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Sunday, February 26, 2017

Beomaster 8000: Update On Burn-in Retest

In my previous post I was reinstalling the output amplifier boards into the Beomaster and planning on a retest of the burn-in test.  I was expecting to power on the Beomaster and start playing music again. I should say hoping. I did consider that I still didn't know what caused the circuit breaker trip when powering on the Beomaster after the first burn-in test and that there was likely still a problem lurking.

When I powered the Beomaster up for this second burn-in test I immediately ran into a couple of problems. I didn't actually know right away that there was more than one problem but the first one was the Beomaster ±55 VDC rail relay would not stay engaged. I also noticed that the clipping lamp flashed when I tried turning on the Beomaster.

To find the cause of the power on problem I started disconnecting board connectors and discovered that removing the cables from the power supply to the FM Interface board allowed the rail power relays to start working again. To verify that was the problem I swapped in a spare Beomaster 8000 FM & FM Interface board set and the Beomaster was able to turn on again.

Feeling that I was now ready to begin the burn-in test again I decided to check the output amplifier DC offset voltages again. All of the rework I did on them involved checking the no-load idle current adjustment but I hadn't messed with the DC offset until I now had the boards back in the Beomaster.
I discovered that I couldn't adjust the DC offset in either channel now. So another problem to trace down.

I connected an oscilloscope to the speaker output jacks to see what the output signal looked like. The voltage with no music source playing was around 400mV.  I started playing some music via an iPod Nano connected to the Beomaster TP2 and saw that the output signal from the amplifier board was at a high level even though I had the Beomaster volume dial set to zero. That meant the Beomaster was not controlling its volume output.

The most likely suspect was the Filter & Tone Control board because the preamplifier routes the selected music source up through the Filter & Tone Control board for bass, treble and volume control.

I can't tell what integrated circuit is bad on the board by inspection so I will have to swap out some components to figure it out. It is possible that IC201 and/or IC202 failed as they control the voltage. That will be unfortunate as those are no longer made.

In the meantime I am able to test this suspected board by swapping in one of my spares. Sure enough, with a different Filter & Tone Control board the Beomaster DC output levels could be adjusted on the output amplifiers again and the volume control function works.

I now need to determine what the specific failures are on the FM Interface board and the Filter & Tone Control board. Should the volume control failure be due to IC201 and/or IC202 I can see if Beolover has any spares or I can use the chips from my spare Filter & Tone Control boards. But then I will need replacement AD7110 chips for those.

The good news is I can retest the burn-in test of the power supply, output amplifier and preamplifier again. This test is needed to make sure that initial burn-in test failure is fixed.
Here is the burn-in retest after an hour running. I will leave the Beomaster playing from the iPod Nano overnight and check it in the morning.

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