. Not sure what happened to Q7 though... Point C is at 1.36V and the relay kicks in at about 3 seconds mark. LM317 outputs 12.8V