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A9 developed a hum through Earphones Jack

N

Nightmare

Member
My A9 amp has developed a 'hum' through the earphone jack. This is the only place that there is a hum. Sound through the speakers (both terminals) and the 'tape rec' and 'tape play' terminals are all clear.

Any suggestions on how to repair this?

TIA
 
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Nightmare said:
My A9 amp has developed a 'hum' through the earphone jack. This is the only place that there is a hum. Sound through the speakers (both terminals) and the 'tape rec' and 'tape play' terminals are all clear.

Any suggestions on how to repair this?

TIA
Click to expand...

The headphone signal comes from the power amp, and with the exception of the resistors limiting the power available at the headphone jacks - there is no difference other than, like speaker a and B, it has it's own relay.

Thus we address the wiring - the headphone jack is directly wired to the gws-303 board, which then immediately leaves the board on the cn-5 assembly and cable going to the gwh-148 power amp assembly. This cable also carries the ground (pin 2) directly back,(l.chan=pin 1, r.chan=pin3, AC for protect pin 5, other lines are prot lamps, power) but there are some extra connection taps to the ground run p.c.b. trace as it heads over to the true ground point on the amplifier board.

These connection taps to the ground run p.c.b. trace could be the problem, because they involve the protection circuit's AC sense leg, where AC voltage comes in from the green transformer wire and FU5 and then to R78 (2.7k), D37 and then the R77 (12k) bleed resistor and the C38 3.3uf integrating capacitor. This circuit senses the power on state of the amplifier, and is a perfect entry point for unwanted hum.

So the capacitor C38 could be excessively leaky, and I would replace it first. Next I would replace C39 220uf, the timing capscitor - which if leaky could have risen the ground voltage on that trace - making it vulnerable to hum from C38's bypass action. After that I would check the integrity of the ground trace between C38 and it's source point nearby.

BUT after that big pad and it's jumpered companion, just HOW the ground gets there is a mystery that the service manual sheds no light on.
 
Thanks for the heads up on where to start. I need to break out my glases, multimeter and scope and have a look and hopefully it will be a small item that can be easily replaced.

Thanks for the trace data.
 
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