Hi!
. . .Thank you for providing the schematics and layouts – it helps us enormously when diagnosing a fault like this!
. . .Apart from some strange Asian semiconductor type nos, it's relatively straightforward to diagnose this!
. . .First of all, is the delayed power–on reset releasing the "MUTE" line? The +55 V main h.t. supply is connected to a reset circuit consisting of C127, R86 and R89, the voltage at the positive terminal of C127 should be about 5.7 V and Q9 should be conducting with its base approx. 0.6 V.
. . .If Q9 is not being turned on, check the +55 V h.t. supply is reaching that voltage, and replace C127, Z2 and Q9 if you're in any doubt about these components – a BZX84–C5V1 will do for Z2 if the package matches!
There is also a 5 V "P_MUTE line needed to turn ON transistor Q11 to release the "MUTE" line once the main power supply circuit has started up – this comes from the 5 V "Reference Voltage" output of the '3525 SMPS controller in the power supply circuit – if the '3525 isn't running correctly, Q11 will not turn ON and the "MUTE" line will remain low – this is a safety protection feature designed to turn OFF the drive to the Class D power MOSFETs until the power supply is running correctly!
. . .Also make sure you have no extraneous DC at the mid–point of the Class D output stage due to defective MOSFETs or a low or missing ± 55 V h.t. supply – problems here will trip the power muting circuit via the d.c. protection transistors Q15, Q16 and Q17 in the power amplifier circuit!
. . .There are also three temperature sensors in your amplifier that need a quick check – TS1 is a normally closed bimetallic thermal switch fitted to the main heatsinks, an PTC thermistor TH2 in the temperature protection circuit, approx. 50 Ω at room temperature , and a DO220 package thermal device TH3 called an "AIRPAX 67L050" which should be a short–circuit cold – this becomes high resistance when its tab reaches 50°C or greater, when any of these sense an over–temperature fault, Q12 in the power supply circuit turns ON, shutting off the main '3525 SMPS controller by way of the '555 monostable U4 which trips the "/SHUTDN" input at pin 10 of the '3525!
. . .Therefore, the first thing to do is check both the +55 V, –55V and the ±15 V supplies as all these are sensed, followed by the power–on–mute circuit, then the output stage itself for DC faults, then the over–temperature protection, roughly in that order!
Chris Williams