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Tuesday, February 20, 2018

Beomaster 8000: Left Channel Output Amplifier DC Offset Resolved

This Beomaster 8000 is almost fully functional. Actually it does function and play music but it doesn't completely pass the service manual DC Offset adjustment procedure. It is very close but no cigar.

The right channel adjusts perfectly. With the Beomaster on and warmed up I have the right channel output amplifier adjusted so the DC Offset measures 0.0V ± 0.1mV.

The left channel could only be adjusted to around 0.040V (40mV). Here is the differential amplifier circuit where this adjustment is made.

The adjustment trimmer (R200) adjusts the current to even out the differences in hFE between TR201 and TR202 (BC546B NPN transistors). As I said, the left channel cannot be adjusted to the 0.0V ± 5mV range specified by the Beomaster 8000 service manual.

I pulled out TR201 and TR202 to measure them with my transistor tester. Sure enough, their hFE measured quite a bit different. TR201 measures 146 and TR202 measures 318.

I checked a few other BC546B transistors and they are all close to hFE = 300. So TR201 appears to be the culprit. I went ahead and replaced both TR201 and TR202 with BC546B transistors as close of an hFE match as I could find.

That did it. I rechecked the no-load current adjustment first then performed the DC Offset adjustment. Now I can get the left channel output amplifier down to a good DC Offset value.

This Beomaster is feeling like it is into the home stretch now. Time to start closing up the cabinet and doing some real play testing.

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