I checked them yesterday with a screwdriver and a multimeter they work well and react correctly when you turn them and read correctly.
tested both with the voltage on and off
So what happens, when you let the amp warm up for like 15 minutes, and then adjust them??
Can you adjust them to zero +-10mV??
Can you adjust them to zero +-10mV??
DC offset is present probably because the base voltages of these transistors are wrong.
When zooming in to the schematics in the service manual PDF it seems to be vaguely readable that they should be 1,2V and -1,2v.
My guess is that once these voltages will be correct then there willl be no DC offset.
The next step "backward" to check is the symmetry of the Q421/423/415/427 chains - besides transistors also the resistors can become defective (or their soldering may have deteriorated).
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When you power up the amp, and then turn those potmeters, can you adjust the output voltage when you turn them? And if that is the case, can you with those potmeters adjust the DC output of the amplifier to somewhere between -10mV and +10mV. That is what we would like to know.
These potmeters are there to adjust that output DC voltage, they have been designed in there to do that. So if you can adjust the output to have a DC offset to around +/-10mV with them, then your amplifier is working correctly.
These potmeters do go wrong. So the chance are that this is all that is wrong with your amp. So check the simple things before starting to go deeper.
These potmeters are there to adjust that output DC voltage, they have been designed in there to do that. So if you can adjust the output to have a DC offset to around +/-10mV with them, then your amplifier is working correctly.
These potmeters do go wrong. So the chance are that this is all that is wrong with your amp. So check the simple things before starting to go deeper.
The -8mv on Q435 looks highly suspect and points to it being reverse biased. Q433 base should be the more positive of the two (Q433 and Q435)
It is much easier and quicker to measure the voltages over the resistors - if they are equal (within the pairs per rail of both channels) then these components should be ok and the cause of the problem is probably upstream of the signal path.
These voltages indicate that the PA isn't even enabled. Per the schematic in post 37, the collector of Q417 should be a few mV negative re ground. The emitter of Q419 should be about -600mV. Same applies to the opposite channel. I suspect the voltages you're observing at other points are leakage voltages and mean nothing because the amp isn't turned on.
Explore the Mute control path from base of Q417; it should be about 0V, but I suspect is not.
Edit. My bad: the mute control has to pull Q417 into conduction, hence a negative voltage. None the less, Q419 is cutoff. The amp in not enabled.
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when I measure on q421 and q425, the right channel that was at -5 v starts to go back to 5 mv and stays there for 5 min then if I measure there again it's the same thing. could it be them who are ghosting on channel right ?
And collector Q417? It should be about 0V, and pull base of Q419 to about 0V and Q419 emitter to about -600mV. Emitter of Q417 should be at ground.
Something very wrong with Q419. Collector junction may be open. Probably Q419 failed. What about Q420 voltages?
q420
base -42v
collector -43v
emitter 42.8v
have a q418 not shown in the schematic ?? sitting in the same place as the 417 but on the left channel and the same function i guess , should i measure it too?
base -42v
collector -43v
emitter 42.8v
have a q418 not shown in the schematic ?? sitting in the same place as the 417 but on the left channel and the same function i guess , should i measure it too?