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Wednesday, November 7, 2018

Texas Beomaster 8000: Readjusting the output amplifier assemblies - Part 2

Continuing with the recheck of the Texas Beomaster 8000 Left Channel Output Amplifier Assembly I confirmed that it is failing. It is able to be adjusted for no-load current and DC offset but it doesn't do its job of amplifying an audio signal.

I pulled the output amplifier assembly back out of the Beomaster 8000 cabinet so I could test it stand-alone on the bench.





























That removal procedure is getting easier to perform but it is still not a fun task.

Here is the left channel output amplifier assembly with the connections to test it.

 The stand-alone bench test requirements are ±55 VDC supply voltages for the rails. A +15 VDC supply voltage to (essentially) turn the amplifier on, an input signal from a signal generator and an 8Ω dummy load resistor to simulate a speaker.

Here are the voltages and the input/output signals when the output amplifier assembly is power up without any input signal.  The oscilloscope shows just a flat line for the amplifier output to the speaker load.





























I used an HP 8903A audio analyzer to supply the test sine wave signal. This was a 1KHz sine wave with an amplitude I adjusted from 0.0 Vrms to around 20mVrms.

Here is the starting point (0.9mVrms, 1KHz sine wave).






























Here is the output with a 20mVrms, 1KHz waveform applied at the audio input to the board.





























The output signal looks bad. Compare that to a spare, good Beomaster 8000 output amplifier board.






























This Beomaster 8000 left channel output amplifier assembly needs more work.

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