so when you switch the EQ off, there is a very brief surge at about +600mv at the speaker output
600mV should not be very loud.
Is this with a scope, or DVM (which will not read it accurately)?
Is this with a scope, or DVM (which will not read it accurately)?
Did you measure the DC voltage at the four points I arrowed. Do it with the switch in each position. That is eight readings. Are they all zero?
Actually that capacitor is preventing the noise, not causing it, when the low filter is on.
^ Maybe.
Working back with the scope from the power amp inputs you need to look at this sudden pulse that trips the relay and see where this voltage is occurring.,
Remember that if a switch contact moves between two points of equal voltage (which can include zero) then there is no change in output from the switch and the operation should be silent.
If the contact is break before make then is it possible that the 'output' of the switch floats momentarily and rises to some unwanted voltage maybe because of leakage somewhere (cap, board...)
Working back with the scope from the power amp inputs you need to look at this sudden pulse that trips the relay and see where this voltage is occurring.,
Remember that if a switch contact moves between two points of equal voltage (which can include zero) then there is no change in output from the switch and the operation should be silent.
If the contact is break before make then is it possible that the 'output' of the switch floats momentarily and rises to some unwanted voltage maybe because of leakage somewhere (cap, board...)
ok i will check it, but the period is very brief, milliseconds and is quite hard to capture
Im sure my scope can record so it is possible i think it should be ok
Im sure my scope can record so it is possible i think it should be ok
If you make the power amp input safe (maybe remove the input coupling caps) you can look at isolating the feeds to and from the switch and see if any float up to some voltage.
Also check if the issue is on one or both channels.
Also check if the issue is on one or both channels.
something odd i have noticed
when the EQ defeat switch is depressed all of the output transistors Q725,727,and 728 remain on, but 726 turns off untill the switch is released out again, but it turns back on before the breaker has come back in
when the EQ defeat switch is depressed all of the output transistors Q725,727,and 728 remain on, but 726 turns off untill the switch is released out again, but it turns back on before the breaker has come back in
yep there is def an issue here as im pnly getting -6v to the base of Q801 and nothing on the collector so ill have a look at that tomorrow now
i should be able to find out whats going on here.
i should be able to find out whats going on here.
Should be an easy to find. The diagram I have is wrong as it shows -25 volts on Q801 emitter. Quite unusual to see a half wave rectifier set up in an amp.
