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BM4000 one dead channel ?

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ALF
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ALF Posted: Tue, Mar 14 2017 9:59 AM

greetings,

currently working on a new arrival: a lovely condition BM4000

but the right channel is dead !

looking at the output amp circuit from the SM download it is different to what mine looks like ?

there are also 6 trimmers on the board - 4 of them according to the SM not to be touched ?

but mine were in terrible condition and had to be replaced - what is their typical setting ?

so far I replaced about 4 trimmers on the main board, 2 on the tuner board and 6 on the preamp board !

most of them were almost rusted through ???!!!

the SM also mentioned a resistance test on the mounted output MJ2501/3001 darlingtons - I should be able to do this with a DMM ?

I will upload a few pics separately as a first impression, inviting suggestions !

ALF


ALF
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before and after the trimmer replacement on the main-board !


ALF
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still two mystery trimmer in the center position on the main-board ? can't find them on the circuit diagram ??


ALF
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more trimmer work on the preamp board - next step will be the "orange revolution" of those ROE caps !!


Dillen
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Dillen replied on Tue, Mar 14 2017 11:41 AM

There is no "typical setting" for alignment trimmers.
All six trimmers on the amplifier board are in the schematics and you will have
to adjust them the way it's shown in the servicemanual.

Why are you placing the preamp input level trimmers on the underside of the board?

Martin

ALF
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ALF replied on Tue, Mar 14 2017 3:31 PM

The question in regards to the 6 trimmers was for the output amp board - two of them are for the no-load current, the others ?? - and not for the preamp board !

as for the placement of the trimmers on the preamp board :

the trimmers used can not be adjusted on both sides, unlike other types.

ALF

Dillen
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Dillen replied on Tue, Mar 14 2017 4:00 PM

Your Beomaster is a late type (after s/n 227250) with the second "upgrade" to the protection circuitry.
The four extra trimmers are part of this upgrade.
Basically, it's all about setting the working point correctly for transistors TR44, TR45..., compensating for
their individual specs.

The following is the procedure for adjusting the four trimmers:

Do the right channel first:
Connect an oscilloscope to the speaker output
Feed 1 kHz sine to the amplifier input, until clipping starts
Capacitor 4.7uF is mounted across C-E of TR43
Resistor 190ohm  23W is mounted across C-E of TR46
Trimmer 5Kohm pos 585 is adjusted until distortion on negative peak
Resistor is moved to C-E of TR47
Trimmer 5Kohm pos 586 is adjusted until distortion on positive peak

Left channel is adjusted the same way

Martin

ALF
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ALF replied on Wed, Mar 15 2017 1:44 PM

Thanks for that  Martin - I assumed there was more to it !

sadly I don't have a frequency sweeper and are unable to perform that exact adjustment procedure.

therefore my question whether a "typical" adjustment of these trimmers would be sufficient, 

as long  they are in the ball-park ?

It is unlikely to meet the optimal setting that way but at least It won't cause any damage to the transistors involved ?!

these old almost cooroded trimmers need to be replaced !

ALF

Dillen
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Dillen replied on Wed, Mar 15 2017 2:46 PM

Well, I have seen people measuring the current set value of the old trimmers and setting
the new ones to the same ohmic values, but obviously this is neither the best nor the right way to do it and
not a method I would recommend.
In many cases, the other components in the circuit containing the old trimmer will have
wandered a bit in value in 40+ years, as will the trimmer itself, and the trimmer would ideally
need adjusting even if left untouched from factory.

A heavily oxidated trimmer can't really be trusted to read correctly either.
It's bad (the reason for replacing it in the first place).

Martin

ALF
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ALF replied on Thu, Mar 23 2017 9:48 AM

sadly at the moment the best I can do after carefully removing the old trimmer from the board to get - hopefully - a "reliable" reading of the current setting !

on another matter:

could you advise what type transistor would be a good substitute for the BC 173B transistor ?

thank you kindly

ALF

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