Beomaster 8000: Output Amplifier Board Initial Testing

The Beomaster 8000 Output Amplifier boards are recapped and the output transistors have been remounted with new thermal insulators. Before using these amplifier boards they need to be checked out and the no-signal current level adjusted to 18mV across the emitter resistors. I will do a final check again on these once the boards are re-installed in the Beomaster cabinet but while the boards are out it is good to do an initial check.

This check will let me know if there are any serious problems before going to the trouble of re-installing the boards.

For this test outside the Beomaster I need ±55VDC power and +15VDC power to the board. I also need a multi-meter to measure the voltage across the emitter resistors.

I can get ±55VDC with my two 0-60 VDC bench power supplies. These supplies are nice because I can set them up to limit their current to 15mA. That will provide protection against burning anything up should there be a hidden short circuit somewhere (i.e. a bad component).

Here is one of the two identical output amplifier boards ready for the no-signal current adjustment.

Adjusting the no-signal current involves slowly ramping up the ±55V rail voltages simultaneously to 55V. This should result in ±55V ready to go on the positive and negative supply rails. The +15V is then slowly applied to P38-2 which controls current in the output amplifier. The current draw from the  ±55VDC supplies should be around 100mA to 110mA when the output amplifier is on (+15 VDC applied to P38-2). The R226 trimmer resistor adjusts the no-signal current of the amplifier.

My bench multi-meter is more accurate for this adjustment than my hand held DMM so I switched to the bench meter to dial in the 18mV emitter voltage setting.

Here is a picture of the bench power supplies with the output amplifier on and the proper no-signal current level set.

I repeated the adjustment for the other output amplifier board and these two boards are ready to be re-installed in the Beomaster.