Thank you for part 1. I could also read that you made some modifications so that the sound does not 'exit' the amplifier to go through the blind plug, saving quite a bit of signal path. Any advice on how to do this?

Finally got around to removing all of those capacitors.

Apparently C67 and C68 were never fitted from Bang&Olufsen, so I guess they must have suspected that they were not necessary.

I am listening to my BM7000 now, and I agree that it takes away a layer of 'muffleness' - it's like removing a blanket in front of the speakers.

Thank you for suggesting this mod. I wonder why B&O fitted the board with those capacitors in the first place.

I also finally got around to adjust the no load current to the specified 11 mV. The trimmers were quite dirty it seemed so I gave them a big splash of contact cleaner, waited some minutes, and then it was possible to adjust the no load current without it jumping like crazy.

However, after I put everything together, I have a rather irritating and interesting problem: After some time, the two buttons on the front panel do not work which means that I cannot turn the BM7000 on with the left button. If I then turn it on with a remote control and turn it off again, the left button will work for some time, after which it gets unresponsive again. Any ideas why?

The tuner board has a hinge to open it 90 degrees... and the hinge has a function to remove the tuner board... turn and push/pull axial ... no need to open bolts on the hinge.

The tuner can be removed and the master is still working without the tuner, so you can test it on your desk.

The black bottom plate is easy to remove... 4+3 bolts. Then you have access to all other boards from both sides without removing them out of the cabinet.

 

The defect buttons: check the button on the board. There can be oxyd inside the switch, especially when there was smoking nicotin. Then you need the switches.

Sometimes the connector ob the display board to the cpu board makes problems. I have also seen keramic caps break near the connector because there is sooooo much solder, there is a filter cap row and the thermic expansion makes them crack.

Hello everyone,

I am wondering if it's going to have Part 2 or maybe more mods for this amp. I also want to thank you Die_Bogener for your advices and great work that you do for this community.

Best Regards
Martin

The insulation capacitors are there for a reason.
I would NEVER remove them.
Actually, they are signs of a very high quality circuit design.

So what are they, and why are they here?
They are small ceramic capacitors placed in strategic places through the signal path, from the signal path to ground.
They are called insulation caps because they insulate the individual signal path stages HF-wise.
They are here because you cannot put a load on f.e. a power supply, without seeing some kind of reaction on its power output, and
because most OpAmps have a very high slew-rate.

If an OpAmp switches fast from off to on (or vice versa), the current it "accellerates" will have to come from somewhere.
In other words, a sudden spike commanded on an OpAmp output will see a corresponding (reverse) spike on its power supply rail(s).
This supply will often be supplying other circuits in the signalpath as well, and these circuits will also see the spike in their supply causing them to instantly produce a spike on their output.
Actually, the OpAmp itself will almost always also cause a spike, as will most semiconductors (and some capacitors).

The spike on the power supply rail will of course also affect the very OpAmp that caused it (with a brief delay), and
the (very!) brief power surge will cause a new spike on its output.
The result is a very high frequency "ringing" (fading burst of spikes) until the new current flow situation has settled (throughout
the signal path!).

As the spike reaches stages with higher and higher amplification and higher currents involved, the corresonding
spikes on the power supply rails will be larger - again feeding back to the earlier stages.
If the power supplies are not suitably decoupled, due to f.e. tired/sluggish filter capacitors, this could in severe cases end in
self-oscillation and a burned amplifier.

Small caps (100nF or so) are often placed directly on OpAmps (and other circuits handling audio signals) power supply pins.
This helps quite a bit when viewed from the power supply side.
But any spike already produced will go on to the next stage, be that tonecontrol, volume or amplification.
If nothing is done, the spike will continue to the next stage and cause the next OpAmp to (attempt) to
reproduce this "signal" which, in case of an amplification stage, would only make matters worse.
The spike will grow through the amplifier, and the more it grows the heavier spikes form on the power supply rail and the
more ringing will be introduced to the signal.

This signal could eventually end at the tweeter (as speakers crossovers usually don't cut off HF), and it can contain quite an amount of energy.
A Zobel-network on the amplifier output would take some of it, but even this small circuit could be put to hard work.
Bad power supply filtering caps (the small 10uF) in f.e. Beomaster 5500 can cause ringing throughout the preamp and output stages,
and the small Zobel network resistor can actually burn from the resulting HF energy contained in the signal.
The output stage will also run warm.

Putting an insulation cap (a few pF) on the signal path to ground will drastically improve matters as it will be seen as a
dead short for the very high frequency contained in the bursts.
A capacitance this small will have no influence on anything within the audible frequency range.
Quite opposite in fact, as it will remove spikes and bursts that are not part of the original signal.

In some cases, - typically early in the signal path, the OpAmps (or whatever circuit handles the signal) power are fed through
a low-pass filter, often seen as something like 47-100 Ohms in series with the power supply/ies and 100uF to ground.
This is done for all stages individually, of course, - or it would be pointless.
The JAMO Promix 200 which is at first glance simple but, by far, one of the best sounding discoteque mixers, uses this in dozens of places.
Beogram CD5500/6500/7000 does too in the analogue stages.

I strongly recommend leaving insulation caps well alone.
It is very high quality circuit design.

Martin

Hi Filip

I saw that you aligned idle current between 2TP200 - 2TP201  for 11 mV  , and similar for the other channel.

I suppose that is across both emitter resistors 0,22ohm?

Cannot find these TP stipulated on diagram or print.