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Monday, November 5, 2018

Texas Beomaster 8000: Readjusting the output amplifier assemblies - Part 1

I was expecting to perform a quick task of verifying the output amplifier board no-load and DC offset adjustments (that I made earlier) were still good. Once that check was made I could start some functional testing of the Beomaster.

The Beomaster units don't always cooperate. In the case of the Texas Beomaster 8000 the left channel no-load current drifted upwards (from 18mV across the output emitter resistors) to the point of the clipping light illuminating and the protection circuit shutting the amplifier down. Disappointing to say the least.

I was able to adjust the left channel no-load current down where it stabilized at 18mV after a warm up period of a couple of minutes. The problem now is that the left channel was starting at around 6mV across the emitter resistors on power up where the right channel starts around 12mV.

I decided to check what music sounded like through the Beomaster. This is the first time playing a music source since the restoration work began.

Success. I can play music through the Beomaster TP2 source input.

A better description would be partially successful.

The volume level had to remain low and I could hear some distortion in the left channel speaker.
To determine what was going on I replaced the speaker connections from my Beovox S55 speakers to my 8Ω fixed dummy loads. I applied a test sine wave of 1.5Vrms, 1KHz to the Beomaster TP1 (Tape 1) source input and setup my oscilloscope to monitor the source signal and the amplifier output signal that is across the dummy speaker load (one channel at a time).

Beginning with the right channel here is what I got with the Beomaster in standby mode.

My measurement through my differential probes is a little noisy but will show me what I am wanting to see here.

This photo is of the Beomaster turned to the TP1 source and the volume level at 2.8.

I can see the 1KHz sine wave on the right channel speaker output as expected.

Now for the left channel.  The 1KHz sine wave looked okay up to around 2.2 on the Beomaster volume display.

Going above that results in clipping on the top section of the sine wave.

You can see the top of the left channel output signal is chopped off (even before the clipping light comes on).  That matches the distorted sound I was hearing when I had the Beomaster connected to my Beovox S55 speakers.

This means I will have to pull the left channel output amplifier assembly again and locate the fault. Hopefully it will just be some connection problem.

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