as you descripted, both channel begin to fade after 2 mins.
It could be something that trigger the protection circuit, the protection circuit controls the input power supply of both channel.
download the schematic from the following website which need no signup.
HARMAN-KARDON PM635 SM Service Manual download, schematics, eeprom, repair info for electronics experts
according to the schematic
check Q1, Q2, Q3, Q4, (protection circuit)
check the voltages when turn on and sounds normal..
check the voltages when the sound fade away.. no sound.
if the voltages appear different, find the bad component.
It could be something that trigger the protection circuit, the protection circuit controls the input power supply of both channel.
download the schematic from the following website which need no signup.
HARMAN-KARDON PM635 SM Service Manual download, schematics, eeprom, repair info for electronics experts
according to the schematic
check Q1, Q2, Q3, Q4, (protection circuit)
check the voltages when turn on and sounds normal..
check the voltages when the sound fade away.. no sound.
if the voltages appear different, find the bad component.
A relay cuts off the output to the loudspeakers depending on the circuit design for large DC offset /over current/delay at switch on to stop "thump " in loudspeakers and various other faults .
In processor controlled amplifiers this can become quite complicated as digital circuits control its function.
You can also get relays in the power supply to safeguard the amplifier or even the mains transformer --many many different circuits exist.
Modern solid state amplifiers have electronic power supplies that do all the functions of a relay.
In processor controlled amplifiers this can become quite complicated as digital circuits control its function.
You can also get relays in the power supply to safeguard the amplifier or even the mains transformer --many many different circuits exist.
Modern solid state amplifiers have electronic power supplies that do all the functions of a relay.
Lynyrd
its very nice of you to say please.
did you follow the suggestion on #22,
(its a suggestion not an instruction, I am old but still learning.)
In the protection circuit, where you locate the difference in voltages between normal stage and no sound stage, the fault will be easier to find.
When overcurrent is detected, the protection circuit can shut off the current supply to the input diff. of both channels and thus no output. this amp is protected in this way,
but when components fail, the protection circuit could cause no sound.
if it happens after 2 mins, C8 and other components in the protection circuit could be the culprit.
According to the schematic, in the protection circuit
Q3 collector pin supply current to the input diff. of both amps via two 68k resistor, R11,R12.
when Q3 is cut off, no current to the input diff. thus no output---no sound.
R8, R9 provide a turn on -0.6v bias to Q3 when normal. if this -0.6v not present then Q3 shut off. .. no sound.
R4, R5, Q1 provide a -2.5v supply to R8 68k to turn Q3 on.
if this -2.5v not present then Q3 shut off. no sound.
when Q2 was turned on by Q4 via R6 4.7k,R7 47k resistor,
minus2.5 volt would be shorted to ground.
no -2.5v, no -0.6v to Q3, Q3 shut off......no sound.
Q5, Q6 detect the output current,
when overcurrent was detected, Q5 Q6 would be triggered to turn on,
Q4 is turned on via R13 R15 R16 R21, provided by the current of Q5 or Q6 or both.
D5, D7, C6 provide a -5.4v supply for the protection circuit and all led.
any failure of the components in the protection circuit could cause malfunction and cause no sound.
(please point out wherever there is not correct .)
its very nice of you to say please.
did you follow the suggestion on #22,
(its a suggestion not an instruction, I am old but still learning.)
In the protection circuit, where you locate the difference in voltages between normal stage and no sound stage, the fault will be easier to find.
When overcurrent is detected, the protection circuit can shut off the current supply to the input diff. of both channels and thus no output. this amp is protected in this way,
but when components fail, the protection circuit could cause no sound.
if it happens after 2 mins, C8 and other components in the protection circuit could be the culprit.
According to the schematic, in the protection circuit
Q3 collector pin supply current to the input diff. of both amps via two 68k resistor, R11,R12.
when Q3 is cut off, no current to the input diff. thus no output---no sound.
R8, R9 provide a turn on -0.6v bias to Q3 when normal. if this -0.6v not present then Q3 shut off. .. no sound.
R4, R5, Q1 provide a -2.5v supply to R8 68k to turn Q3 on.
if this -2.5v not present then Q3 shut off. no sound.
when Q2 was turned on by Q4 via R6 4.7k,R7 47k resistor,
minus2.5 volt would be shorted to ground.
no -2.5v, no -0.6v to Q3, Q3 shut off......no sound.
Q5, Q6 detect the output current,
when overcurrent was detected, Q5 Q6 would be triggered to turn on,
Q4 is turned on via R13 R15 R16 R21, provided by the current of Q5 or Q6 or both.
D5, D7, C6 provide a -5.4v supply for the protection circuit and all led.
any failure of the components in the protection circuit could cause malfunction and cause no sound.
(please point out wherever there is not correct .)
According to the schematic,
in the protection circuit
A: Q3 supply current to the input diff. of both amps via two 68k resistor, R11,R12.
B: when Q3 is cut off, no current to the input diff. thus no output--- no sound.
C: R8, R9 provide a turn on -0.6v bias to Q3 when normal.
if this -0.6v not present then Q3 shut off. .. no sound.
D: R4, R5, Q1 provide a -2.5v supply to R8 68k to turn Q3 on.
if this -2.5v not present then Q3 shut off. no sound.
E: when Q2 was turned on by Q4 via R6 4.7k,R7 47k resistor,
minus2.5 volt would be shorted to ground.. no sound.
( if Q4 was defective, leakage...., it would turn on Q2..) no sound.
F: no -2.5v, no -0.6v to Q3, Q3 shut off......no sound.
G: Q5, Q6 detect the output current,
when overcurrent was detected, Q5 Q6 would be triggered to turn on,
Q4 would turned on via R13 R15 R16 R21,
provided by the current of Q5 or Q6 or both.
H: D5, D7, C6 provide a -5.4v supply for the protection circuit and all led.
if no -5.4v, no sound.
I: R22, D6 zener, if no -5.4v..... no sound.
any failure of the components in the protection circuit could cause malfunction and no sound.
(please point out whatever incorrect .)
in the protection circuit
A: Q3 supply current to the input diff. of both amps via two 68k resistor, R11,R12.
B: when Q3 is cut off, no current to the input diff. thus no output--- no sound.
C: R8, R9 provide a turn on -0.6v bias to Q3 when normal.
if this -0.6v not present then Q3 shut off. .. no sound.
D: R4, R5, Q1 provide a -2.5v supply to R8 68k to turn Q3 on.
if this -2.5v not present then Q3 shut off. no sound.
E: when Q2 was turned on by Q4 via R6 4.7k,R7 47k resistor,
minus2.5 volt would be shorted to ground.. no sound.
( if Q4 was defective, leakage...., it would turn on Q2..) no sound.
F: no -2.5v, no -0.6v to Q3, Q3 shut off......no sound.
G: Q5, Q6 detect the output current,
when overcurrent was detected, Q5 Q6 would be triggered to turn on,
Q4 would turned on via R13 R15 R16 R21,
provided by the current of Q5 or Q6 or both.
H: D5, D7, C6 provide a -5.4v supply for the protection circuit and all led.
if no -5.4v, no sound.
I: R22, D6 zener, if no -5.4v..... no sound.
any failure of the components in the protection circuit could cause malfunction and no sound.
(please point out whatever incorrect .)