For the smaller A400 it is 6.5mV. Before replacing the power transistors (did you still get them?), replace the variable resistor with a multiturn one. If you turn the poti and dirt increases the resistance value or opens up the connection for a short time, the bias rushes away and might blow your trannies again... I just soldered out the original poti and soldered in an adaptory pcb with the multiturn resistor.
Happy new year!
Matthias
Happy new year!
Matthias
Hi Matthias,
For emitter resistors of 0.22 R each, 6.5 mV across the pair indicates around 15 mA of bias current. This looks good. There is no need to run these amplifiers any hotter as it will not improve the sound quality.
The bias control settings can be touchy with Yamaha product (too much range built in). The use of multiturn pots may help with this, but they are no more reliable than the original 270 ° controls that are original. A 10 turn control may actually be less reliable than the original part over time. Anyone who works with instrumentation can attest to this. Remember that you are passing current through this control, higher than there normally is in a normal application for this control type.
Hi Boris,
You may want to replace the control with a similar type, or at least work it back and forth a bit before setting it. If you turn gently, you will feel a detent where it was set.
-Chris
For emitter resistors of 0.22 R each, 6.5 mV across the pair indicates around 15 mA of bias current. This looks good. There is no need to run these amplifiers any hotter as it will not improve the sound quality.
The bias control settings can be touchy with Yamaha product (too much range built in). The use of multiturn pots may help with this, but they are no more reliable than the original 270 ° controls that are original. A 10 turn control may actually be less reliable than the original part over time. Anyone who works with instrumentation can attest to this. Remember that you are passing current through this control, higher than there normally is in a normal application for this control type.
Hi Boris,
You may want to replace the control with a similar type, or at least work it back and forth a bit before setting it. If you turn gently, you will feel a detent where it was set.
-Chris
Hi Matthias, hi Chris,
already got the transistors. At www.lobtron.de you can order a lot of semiconductors via internet.
I think that it was exactly the thing that the resistor opend up for a short time and so the power transistors blew up. I turned the resistor back and fourth to remove the dirt on it.
I think replacement is not necessary. But resistor-adjustment I will only do when amplifier is turned off in the future.
Thank you both for your hints
and happy new year!
- Boris
already got the transistors. At www.lobtron.de you can order a lot of semiconductors via internet.
I think that it was exactly the thing that the resistor opend up for a short time and so the power transistors blew up. I turned the resistor back and fourth to remove the dirt on it.
I think replacement is not necessary. But resistor-adjustment I will only do when amplifier is turned off in the future.
Thank you both for your hints
and happy new year!- Boris
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