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Thursday, October 27, 2016

Beomaster 8000: Step One - Rebuilding the Output Amplifiers

While we wait for Nick's awesome specially made pulley for the Beogram 4002 (5501) that I just finished up, it is time to get started on the Beomaster 8000 that will also go on to the UK once restored. It recently arrived and I gave it an external inspection, which suggested this 8000 is an excellent starting point for a full restoration.
My first step in any Beomaster 8000 project is to rebuild the output amplifiers. They are the most crucial part of the restoration of the 'power' part of the unit. A failure of their often corroded quiescent current trimmers usually neatly kills all output transistors with a bit of smoke emission on that channel before the main fuse protects the transformer (and the breaker of the house grid on which the 8000 resides while this burnout happens). These old trimmers often go open circuit during transport, so my approach is to not even turn the unit on when I receive it, but I go straight to the amplifiers and rebuild and test them to make sure that the above does not happen.

Here are a few impressions from this effort on this unit:

This shows the right channel as it came:
We see from the two clunky (white) emitter resistors that the above mentioned disaster already must have happened at some point. When the transistors burn out, the emitter resistors usually brown like a chicken in the oven due to the immediate heat emission. But usually this does not affect their performance down the road since they are wire wound types. An ugly replacement like seen here is definitely not an improvement in the Beolover's eyes! Also, whoever did this did not learn 'The Lesson'...he did not replace the trimmer! I always put in 25 turn precision encapsulated units to prevent this from ever happening (again). Multi-turn trimmers drift only very little and so the quiescent current adjustment is very stable over time. This is not the case for standard single turn trimmers, and that is one of the reasons that the 8000 often presents with one or two hot heat sinks, if it runs at all.

This shows the rebuilt board with new electrolytic 105C type capacitors and prettier resistors and the 25-turn trimmers:
Beautiful! I did the same to the left channel, which did not have new emitter resistors, but (slightly browned) original resistors. And then it was time to run these babies from external power supplies to test for any silicon failures, cracked traces and the like. This shows my hookup:
And here with the multimeter connected to the test points at the emitter resistors after powering the board up:
When installing a new trimmer make sure that the resistance is close to zero. This may be counterintuitive, but in this setup it turns off the output transistors preventing significant current flow between the +/- power rails. This shows my bench supplies at this point:
The left two are the - and + rails, which draw 60-70 mA while the quiescent current trimmer is close to zero Ohms. The right supply provides the 15V control voltage that is used by the Beomaster to enable the output by controlling the constant current source used for biasing the output transistors.

Adjusting the quiescent current means to ramp up the resistance in the potentiometer to set the working point of the output transistors that there is a 18mV voltage across the two emitter resistors:
At that point the power rails draw 0.1 (+) and 0.11(-) A:
I did the same for the right channel:
All good now in the output amplifier department! On to rebuilding the power supply board before giving this unit a first spin!

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