Subwoofer amplifier MIK

[Perry Babin]

It appears that these are being used as ±15v regulators. Everything appears to be OK. They get hot because they're dissipating power from the voltage drop (26-15v) and the current flowing through them.

You should not have had the same reading on the emitter pads with the transistors out of the circuit.

 

[Nite]

I checked the voltages on different transistors and have -0.6V at the base of 2sc3198. This can't be correct? The base of NPN should always be positive, right? The base of this transistor is connected to the emitter of KTD1303, which has negative voltages on all legs.

 

[Perry Babin]

That sounds like it may be a protection circuit transistor. The protection circuit transistor will be connected across one of the emitter resistors. There will generally be a resistor between the base of the 3198 and the emitter resistor.

 

[Nite]

I decided to remove the regulator pair to measure their pads again. NPN reads B=15.3V, C=38V, E=-0.7V; PNP reads B=-16.5V, C=-39V, E=-0.7. Is the PNP supposed to have negative voltage on all pins?

 

[Perry Babin]

Yes, negative voltage on all pins.

 

[Nite]

I measured voltages on various transistors and have all negative voltages on 2sc1027. E=-38V, C=-0.4V, B=-35V. How come they all have the same polarity. Isn't the base of a NPN transistor driven by a positive voltage and the current being controlled flows from collector to emitter? Also, the the driver KD600K reads E=0.8V, B=36V, C=0.3V. Isn't emitter supposed to be negative? I'm sure that all the transistors are where they are supposed to be, since I wrote their location on a paper before removing or testing them. What would be worth checking next? Originally, the amp had two blown PS mosfets and one output driver. I think I'm going to remove all the diodes and test them one by one.

 

[Perry Babin]

The voltage on the transistors will depend on the way that they're used in the circuit.

 

[Nite]

Thanks for your time, Perry. I have already spent too much time on this amp. Since it is built into the subwoofer box, I'm going to find another suitable amp and put it in place of the old one. The original board can still be used for parts.