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Beomaster 901 Mexico

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Søren Mexico
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Søren Mexico Posted: Tue, Apr 17 2012 4:38 AM

A question to my BM 901 type 2602, I downloaded the manual and schematics(two different downloads) from the site, when I started repairing I went with the big manual and the schematics in it, As I made a short during last adjustment I had to go in deeper to find out what I had destroyed.

I found differences, between my 901 and the manuals schematics, in some resistors, when I started checking for failures I went through left channel end, checking all components on board and found two resistors that in the manuals schematics showed 820 ohm, and I measured one 570 and the other 270, of course I thought that I had found a failure.

Later I opened the downloaded schematics (not the manuals schematics) and found that the 2, 820  ohm in that scheme was 560 and 270, I also found a preset with 1K ohm, where in the manual it was 500 ohm

Here the pic from only schematic.

 


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Søren Mexico
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And here from the manuals schematic, now please explain to me why this change and what is it good for ?? and if you look at the 560 ohm resistor, there is a bridge to the base of 2TR21 ??

 


Collecting Vintage B&O is not a hobby, its a lifestyle.

Søren Mexico
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Søren Mexico:
And here from the manuals schematic, now please explain to me why this change and what is it good for ??

Anybody out there ??

Collecting Vintage B&O is not a hobby, its a lifestyle.

Dillen
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Dillen replied on Tue, Apr 17 2012 9:57 PM

It was probably changed in order to achieve a slightly higher cross-current for 2TR21, it makes for
a slightly less signal-sensitive setting.
It could also provide a broader range of the idle current trimmer but I don't think that was the main reason
since the trimmers usually have more than enough span and rarely go far off center.

If you replace components, I suggest you put in values similar to those fitted.
If you shorted the output transistors or something in the diode string and the 2R139 and 2R140 resistors burned,
both the diodes, the output stage transistors, 2TR20, 2TR21, 2R141 and the trimmer itself could also be damaged.

Martin

rolfzetterberg
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The reason for the "bridge" to the base of 2TR21 is that if the wiper is loosing it's contact with the carbon track,the higher voltage at the upper end of the pot causes the transistor to conduct more,thus clamping the bases of 2TR22 and 2TR23 together and thereby reducing the current.

Quite smart really,and this trick can with advantage be used in power supplies,for example.

Søren Mexico
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thank you for the answers, yes I changed with similar components, and yes diodes and 3 big and one small transistor got damaged.

I have been thinking about this change for a while, and as didn't find out why, I just wanted to know, still learning Smile

Collecting Vintage B&O is not a hobby, its a lifestyle.

Dillen
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Dillen replied on Wed, Apr 18 2012 12:07 AM

If the wiper loses contact, the base of 2TR21 will be floating and the idle current will be out of control (typically very high) regardless of the voltage on the top of the trimmer since those two circuits will then no longer be connected.

The circuit with the trimmer and 2TR21 is fed from the two output stage bases to give stabile and balanced
working conditions while at the same time regulating or 'clamping' the voltage difference at these two points.
The larger voltage difference, the more open the output transistors.

There's also the fact that the idle current circuit in this amplifier runs
without any constant current regulator to speak of. No zener to lock anything precisely so the signal, (mainly
low frequency contents) can have slightly more influence on the idle current, if you can speak of idle
current when actually playing. Not a lot but enough to correct for in more expensive amplifier types.

The only means introduced to hinder a thermal runaway is basically 2TR21 itself in that it has been
fitted so that it has thermal contact with the collector pin (and thereby the cooling fin) of one of the output stage transistors.
In other words, its temperature is varying with that of the output stage transistors (and the cooling fin).
A transistors amplification rises with the temperature and a higher amplification of 2TR21 will cause a
closer clamping (closing) of the two output stage transistors.
So it will take down the idle a bit if warm.
The output stage transistors will of course also conduct and amplify more when warm - they are transistors too -
but 2TR21 has a good grip of the bases (the control "handles" of the darlingtons), it's influence
will be amplified by the darlingtons themselves so will easily compensate for the rise in their amplification when warm.
Simple and clever. And still incredibly well sounding in my opinion.
In other types of amplifiers, one of the transistors in the constant current generator is often
seen in this job to the same effect.

However, it does not explain why the resistors were changed but it's quite common to see actual component
values differ from the service manuals.

Martin

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