Onkyo A-9130 Volume Increase Causes Protection

[electrotek]

Hi legends! I recently acquired a 5 year old Onkyo A-9130 amp from a first-time owner who found it would cut out (assumably protection circuit) after the volume reached above about 10 o'clock (1/3rd power) if, and only if, music ("input") is playing. However, using headphones does not cause the issue. Amp details: https://www.ambertech.com.au/Documents/brochures/ON-A-9130B.pdf

Research

1. I have been unable to find a service manual for this model, or its little brother (A-9110). I have been able to find a service manual for the A-9150 (on hifiengine), but the model is from a year prior.
2. Forum searching has found issues about volume increasing triggering circuit protection on other models (though no specific mention of music needing to be playing, or using headphones being a workaround). Main culprit appears to have been issues with bad volume control circuitry on a previous model - A-9010. This model does not appear to have the same circuitry.
3. Turning volume up on the remote will make the volume dial on the unit physically go up (i.e. there's a motor control in the volume circuit).

Testing

1. I've been able to reproduce the issue identically to the previous owner, so I think safe to rule out speakers or speaker wire
2. I've also been able to reproduce that using headphones works
3. I've reset the unit - no change
4. I've changed the setting to 4ohm speakers from its default of 8ohm (I'm using 6ohm bookshelf speakers so it instructs to make this setting change) - no change
5. I've tested digital and analog inputs - no change
6. I've tested turning it on at higher volume - it will turn on, few seconds later it will "lock in" the input, shown visually by it flashing then going solid when locked in (from memory, clicking noise too), play for no more than 5 seconds, then power will cut out with another clicking noise. It doesn't turn back on of its own accord.
   1. If you try and turn it on immediately after display will show for no more than 1 second, then it powers off again (input does not lock in)
   2. If you leave it for about 2 minutes, it will turn on again, and either repeat the issue (same volume) or remain on if volume was reduced below 10 o'clock or nothing playing
7. I've opened the top cover and visusally inspected the circuit board. Clean inside, no sign of bulging capacitors, no sign of components loose/burnt or bad solder. Large capacitors look glued to the PCB (white). Looks spick & span! When it cuts out, the "click" is coming from the back left (i.e. where the power circuitry is) - no surprise there.
8. I've not played around too much here, but I believe selecting a different input when it's at a higher volume (nothing playing) can also trigger it

Protection (Reference Only)

Below is the protection documentation from the service manual for the A-9150, for reference only. The A-9130 is not identical - for example, the A-9130 has a 5-wire thermal section (looking at the circuit board) whereas the A-9150 has a 3-wire.

[Voltage Protection]

The Main MCU watch "VPROTECT" port about each 250uSec. When the MCU count twice this port error, the MCU will be STANDBY mode
When the MCU detect Voltage protection, MCU control to "LOW TAP" mode after next power on until speaker relay on.
LOW TAP mode means supplied voltage for final device of the power amplifier circuit will be supplied low voltage (about +25v)

[Current Protection]

The Main MCU watch "IPROTECT" port about each 250uSec. When the MCU count twice this port error, the MCU will be STANDBY mode.
When the MCU detect Current protection, MCU control to "LOW TAP" mode after next power on until speaker relay on.
LOW TAP mode means supplied voltage for final device of the power amplifier circuit will be supplied low voltage (about +25v)

[Thermal Protection]

Main MCU watch the port of Thermal sensor output (P409A 2pin). 5 possibilities:

1. Shut down if the state is maintained for more than 3 seconds [overheating]
2. Shut down if the state is maintainted for more than 5 seconds [very high heat]
3. Forcibly enter LOW tap mode [high heat]
4. Normally work [around -30 to 50]
5. Shut down immediately [overcool]

Would welcome any thoughts. Thanks!