Beomaster 1200 - dc on output

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baek47 Posted: Sun, Oct 11 2020 10:39 AM

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Hi

I bought a white BM1200 which is so good looking.

I bought it as "functioning" but i had 13V dc on the output. I never attached speakers.
I changed the 3000 uf output caps - , that didnt to it - now the dc is 18V meassured between pos. and ground on the speaker terminal.....

The dc powersupply is adjusted to 33V.

I didn't adjust the bias yet, it is about 6.6 mV.

Do I miss something - or is something wrong…?

Br Torben

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Dillen replied on Sun, Oct 11 2020 10:56 AM

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Burned output stage is the most likely.
Check the output stage transistors and the emitter resistors.

Martin

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baek47 replied on Sun, Nov 8 2020 7:48 PM

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Hi

Martin, I followed your advice of checking the output transistor, they were all good - nice rca's.

I changed all capacitors in the power supply and oytput stage, also emitter resistors....all adjustment on the 33V voltage and 7,5 mV on emitters are good.

I still have 0 v ac, but 18,4 v dc on both outputs.....hmmmm. The same dc amount on both sides of the output caps.

The schematic is so that there is a 3000 uF at the end of each output stage - just before headphones/speakerterminals...…..

When I meassure with my multimeter for dc on the outputs with no speakers/headphones connected, isnt it so that I will meassure dc voltage "hold" by these output capacitors?
And if speakers where connected, the dc "hold" would dissapear.due to 4-8 ohm circuit? Perhaps with a little Beomaster thumb at power on?

Shouldnt I be good and done repairing - ready for speakers? Im still a beginner in this…..

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baek47 replied on Sat, Nov 14 2020 12:10 PM

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Solved….

DC offset (a lot) is normal for the construction due to the output caps.

I found this

https://audiokarma.org/forums/index.php?threads/bang-olufsen-beomaster-1000-dc-offset-problem.736246/

When speakers are connected there is no dc offset....

But I have no sound on left channel from tuner!
From recordinput I have both channels.
When "mono" is on I have sound in both channels from all sources.
I resolderi everything on the tuner board - + the input switch board - that didnt make it.

Its connected with a not nice "thumb" changing input - especially from tuner to record/tape...