There is a service manual available here after free sign-up. Trimpots TVR1, TVR2 adjust bias which you can monitor as the voltage drop across either R51 or R52 and R134 or R135 in the respective channels. Unfortunately, this scan of the manual doesn't show the specification for bias current nor the corresponding mV drop across the resistors.
Measure and make notes of the existing mV readings before adjusting anything. First, ensure the speakers are removed and the amplifier is quiescent (no signal) then. The existing bias current is likely still correct if the amplifier works properly and that might show a reading of 10-15 mV and could be your best or even the only source of reliable information.
Adjusting bias away from the factory setting in AB amplifiers is often problematic because the current won't stabilise until the heatsink has reached equilibrium and that may no longer happen if you increase it much above the design spec. Unwise bias increases usually end in smoke and tears now or maybe later, when you turn it up for partying etc. Simply, you cant turn a class AB design into a class A or even a high-bias class AB because the heatsinking usually isn't enough and the semis and other components become stressed.
Measure and make notes of the existing mV readings before adjusting anything. First, ensure the speakers are removed and the amplifier is quiescent (no signal) then. The existing bias current is likely still correct if the amplifier works properly and that might show a reading of 10-15 mV and could be your best or even the only source of reliable information.
Adjusting bias away from the factory setting in AB amplifiers is often problematic because the current won't stabilise until the heatsink has reached equilibrium and that may no longer happen if you increase it much above the design spec. Unwise bias increases usually end in smoke and tears now or maybe later, when you turn it up for partying etc. Simply, you cant turn a class AB design into a class A or even a high-bias class AB because the heatsinking usually isn't enough and the semis and other components become stressed.
I neglected to say above, that there are 2 pairs of output transistors in each channel. In an amplifier that has an unknown history, you should also check for a similar current (i.e. voltage drop) across the second pairs of emitter resistors at R60 or R53 and R53 or R136. If the transistor pairs are original and sufficiently matched, the total bias current should be near equally shared between pairs, so you should only need to check one. It's a different story if the amplifier has been hacked.
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