R!f@@: U123 (7805), Input 7.22 V, output 4.81 V, U125 (7805), Input 8.09 V, Output 4.91 V. Voltages across the main board are all within specs. This all seems reasonable to me.

@Lestraveled: U123 and U125 are warm to the touch, but that is all. U121 (heatsinked) is quite hot, painful when held for more than 5 secs (but this regulator outputs a flawless 3.3 V, so I guess it's not defective).

When I disconnect the AC3 board from all the analogue parts there is, of course, no hum (no connections on J604B or J601A). The hum starts when I connect the connector J604 (J7001) on the mainboard, which connects the ground, the Front Left signal and the Front Right signal to the AC3 board (J604B on the AC3 board). As far as I can see this feeds the LR signal into the AC3 board, but also couples the hum (through ground, perhaps?) back into the audio electronics.

Thanks so far, I will do more measurements and tests tomorrow!

Greets, Maarten

 

Exactly the same problem here with a T761. Awaiting your solution ;-)

Thx. Wieger in Amsterdam

 

Use an ohm meter and verify continuity of ground across all plug in connectors that carry a wired ground across boards.
Edit: try to check at a PCB trace point and not by putting a probe into the back of the connector

 

Check for voltage drop across the Pin In/Out switch. If it doesn't make solid contact, it can cause a sag in the input voltage to the 7805s.

 

Just to make sure: my T761 has hum with only speakers attached. So nothing connected to one of the inputs. The hum only isn't present when you choose the external 5.1 audio input.

 

May be the Audio Analog selector IC is generating the hum. I have had this on 2 occasions.

 

R!f@@: U123 (7805), Input 7.22 V, output 4.81 V, U125 (7805), Input 8.09 V, Output 4.91 V. Voltages across the main board are all within specs. This all seems reasonable to me.

@Lestraveled: U123 and U125 are warm to the touch, but that is all. U121 (heatsinked) is quite hot, painful when held for more than 5 secs (but this regulator outputs a flawless 3.3 V, so I guess it's not defective).

When I disconnect the AC3 board from all the analogue parts there is, of course, no hum (no connections on J604B or J601A). The hum starts when I connect the connector J604 (J7001) on the mainboard, which connects the ground, the Front Left signal and the Front Right signal to the AC3 board (J604B on the AC3 board). As far as I can see this feeds the LR signal into the AC3 board, but also couples the hum (through ground, perhaps?) back into the audio electronics.

Thanks so far, I will do more measurements and tests tomorrow!

Greets, Maarten

Hi Marten,

I'm bringing up this old subject as I have the exact same problem with the same amp and I was wondering if you could reach any solution to this annoying noise?!

Thanks,
Shahin