Beomaster 1900-2 - very strange behavior

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livingstone2011 Posted: Mon, Sep 14 2015 10:02 AM

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Hi all ;)

Over the past weeks I have been refurbishing a very nice BM1900-2 but I have now run into a problem wich puzzles me a lot.

FM2, 3 and 4 works perfectly - very crisp and playing both channels. Stereo, loundness and afc works.

But FM1 and 5 - no sound at all - not even hissing or cacking. Adjusting R1 and R8 does not produce anyway near 4.5 (4.6) volts over TP1 (messured between R1 and ground).

Secondly, Tape and Phone plays nicely in right channel only with only a faint sound in left channel.

The receiver has been recapped, balance, treble and bass sliders cleaned and are working. New blubs - and all working. Idle trimmers have been replaced and bias adjusted to 12mv across R256/356 as per manual.

So - in theory - erverything should be fine.. but it isnt..

I am really in the dark here so any help or advice would be very much appreciated. Thanks

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Dave Farr replied on Mon, Sep 14 2015 10:12 AM

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May seem a strange question but - where did you get your bulbs?  Unless they are exactly the correct type (from member 'Dillen') you cannot trust them. The wrong bulbs can cause all sorts of strange problems.

Dave.

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livingstone2011 replied on Mon, Sep 14 2015 10:33 AM

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The bulbs are of the correct specs - I am aware that they are critical for the functionality of the receiver.

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Dillen replied on Wed, Sep 16 2015 9:36 PM

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Tuning problem;
Check for shorts under the tonecontrol/FM presets board.
Depending on board type, the wires coming from below is not particularly well insulated and can cause problems like this.
Could also be a problem with the muting. A too high source-lamp current can keep the muting signal active for the source in question.

Line in problem;
Here you will have to put a scope to the signal path and see where the signal gets lost. Somewhere before the diode-switch.

Martin

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livingstone2011 replied on Thu, Sep 17 2015 11:22 AM

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IMG_0425.jpg

Update. Well Martin beat me to it ;) Thanks.

Anyway. I have now got correct tuning voltage 4.7 and 4.8V respectively over TP1. Turns out if was a defective -new- trimmer. (Note to my self: Always mesure new components before soldering )

But still no cigar. FM2,3,4 playing crisp both channels - FM1 and 5 tuning well (according to indicator and stereo lamps) but no sound at all.

Tape and phono good sound in right channel but no or only faint sound in left.

I'll keep measuring. The errors must be connected somehow - a failed component somewhere in the signal path. 

Picture of the tuning voltage culprit..