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I have repaired a few Beomaster 8000. In some of them when I set the bias to 18mV it will raise a few mV after some minutes. The 100ohm potentiometers on the amplifier boards are exchanged. So are the capacitors on the boards.
Any ideas to reduce the drift? Currently I adjust them a bit lower to compansate. I have tried to replace the involved transistors without success.
A little drift upwards within the first minutes is normal, as the components warm up.
That's why you set the idle at power up, keep an eye on it during the first 4-5 minutes and correct it if it drifts.
After this couple of minutes it will usually have settled and you can do the final check and it will be set correctly.
- Yes, it will then start up a little low at each "cold" power up, but after a few minutes it will again have reached the correct idle (and stay there if everything is fine), and that's how it should be.
If it keeps drifting slowly higher and higher, even after a few minutes, it could be a sign of bad thermal contact from the output stage transistors (and temp. sensing component) to the cooling fin (never run without cooling fins fitted), a leaking driver or temp. sensing transistor or some other problem related to the constant current generator.
If it runs completely astray, power down immediately and diagnose.
Martin
Thank you for your answer. Then it is no worry🤗.
I think it could have to do with the temperature on BF857 which gets quite hot.
BF857 which gets quite hot.
That's also normal.
Martin
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