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Member Since 18 Nov 2009
Offline Last Active Sep 15 2014 06:02 AM

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In Topic: D75 Bias issue

06 September 2014 - 05:51 AM

Thanks to all the responders.

The caps have been replaced. Remember, the amp is putting out 40w per channel into 8 ohms with under 0.01% distortion from 10 Hz to nearly 10 kHz. If anything was open or not working, it wouldn't have such good performance.

I am measuring 5-6 ohms from B-E of each of the four TO-3 transistors. I even pulled Q212 out, cleaned it, checked it separately with an ohm-meter and got the same values as with a new MJ15015. I also measured the resistance across the B-E solder pads on the output board: 5-6 ohms.

I also checked the voltages on the four TIP-47 transistors and the only thing I found was slightly lower base and emitter voltages on the one that drives Q212, which tells me the problem is on the main board. I tried moving backwards into it but it's nearly impossible to locate things on the old board because the parts are oriented in whichever way there was room for them. The newer board has all the parts nicely aligned.

Bob M.