Ampeg BA115 - Max VDC on Speaker Out?

[Jecarroll7]

Amp was blowing fuses, I replace the common parts (Q1, Q2, Q4, Q5,) and all 4 Zener Diodes. I have not replaced U3 - 5532.

Connected it to my current limiter with a 40w appliance bulb, flash then dimly lit. No smoke. The bias was 9.1MV drop across R61, did fine adjustments with amp off until I got to 5mv.

I still have it connected to my CL. My question is I'm reading 29mvdc RMS at the speaker out connection (no load).

Is this too much? If so, any thoughts on likely suspects?

[DrGonz78]

After going through this on another project I was recently told by MarkusBass that DC offset in the range of 30-50mv DC is not a problem. Obviously, I guess you can tinker with this to get it better and better. Or it might get worse and worse if you tinker in a bad way...

So this should be good and time to get it off the current limiter to do more DC offset stability testing, then add a load to test that too. Why not right?

Also, I have question for you... Jecarroll7 when you set the bias on this amp how did you connect multimeter...? What side of R61 do you test voltage? And it just goes to chassis ground for readings right? I ask because just acquired this exact amp and the Mosfets are shorted and I plan of replacing the drivers too. Let me know which side of R61 you tested on. Thanks.

Edit: Just had to add that maybe you should be using a 100 watt bulb to test the DC offset on the speakers before going to full voltage. Just as one more step up between light bulb tester to full mains voltage.

[Jecarroll7]

DrGonz78,

Thanks for your help. I agree, will remove from the limiter later today and test with no load.

I believe the bias reading is taken as a voltage drop across R61, at least that's how I did it. The leads of R61 were covered with silicone, I cut away enough from both sides of the resistor to get my DVM leads in there.

The bias pot is ultra sensitive. Take your time and make tiny adjustments with the power off as some have reported blown outputs due to live adjustment. Clockwise will increase the voltage.

[DrGonz78]

Jecarroll7>>
Thanks for the tip on where to place the leads for setting the bias much appreciated!!

[DrGonz78]

The bias pot is ultra sensitive. Take your time and make tiny adjustments with the power off as some have reported blown outputs due to live adjustment. Clockwise will increase the voltage.

Just wanted to let anyone know that it is Clockwise to increase resistance and thus DECREASES voltage. Just in case this thread is used to help out other people in search of this topic. Thanks again for the help Jecarroll7 it was much appreciated.