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Monday, February 13, 2017

Beomaster 8000: Unexpected Burn-in Test Results

I successfully played music through the Beomaster for about eight hours. That felt like a good test and I put the Beomaster into Standby mode.

The next morning I wanted to check the Beomaster again. When I switched the Beomaster on (from Standby to TP2) the house circuit breaker for the outlet I was using tripped. What a surprise. Not wanting to take any chances I plugged the Beomaster into my dim bulb tester, reset the house circuit breaker and turned the receiver on again. Now it would try to switch on but immediately go back to Standby.

So trouble-shooting this power problem became the order of the day. This is why burn-in testing is necessary.

I suspected something with the output amplifiers so I unplugged the rail voltages. The Beomaster would now switch on and the ±55 VDC rail voltages measured okay (when not connected to the output amplifier boards). I will note that I also had to take the Beomaster off the dim bulb tester at this point because it interfered with the power up.

After some more trouble-shooting I determined that the left channel output amplifier board appears to be good. I was able to hook the left channel rail voltages back up and check the no-load idle current and DC offset. This was not the case for the right channel output amplifier. I re-connected the right channel rail voltages again and they measure correctly when the Beomaster turns on  However, there is no right channel output. The no-load idle current stays zero volts.

The problem is in the right channel output amplifier so I will need to examine that whole assembly.
Here is the right channel output amplifier assembly removed from the Beomaster chassis.

Here is the output amplifier with the components removed from the heatsink -

I discovered that TR208 (NPN) was not securely fastened to the heatsink. The mounting screw was loose and it was free to move. That doesn't mean it is the culprit but I probably need to make removing and checking this assembly part of my normal Beomaster 8000 restoration process.

The next step is to check the transistors and diodes of this circuit to see if there are any failed components.

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