AIWA Mini-Hifi audio amp protection circuit - how to diagnose cause of tripping

[Lorenzo_1]

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.     

[singapol]

The DC and overload inputs come from the power amp outputs so it's like that STK433 may have faults...in other words dead. It could also be the speakers. Are the 4 0.22 2W resistors good? Unless the protection ciruits are faulty this is the most likely cause.

[Lorenzo_1]

Hi singapol.  Thks for the suggestions. The resistors are all good so I decided to hook some audio signal into the amp board input - and I get perfectly good output on both channels of the headphone output jack.  So it looks like that part of the STK433 is working fine. Also, for reasons I can't yet fathom, the INH line is now staying high when connected to the main board - with or without the headphone board connected.  So the original problem that beset the unit when it was whole and in its stripped down version seems to have disappeared - at least temporarily.  I have done a bit of temporary soldering round some of the connectors, so perhaps there was a bad joint somewhere?  Would be nice to fathom the cause, but not really worth the extra effort I think.  I'll see if it re-emerges as I connect the remaining subsystems and try to get the whole thing back to life. I'll do that on the bench if possible as it's hard to get at anything once you strap it all back into the box.  Looking a bit hopeful at this stage unless something sets it off again.

[mzacharias]

The resolution of the JPG's makes the component numbers a bit ambiguous, but check to see if that line is being grounded (held low) by Q304 or Q308 (?). One is apparently controlled from elsewhere (the microprocessor for example) The other senses DC from the two amp channels, via a resistor summing network. Still most likely a bad output IC, but it would be nice to rule out other causes.

[Lorenzo_1]

Thks for the tips. Turns out the low 17V supply rail was b/c I had too low wattage globe in my newly built series light-bulb current limiter.  Upped that fm 25W to 100W and rail came up to 16.9V, on spec .  Q304 & Q308 are now behaving as expected. Took another look at the INH1 line that was causing shut-down - it should be sitting at grd under normal operation so the +5V I was getting at the audio amp input was doubtless causing Q308 to trip (easily emulated with a 5V pulse from external power supply). So I now suspect the problem might lie in or around the 5V Buck converter on the main board IC301 (an AP1501) as there's really nothing else that I can see likely to affect this bit of the circuit. The switching on IC301 is working fine - it turns on/off as expected when DVD/CD button pressed and supplies 5.2V at Vout from 17V input.  Will check surrounding caps and then try connecting MPEG board to see if there's a short somewhere there causing overcurrent (though I was earlier getting shutdown without the MPEG board connected).

[mzacharias]

OK, well, we need to see the relevant portion of the schematic that pertains to the micro-controller so we can see what else might be triggering it.

[Lorenzo_1]

This problem seems resolved - but have attached the full schematic for the main board.  Replaced IC301 (AP1501 5V regulator) as I had one to hand.  Have now got the unit largely reconstructed and the power is stable, it switches functions fine and outputs good audio to headphones and speakers.  So, despite being unsure precisely what caused the protection circuitry to trip, it seems to be fixed.  Now have to wrestle with the DVD that's not opening to take a disc even though the front board controller seems to be talking to it OK.  It's getting 5V power from the AP1501 and all the inputs/outputs there look sensible. If I install it with the tray out, it closes automatically on start-up.  Just won't eject the tray once it closes. Out of the frying pan into the fire on this job!