Texas Beomaster 8000: Testing the Output

I left off with the Texas Beomaster 8000 playing music for several hours as a check of the amplifier rework and a burn-in test. That test play of the Beomaster went great. I ended up running the Beomaster amplifier for eighteen hours without a hiccup.

Before connecting up some test equipment to measure the Beomaster output amplifier total harmonic distortion (THD) at 1KHz and maximum output I needed to take care of a couple of minor things.

The first thing was to adjust out the "clicking" noise that was occurring when altering the volume level. This is something that can happen due to DC offset voltage in the volume control circuit. The Beomaster has separate left and right trimmer resistors to remove that offset. I typically have to adjust those trimmers after replacing components on the Filter & Tone Control board (as part of the restoration).

Along with the left and right trimmer resistors for the DC offset there are two fixed resistors per channel that can be used or removed to extend the range of the offset adjustment.  In this case I had to remove the R145 resistor in the left channel and the R243 resistor in the right channel.

I verified the Beomaster 8000 remote control functions work so I re-attached the metal shielding box for the remote control receiver circuitry on the power supply board.

It was also time to finish re-assembling the Filter & Tone Control board. It is fairly common to find broken plastic tabs that hold the push button board. I repair those using black hot glue. That glue will do the job of securing the board but can be peeled off later if needed.

On the problem with the FM tuner not functioning I found a loose wire on the cable between the FM board and the microcomputer board. An easy fix and the tuner is working great again.

Now to check the output amplifiers and their rated maximum 100W output across my 8Ω dummy loads.

For this test I will use a signal generator to apply a 1.5Vrms, 1KHz sine wave to the Beomaster TP1 input connectors (L & R).  The speaker outputs will go to my 8Ω dummy loads where I will measure the output voltage with a bench DMM and the THD with an audio analyzer.  I also connected my Fluke multi-meter across the emitter resistors to see what the DC voltage levels do there when the amplifier is driving the speaker load.

The THD is measured using the QuantAsylum QA400.  Here is the left channel output amplifier THD with 9 Vrms output across the dummy speaker load. That is about 10W. That was with the Beomaster volume control set to 5.0.  The THD was about 0.013% which is a good number for this amplifier.

Continuing on I increased the Beomaster volume until I got 28.2 Vrms across the speaker load. That corresponds to about 100W which is the rated output of the Beomaster.

I get just under 0.05% THD (not weighted).  While I can't precisely duplicate the methods Bang & Olufsen used back in 1980 to make these performance measurements my results appear to be in line with what this amplifier is advertised to do.

The right channel measures pretty close to the same.