B&K 747 troubleshooting
 

I got one of these beasts. It powered on and the transformer was making a nice chopping style hum and gets hot within a couple minutes. so i figured there was a short somedwhere.

pulled all the electrolytics and ESR, capacitance and leak tested them. they are all fine. Checked all the diodes and found one shorted. So i replaced that diode.

neither of the 2 transistors read shorted.

I checked all the resistors and they appear to be within tolerance.

But the transformer still makes that not so quiet chopping style hum. and gets hot.

if i press test 1 or 2, the needle slams all the way to the other side of the scale. regardless of the tube or sensetivity.

I checked the bias according to the manual, the AC bias is roughly around 1.5v which is fine.

DC bias is supposed to be 2.5VDC, but I am getting roughly -37.2VDC. and the bias pot only moves that voltage around by a few volts.

what gives? thanks.

nevermind, i fixed it. turns out the diode i was using was only rated for 200v. the tranny is putting about 250 to 300v out. whoopsie. sometimes i forget the voltage difference between transistor circuits and tube circuits.