Speaker Protection Circuit Troubleshooting

[Ken1]

Hi, I am trying to fix a Yamaha AX-400 stereo amplifier in which the relay that powers the speakers won't turn on. The output transistors are good, B+ voltages are correct and the idle current for both channels are exactly what the service manual calls for. When I touch the collector to the emitter on Q135, the relay switches on and sound is normal on both speakers. I replaced both electrolytics in the protection circuit as they were a little low in capacitance, however this made no difference. Q135 is only getting 0.5v on the base, not 1.3v as per schematic. Anybody have any experience with these circuits?

 

[Nigel Goodwin]

They rarely (pretty well never) go faulty - they are under no stress, so aren't likely to fail. Almost always, if it's triggered, then there's an actual fault on one on the amplifiers, usually a DC output, so the protection circuit protects the speakers as it's meant to. There may well be over currnet protection as well, I'd start off by checking what's coming in to the protection circuit.

 

[Diver300]

Have you got a circuit diagram that you could share?

 

[Nigel Goodwin]

The manual is available from:

Elektrotanya for electronics experts

Electronics forum for electronics experts to repair defective appliance.

elektrotanya.com

I suggest the OP checks the voltages on the bases of Q133 (over current) and Q134 (DC voltage on the output), these both should be around zero volts.

 

[Ken1]

I am getting -7.4v on Q133 base and +0.6v on Q134 base. The collectors measure 0v. I am also getting +5.1v on both + speaker terminals (R201/R203 junction and R202/R204 junction). Diode test shows all 4 output transistors ok, but could they be leaky when power is applied?

 

[Nigel Goodwin]

I am getting -7.4v on Q133 base and +0.6v on Q134 base. The collectors measure 0v. I am also getting +5.1v on both + speaker terminals (R201/R203 junction and R202/R204 junction). Diode test shows all 4 output transistors ok, but could they be leaky when power is applied?

There's your problem then

It's EXTREMELY unlikely to be leaky output transistors though - that doesn't happen.

Interesting it's on BOTH channels, so presumably must be something that's common to both channels.

You might try checking the supply rails, make sure the +/-50V rails are both pretty equal (could be a reservoir capacitor failure if one was low), but also check the +/-16V rail, which is used to feed the long tailed pair at the front of the power amplifier, and could unbalance the power amplifier.

 

[Ken1]

power supply voltage measurements: +51.5v, -51.5v, +16.3v, -16.5v. I will have to do some head scratching on this one.

 

[Nigel Goodwin]

Check the voltages on the bases of the long tailed pairs - one is the reference voltage, should be zero volts (ground), the other is the feedback from the output stage, and should also be zero. It's essentially all just a big opamp, with inverting and non-inverting inputs. It should maintain itself to keep the base voltages equal to each other.