I have been looking at a Toshiba mini-hifi system CX-LFA880 that has developed a fault that causes the power supply to shut down as soon as it goes from standby to power on - the whole device then has to be powered up again to restart it. After spending some time working out how it functions, I've stripped it down to the power supply (240V Australian supply), the main board, the audio amp board and the front control/keypad board and it now powers on OK and all the keys and display functions work - as long as I don't connect the INH line from the Main Board to the audio amp board. Connecting it results in the 5V INH input from the main board being grounded and the power supply immediately shuts down. I've attached the audio amp schematic and a couple of photos. I have the whole service manual. I can post other extracts if that helps.
I've checked all the power rails and they look OK, including some that only switch on when the DVD select or other keys are activated. The main board supplies +/-28V on +/-B and 16V on the 17V supply line when the audio amp board is disconnected. With the power connector (CN102) and control lines connector plugged into the audio amp board and the system switched out of Standby mode, I get +/-23V on +/-B, the '17V' supply sits at 13.2V, there's -5V at Pin10 on CN107, and I get 11.3V back to the main board from the Q311/Q312 voltage reg circuit on the amp board. PON sits at 4.5V and PON1 (from Pin13 on the STK433 power amp module) at 2.5V. The fan is spinning - it gets 5.4V from Q350 and feeds 0.42V into the base of Q355 that's part of the fan current detection circuit. Q304 has -5V on its base and emitter as expected. The voltages on the main audio amp (Sanyo STK433-040) look sensible - +/-23.6V on the +/-Vcc pins, +/-22.3V on +/-Pre pins and nothing elsewhere - except Stdby at 2.5V. All that is pretty much as expected as far as I can see, except perhaps the low 17V supply rail. I've presumed that because the stripped down system behaves exactly like the whole system, that disconnecting tuner, MPEG board, DVD player, tape player etc hasn't introduced any new behaviour.
The main 'symptom' I've located is that the INH line sits at +5V on the main board and -1.3V on the audio amp board (when disconnected). When connected momentarily the 5V gets grounded and disables the power supply. However, I've not been able to work out what's grounding this line. There are overload, DC, and fan current protection circuits and the Q308 protection switch that look likely suspects. I've been struggling to work out how these interact. Q301 has 0V on B&E, so I presume the overload protection circuit isn't grounding INH. I've done quite a bit of measuring transistor voltages, but haven't found an obvious cause. Perhaps there is some way to make the diagnostic process a bit easier, given there's no obvious sign of damage to the hardware anywhere.
I'd be grateful for any suggestions on how I might break down the protection circuit diagnosis to identify the cause of the grounding. This is a bit of learning exercise as I've not worked much with audio circuits before. I'd like to get the root cause and fix it if possible. The SMD components make the diagnostics a bit harder than other stuff I work with.