I just fixed a Technics SU-V85A which had a bad VAS transistor Q401. The result was that the "voltage amp" railed and the "current amp" tried to counteract this. Because of the former's current limit circuit it even succeeded. After fixing the fault I wondered why the speaker relay came in during the fault condition. As I did not measure the output voltage at this point I don't understand why that was and if it even really happened. Nonetheless, I wanted to check the protection circuit and injected voltage into the VAS driver opamp IC401 - input (pin#3). With a resistor of 120k this becomes an inverting amplifier mirroring the injected voltage. I found that the protection IC601 AN7073 switched off the relay at -3.7 V and +6.7 V. I don't understand the large difference and could not find a datasheet for this device. Very annoying. OTOH, the circuit works as intended and will protect precious speakers in case something goes phut.
Now this is the first time I ever checked a protection circuit. Question is: good thing or a waste of time?
Tjerk, 9ZZ
Now this is the first time I ever checked a protection circuit. Question is: good thing or a waste of time?
Tjerk, 9ZZ