Beomaster 6000 type 2251 - 1 channel DC offset voltage high - possible reasons ?

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stecknuss Posted: Mon, Mar 18 2019 1:05 PM

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Dear reader, 

I have just updated my BM 6000 with Dillen's  capacitor kit (and new trimmers on the output Amp board) . Powering on I now found one channel of the output Amp can be adjusted well (no load current 22mV  / DC offset about 0)   but the other channel gets no load current also to 22mV  but DC offset  is at 450 mV  (I can regulate with the new trimmer between 350 and 550 mV ) 

I have swapped the left and right input signals  but the faulty and good reading stays the same at the output Amp . (when left was bad before it is still left showing bad after swapping input channels)  

Is there anyone who has a hint about the area to look at ?  Are there "usual suspects" for this behavior ? 

Tks & regards, 

Stecknuss

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Dillen replied on Mon, Mar 18 2019 1:29 PM

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Are all plugs and connectors correctly seated?

Martin

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stecknuss replied on Mon, Mar 18 2019 3:11 PM

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Hello Martin, 

.. and thanks first of all for the cap kit being complete in all ratings. 

Plugs / connectors seated correctly ?  Well , I'm tempted to say "for sure" but one never knows really...  I just had several rounds of  "Output Amp" -board in and out so that likelihood is not too high that connectors were wrong in all these runs. 

I have also tried to figure out if the "remote side" of connectors is ok  .  e.g. the DC offset of both channels gave strange values  when the audio signal input  connectors were not connected to the Output Amp  - this is why I then tried to swap these audio signal inputs (changing left and right line coming from "Tone-control board 03") .   But result was as described :  still the same channel on output Amp board showed the 450mV reading.

I am now confused where to start further search... and appreciate any hint of further logical sequence.. 

Tks 

Stecknuss

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Dillen replied on Mon, Mar 18 2019 9:37 PM

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Check connector P14 at the tonecontrol.
It handles the ground connections to the amplifier sections and it's easily pulled unnoticed.

Else - most likely a problem with one or both of the small signal transistors in the differential amplifier circuit at the amps input.

Martin

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stecknuss replied on Wed, Mar 20 2019 8:50 AM

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Hello Martin,

Thanks.  P14 should not be the reason -  as mentioned I had swapped the corresponding input connections  (P31 and P32) at the "output amp". 

I had measured some voltages in the meantime  and in the area  of  one of the power Transistors (IC204 connection to D205) the base voltage seems too high (-2.4 V iso -1.1V)  . Does this point to the same direction as you mentioned  and which  transistors you were referring to ? (can I post a snapshot of the  diagram here to simplify your life  / avoid search on your side ) ? 

Regards,

Stecknuss