Hi,
I need some advice regarding repair of a Beomaster 4000 (2406).
It came to me as a botched repair with right channel darlingnon transistors burned out (TR43/46/47). TR44/45/41/42 were also burned out. I replaced many out of spec resistors and all electrolytic capacitors. I used BC114 for TR44 because I could not source a BC113, but it is pretty much the same device with minimal differences in frequency and noise levels.
I started testing with output darlingtons disconnected to ensure all voltages are correct. The left channel would balance out with correct voltages, but the right channel would not – the TR43 would stay on VCC (70V), instead of about half that value.
I noticed that TR44 does not conduct during the power up sequence – the Vbe is about 0.3V at all times. TR41/42 never get to conduct, which in turn never sets correct base voltage for TR43.
I then swapped TR44/45 pair with TR55/56 (left channel) to ensure that the replaced components and board work in the right channel is correct. Sure enough – the voltages on the right channel are now measuring correct, but the left channel now has the same problem.
This lead me to a conclusion that the difference is in BC113 and BC252B pair between two sides. Using a transistor tester I tested all four transistors. The BC113 on the side that works and it tests with a very low Vbe forward voltage (0.2V) compared to the BC114 I purchased (0.66V). This does not make sense since the BC113/BC114 specification sheet shows Vbe of 0.64-7.0V for both.
My question for you is: is there anything special about BC113 transistor used in Beomaster 4000?
I see that service manual shows two revisions of the bias circuit for these two transistors. Is it worth implementing these revisions to fix this problem?
I must say I am a bit perplexed by the complexity Beomaster 4000 feedback loop and the fact that BC113 the working channel does not measure as specified.
Thank you for your help.
Vlad
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