Hi!
I've got the strangest problem! I have 11v dc on the right output of a Technics SA-5460 selector switch when connected to aux input, 5.6v when connected to phono, 3.2v when FM is selected and 2v when AM. It goes together with a little crackle (except on phono). First I suspected a bad transistor somewhere but no, then I thought maybe an internal short in the switch but no, I tried disconnecting every input directly on the selector switch but the output stays at 11v. same story when I disconnect the output pin from rest of the amplifier. I checked for shorts to any power supply rails nearby on always get open circuit. Now comes the strange part : when I remove the shielding of the switch the voltage and noise go away (the shield is connected correctly to the chassis and doesn't touch anything else.
. shorting the output to ground doesn't disturb any other voltages around the amp and when I disconnect the short the voltage creeps up again like a cap charging, seems like a static charge is building up on that pin but I have no idea why. Other than that the amp and fm/am sections work fine even if the dc on the switches tend to make loud pops whenever I use them...
I've got the strangest problem! I have 11v dc on the right output of a Technics SA-5460 selector switch when connected to aux input, 5.6v when connected to phono, 3.2v when FM is selected and 2v when AM. It goes together with a little crackle (except on phono). First I suspected a bad transistor somewhere but no, then I thought maybe an internal short in the switch but no, I tried disconnecting every input directly on the selector switch but the output stays at 11v. same story when I disconnect the output pin from rest of the amplifier. I checked for shorts to any power supply rails nearby on always get open circuit. Now comes the strange part : when I remove the shielding of the switch the voltage and noise go away (the shield is connected correctly to the chassis and doesn't touch anything else.
. shorting the output to ground doesn't disturb any other voltages around the amp and when I disconnect the short the voltage creeps up again like a cap charging, seems like a static charge is building up on that pin but I have no idea why. Other than that the amp and fm/am sections work fine even if the dc on the switches tend to make loud pops whenever I use them...Schematic here:
TECHNICS SA-5460, Service Manual, Repair Schematics
So, the switch is connected to the chassis... but the chassis isn't connected to ground ... You are measuring a floating voltage. Do You also measure 11V between the pad on the PCB and main star ground without the switch connected ?
TECHNICS SA-5460, Service Manual, Repair Schematics
So, the switch is connected to the chassis... but the chassis isn't connected to ground ... You are measuring a floating voltage. Do You also measure 11V between the pad on the PCB and main star ground without the switch connected ?
Hello Maac0,
I've got the schematics thank you, the whole power amp stage was dead so i needed it to repair that.
The chassis is definitely connected straight to ground so that 11v is measured according to the circuit ground. With the switch removed there is no voltage at all on the circuit pad and even with the whole switch in place but without the metal case there is nothing. When plugging a source in the right channel rca the voltage drops to about 500mV.
I've got the schematics thank you, the whole power amp stage was dead so i needed it to repair that.
The chassis is definitely connected straight to ground so that 11v is measured according to the circuit ground. With the switch removed there is no voltage at all on the circuit pad and even with the whole switch in place but without the metal case there is nothing. When plugging a source in the right channel rca the voltage drops to about 500mV.
Have you measured the voltage on the metal case over the switch?
It could be shorting to some points in the circuit.
Yes, that's what I thought but it is at 0v. When I short the pin to ground it discharges but as soon as I remove the short the voltage gets back to 11v within a few seconds (just like something charging). Could this be static electricity building up ? But why there and why 11v ? could I just add a high value resistor from there to signal ground without altering the specs to much ?
It's driving me crazy ! I put a 2.2µF cap on the of the switch just to take care of the dc voltage that made all the other switches pop or even bang when used and for about 15 minutes the cracking noise was gone so I was happy but now it back, and it seems to be on both channels. It reacts to my finger pressing down on the switch. Could it be that the switches pcb got conductive or something (a separate section of the switch carries the supply voltage for the tuner section : 13.5v which is exactly what I measure on the output pin when disconnected from everything else) ? Anyone ?
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