Hello. I have a pair of b-12 mono amps running very hot. One amp is running at 152mv of offset and the other is at 720mv. Proper settings (found in another post) appear to be 120mA bias/5mV offset. I have searched extensively and cannot locate a service manual, just schematics. It seems clear how to make the offset adjustment but I can not figure out how to make the connections to test the idling current (bias). Does anyone know how to adjust the idling current including the location of the test connections? Attached are a few pics of the amp where the idle current pot is located and a schematic.
Here is a link to the schematic also
LUXMAN B12 SCH Service Manual download, schematics, eeprom, repair info for electronics experts
Here is a link to the schematic also
LUXMAN B12 SCH Service Manual download, schematics, eeprom, repair info for electronics experts
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Adjustment is for a total of 120mA bias, and <5mV offset.
There are three outputs per side, so 40mA per transistor.
That is 0.04A x 0.18R = 7.2mV across each emitter resistor.
Check the emitter resistors for their resistance values first,
and replace them if they are much off.
There are three outputs per side, so 40mA per transistor.
That is 0.04A x 0.18R = 7.2mV across each emitter resistor.
Check the emitter resistors for their resistance values first,
and replace them if they are much off.
Luxman b-12 bias
1) people dismantle the heatsink to gain access to the output transistors' emitter resistor to measure.
the voltage on both ends of that resistor reveal the current flow.
or just one lead of the multimeter contact the joint of transistor emitter and resistor,
the other lead contact the output terminal to measure.
(make sure the leads won't touch any other thing.)
or
2) de-solder the cable that supply power to the output transistors,
and solder a 0.1 ohm or 0.2ohm 5w resistor in series between that cable and the 15000u bypass cap. the voltage on the 0.1 ohm resistor reveal the current.
the 1k-B next to Q201 set the bias,
the gear must be off power, measure the value of the 1k-B,
the smaller the less current, you can turn it to 0 ohm to begin.
Its a vintage good amp,
I guess you'd changed all the bypass caps other than the two 15000u caps.
1) people dismantle the heatsink to gain access to the output transistors' emitter resistor to measure.
the voltage on both ends of that resistor reveal the current flow.
or just one lead of the multimeter contact the joint of transistor emitter and resistor,
the other lead contact the output terminal to measure.
(make sure the leads won't touch any other thing.)
or
2) de-solder the cable that supply power to the output transistors,
and solder a 0.1 ohm or 0.2ohm 5w resistor in series between that cable and the 15000u bypass cap. the voltage on the 0.1 ohm resistor reveal the current.
the 1k-B next to Q201 set the bias,
the gear must be off power, measure the value of the 1k-B,
the smaller the less current, you can turn it to 0 ohm to begin.
Its a vintage good amp,
I guess you'd changed all the bypass caps other than the two 15000u caps.
Q201
change the thermal grease under Q201,
clean the surface of the heatsink,
apply new thermal grease to have better heat transfer.
sometimes the thermal grease dried up and can hardly conduct heat to Q201.
before changing the value of the 1k-b,
clean it with 3 in 1 spray to make sure the contact is good.
check D201, R201, R202, check their conditions.
here is a link to the input IC, you might interest in.
Luxman audio products
change the thermal grease under Q201,
clean the surface of the heatsink,
apply new thermal grease to have better heat transfer.
sometimes the thermal grease dried up and can hardly conduct heat to Q201.
before changing the value of the 1k-b,
clean it with 3 in 1 spray to make sure the contact is good.
check D201, R201, R202, check their conditions.
here is a link to the input IC, you might interest in.
Luxman audio products
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