audiolab 8000d bias

anatech · 30th October 2005, 08:34 PM

Hi all,
I am repairing an Audiolab 8000d that has been worked on by another shop. At this point I'm having trouble setting the bias. The Vb-e voltages on the output stage reduce as the voltage increases and is below the value calculated from the current flow through the driver emitter resistors. The protection circuit has had it's components pulled, as well as the reversed biased diodes from output to supplies.

At this point I am working from an 8000a schematic. It's mostly close. I've pulled the TL071 DC servo, it's blown & I don't have another yet. DC offset is running around -60~70 mV. The diff. fet seems be be in good order.

Any hints would be welcome.

-Chris

anatech · 30th October 2005, 09:01 PM

Hi,
I replaced the Motorola MPSA42 / 92 the other shop used with the Zetex equivalents from the protection circuit. The Motorola's when into the old Zetex locations.

At full line voltage I have low bias on that channel - but there is some bias (about 24mA) while the good channel is sitting at about 80mA.

What worries me is that the Vb-e on the repaired side comes up faster than the good side, goes down (good side keeps increasing smoothly) and finally come up to around 24mA. The DC offset without the servo is sitting at -4.2mV with AC line noise on it. (not bad) The good channel is sitting at +4.1mV with just broadband noise (expected).

Anyone familiar with this amp?

-Chris

30th October 2005, 08:34 PM	#1
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	audiolab 8000d bias Hi all, I am repairing an Audiolab 8000d that has been worked on by another shop. At this point I'm having trouble setting the bias. The Vb-e voltages on the output stage reduce as the voltage increases and is below the value calculated from the current flow through the driver emitter resistors. The protection circuit has had it's components pulled, as well as the reversed biased diodes from output to supplies. At this point I am working from an 8000a schematic. It's mostly close. I've pulled the TL071 DC servo, it's blown & I don't have another yet. DC offset is running around -60~70 mV. The diff. fet seems be be in good order. Any hints would be welcome. -Chris

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30th October 2005, 09:01 PM	#2
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	Hi, I replaced the Motorola MPSA42 / 92 the other shop used with the Zetex equivalents from the protection circuit. The Motorola's when into the old Zetex locations. At full line voltage I have low bias on that channel - but there is some bias (about 24mA) while the good channel is sitting at about 80mA. What worries me is that the Vb-e on the repaired side comes up faster than the good side, goes down (good side keeps increasing smoothly) and finally come up to around 24mA. The DC offset without the servo is sitting at -4.2mV with AC line noise on it. (not bad) The good channel is sitting at +4.1mV with just broadband noise (expected). Anyone familiar with this amp? -Chris

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