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Thursday, November 8, 2018

Texas Beomaster 8000: Readjusting the output amplifier assemblies - Part 3

The left channel output amplifier assembly on the Texas Beomaster 8000 is working again.

I removed the heatsink assembly darlington output transistors (TIP 141 and TIP 146 devices) and replaced them with spares I had in stock. I decided it was better to replace the full set of output transistors and I decided to replace the two output transistor emitter resistors.

The replaced parts are outlined in the photo below.

Here are the replacements installed.

Now it was back to the bench to check and readjust the no-load current and DC offset.
Here is the test setup for that.

There were no problems on those adjustments.

It is worth noting that when output amplifier components are replaced the no-load current is often way off. After I ramp up the ±55 VDC rail voltages the current draw on those supplies should be around 30mA.  As I turn the output amplifier on by slowly ramping up the +15V I monitor the current draw of the ±55VDC bench power supplies. When the no-load current trimmer is way off the current draw on the ±55 VDC supply rails will sometimes go beyond 130mA way before the +15VDC control voltage is reached.  If that happens I turn no-load current trimmer to bring the ±55VDC supplies current draw back down to 100mA.  Eventually I get to the full +15VDC control voltage with the ±55VDC supply currents at around 90mA to 100mA and the voltage across the emitter resistors at 18mV.

Now for what I learned on this whole exercise...
It is worth doing a quick functional check of the output amplifier assembly to make sure it can actually amplify an input signal.

The setup for this test is to remove the jumper I temporarily installed across the audio input connector and plug in a cable to my audio tester (the HP 8903A audio analyzer in this case). The output amplifier ± speaker connections are connected to my 8Ω dummy load where I measure the voltage with an oscilloscope (through a differential probe).

I begin the test with the audio analyzer outputting a zero amplitude sine wave.  I raise the amplitude on the signal in 1mV increments to 20mV while monitoring the output on the scope.

Here is the test setup again. I have marked the input signal and output signal.

Finally the left channel output amplifier is showing a good, amplified sine wave at the speaker output.

Now comes the fun part of re-installing the output amplifier assembly back inside the Beomaster cabinet.  Then I can get back to functional tests on this unit.

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