[vtech]

I use to come across this on variety of different brands. Sort of a catastrophic failure where the detection circuit suppose to protect your speaker(s) from DC in case of an output failure. At the same time it is meant to protect the output stage from an overload from the speaker end.
While this failure can rarely be caused by a power surge, it is almost always the result of "cranking the volume just to see how high it will go". In looking at the circuit, you will see that either channel's output current is monitored by Q622 and Q621 as a differential amp. the resultant output is sampled thru R625 which inturn monitored by Q601& Q602 tied to the speaker relay control Q604, buffered by Q603.
In ideal conditions, Q621 & Q622 are held constant and there is no over current or DC detected and everybody is happy and ready to react. Problem starts when there is a massive output failure, causing the high rail voltage(s) to be supplied directly to the sense lines and often results in fireworks.
This is a circuit operating with millivolts and is instantly exposed to rail voltages which can be as high as 60~150 volts. You can never tell how extensive the damage may be?--sometimes it may not be as bad as it looks? It is a good possibility that the speaker(s) may have also been damaged. Keep in mind that in order for a device to burnup, it MUST find it's current path to the ground. Where is R662?

[bob54]

This unit was given to me not working and when I removed the cover I found R622, R625 and Q622, burnt. Q622 just had the three legs and R662 was a pile of char. (see page 15 of the attached schematic) I know this is the overload dectection circuit. Naturally the power amp transistors are bad and probably several other componets. I was wondering if anyone else has run accros this before or have any idea what could have cause it. All the fuses are good and no discloration in the power supply circuit.

I think the file was to big to attach so here is a link

https://sendnow.acrobat.com/?i=aS8lnPHgO3SCloMoeLSKxw