AMB Laboratories DIY Audio

[gurubhai]

hi there
i am building a 3channel beta 22. However i am not able to set the bias.Initial readings are..
R9 ----> 4.49V
R10----> 4.59V
R11----->4.49V
R12 -----> 4.59V
these are the other readings i took for the same boards.

Please help

[amb]

Looks like D9 is bad -- it doesn't have 12V across it.

[gurubhai]

The D9 had 2.8V across it so I replaced it but the result is still the same.
I checked the diode with DMM before installing and it was allright( 0.707 & infinity).

[amb]

When you checked the diode with the DMM, you were measuring the forward voltage drop. But this is a zener diode and is used in reverse, which should be 12V. Most DMMs can't measure beyond 3-4V in diode check mode, hence your infinity result.

Are you sure you installed the correct diode? Check the markings on it. What does it say?

[gurubhai]

I am certain that the diode is correct, because all BZX55C12 diodes were taken from the same strip & they are working fine elsewhere & also in sigma22(which is working perfectly).

Edit: I apologise. I checked with a magnifying lens & the markings are TZX12B. It seems that my supplier sent me a replacement part instead of the BZX55C12 that I had ordered.

[amb]

TZX12B is still a 12V 0.5W zener diode... while it's not the same part number as specified, it should still work and have 12V across it. Weird.

[gurubhai]

Any suggestions where to start looking?

Are the input & VAS stages ok? Does the problem lie only in output stage?

[agdr]

Q23 is probably shorted.

Your voltage measurements show that your Q19 current source is producing (3.95V - 2.81V) / 75R = 15.2mA of current. That has to go somewhere and can't flow through D9 if the voltage across it is less than the zener voltage (it is off), and DC current can't flow through a capacitor C8, so the only place left that current can go is through R32. Sure enough, your measurements show (2.81V - 1.66V) / 75R = 15.2mA. And that is going right into the gate of a mosfet, Q23, which shouldn't have any current flow. Also take a look at R32 and see if it looks burned, and measure it once Q23 is out and make sure it is still 75R.

[gurubhai]

I checked the Mosfets for shorts & there are none.
I think I might have switched the readings for Q23 leads(G & S), I shall recheck it after connecting.

R32 is 75 ohm(checked).

[gurubhai]

^Checked it again & the Q23 measurements were correct. I am thinking about replacing it.

I also checked a second board today & that one measured fine.