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This is a follow up to my recent post about the redesigned Beogram Commander remote control board, which now works in both (DC-motor) Beogr...

Sunday, June 1, 2014

Beomaster 8000 Output Amplifier Restoration - Most Capacitors Out of Spec

Another Beomaster 8000 arrived for restoration. As usual, I opened it up for rebuilding the output stages before turning it on. This is a preventative measure against burning out the output transistors due to potentially corroded quiescent (quiet) current trimmers. And I like my 8000s with their original Texan transistors...;-).
I put in closed 12-turn cermet quiescent current trimmers, new differential input offset trimmers and new 105 C electrolytic capacitors. It definitely needed new caps. All of the 'red caps' except one of the 220 uF and the two 4.7 uF signal path caps were dead. Here is a capacitance and Equivalent Series Resistance (ESR) measurement on one of the 220 uF cans:

413 nF and 149 Ohm ESR is definitely missing the mark! This suggests that this Beomaster definitely needs a full recap. It is great that these red caps at least seem to die open-circuit instead via short circuits...Interesting to note that the axial leaded 100uF caps were all o.k....it seems it is always the red/orange types that go.

Here are the pics of the right and left channels before and after:

Right channel before:

and after restoration:

Left channel before:

and after restoration:

After putting in the new parts I measured the quiescent current trimmers, and indeed one of them had a too high resistance between swiper and track (~200 Ohm). This would have caused at least a pretty hot heat sink if not fried the transistors. But first things first. I hooked the output stages up to external power supplies and fired them up with the quiescent current trimmers turned to 0 Ohm (all the way CCW), thereby initially turning off the current through the outputs. See my post at http://beolover.blogspot.com/2011/09/output-stages-testrecap.html for details. Then I carefully adjusted the trimmers to yield the prescribed 18 mV across the emitter resistors (the big green wire wound resistors in the center of the photos, R236/7 in the diagram)

The currents into the the power rails were:

+55V --> 0.10A
-55V --> 0.11A

The +15V control voltage for the constant current source (TR208/11) yielded 0.01A.

After the above soft-ramp-up, I turned everything off, and reconnected the power rails, and the signal and control connectors to the output PCBs, and turned the Beomaster on. It came on without fuss and the radio immediately tuned in a local station. Both channels seem to work, and all other functions seem to be working, as far as I can tell at this early stage. Rather remarkable in light of the dead caps in the outputs. It will be interesting to see if many of the caps on the other boards are also dead, or if the ones in the outputs lived a more stressful life...they probably did experience higher temperatures than those on the other boards. At any rate the next step is to rebuild the power supply board...it seemed that one of the lower voltage reservoir caps got a bit warm during the above test, indicating a potential near-short in it.

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