Just finished setting the bias to spec (no troubles) and then measured some surprising dc offsets. most channels are fine however there are two very bad
+1.2v
-2.2v
+0.6
the rest are under 100mv.
Is there an obvious component to check for such a large imbalance? I guess obvious is checking how closely matched the driver & power transistor pairs are.
But if those measure close, what next? Resistor matching? though out the path to outputs?
I bought the amp as is, in protection mode as per seller.
I never tried it out myself, just cleaned of dust and more remarkably cleaned an odd substance from between the power transistor solder joints. didn't measure if it was conductive but it did seem to have some degree of moister content (wasn;t dusty / flaky like old flux)
then reassembled and it powered up fine.
Then did bias adj.
Then checked dc offset.
then tried to put it into protection by driving 8ohms on the -2.2v channel to 0db. and was fine. not even distorted as I thought.
An odd thing about the high dc offset channels; while there is a noticeable "pop" when connecting to a speaker (crappy one that is mostly a resistor) there is also noticeable "noise" at -90db / zero volume. It's not clean like a hum (like I hear from the transformer lol) it's noise of some sort and a wide spectrum of frequencies somehow. similar to white noise.
Would looking at the offsets with a scope help determine likely cause?
I had bought it just for the hdmi board which seem to work fine. fixing the dc bias just means I can use the avr as is an not swap hdmi board. (am affraid is a power issue and may spoil the good hdmi board, so prefer to try and fix dc offset instead)
+1.2v
-2.2v
+0.6
the rest are under 100mv.
Is there an obvious component to check for such a large imbalance? I guess obvious is checking how closely matched the driver & power transistor pairs are.
But if those measure close, what next? Resistor matching? though out the path to outputs?
I bought the amp as is, in protection mode as per seller.
I never tried it out myself, just cleaned of dust and more remarkably cleaned an odd substance from between the power transistor solder joints. didn't measure if it was conductive but it did seem to have some degree of moister content (wasn;t dusty / flaky like old flux)
then reassembled and it powered up fine.
Then did bias adj.
Then checked dc offset.
then tried to put it into protection by driving 8ohms on the -2.2v channel to 0db. and was fine. not even distorted as I thought.
An odd thing about the high dc offset channels; while there is a noticeable "pop" when connecting to a speaker (crappy one that is mostly a resistor) there is also noticeable "noise" at -90db / zero volume. It's not clean like a hum (like I hear from the transformer lol) it's noise of some sort and a wide spectrum of frequencies somehow. similar to white noise.
Would looking at the offsets with a scope help determine likely cause?
I had bought it just for the hdmi board which seem to work fine. fixing the dc bias just means I can use the avr as is an not swap hdmi board. (am affraid is a power issue and may spoil the good hdmi board, so prefer to try and fix dc offset instead)
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suggestion:
as all channels are identical, I'll take SR channel for ref.
A) check the value of R579, 100 ohm, the 100r got one end connect to ground, make sure its continuity to ground.
b) form the other end of R579, check the conditions and voltages of C634, C639, (100u/63v).
C) check R634. R639, 18 ohm, check their values.
D) check C654, 100u/16v
E) one easy way is to use a digital multimeter, to check the same spot of the abnormal channel on the circuit and compare it with the same spot on a good channel. when the voltages is different that would provide you a clue to the fault. must be careful not to short the leads to other components.
F) check if there were any glue get in touch with the components. because the glue could be conductive after years of service.
G) check all the supply voltages if they are up to the design value according to the schematic.
H) the service manual
HARMAN KARDON AVR-3600 SM Service Manual download, schematics, eeprom, repair info for electronics experts
hope you fix it soon.
as all channels are identical, I'll take SR channel for ref.
A) check the value of R579, 100 ohm, the 100r got one end connect to ground, make sure its continuity to ground.
b) form the other end of R579, check the conditions and voltages of C634, C639, (100u/63v).
C) check R634. R639, 18 ohm, check their values.
D) check C654, 100u/16v
E) one easy way is to use a digital multimeter, to check the same spot of the abnormal channel on the circuit and compare it with the same spot on a good channel. when the voltages is different that would provide you a clue to the fault. must be careful not to short the leads to other components.
F) check if there were any glue get in touch with the components. because the glue could be conductive after years of service.
G) check all the supply voltages if they are up to the design value according to the schematic.
H) the service manual
HARMAN KARDON AVR-3600 SM Service Manual download, schematics, eeprom, repair info for electronics experts
hope you fix it soon.
Last edited: