Sunn SR-8200 Powered Mixer Power Amp Repair

[telecaster]

Previously repair attempt by someone else did not work, they fixed some burnt resistors, circuit traces and replaced the power transistors (schematic attached). Came in with some shorted transistors and blowing one of the 10A fuses. I found that the NPN and PNP transistors were in the wrong place (swapped) - Q18,Q17,Q19 and Q10,Q9,Q8. Also Q13 was shorted. I replaced Q18,Q17,Q19 with MJ15025G and Q10,Q9,Q8 with MJ15024G. I didn't have a direct replacement for Q13 (2N4250) - I replaced it with an ECG294 which exceeded all specs except hfe which was 120 min vs 2n4250's 250 min. I had to transpose the leads to correctly connect things. Brought up ok on light limiter without a load - no shorts, no DC on output. Hooked an AC ammeter up and took it off light limiter, still no load. Power up, draws about 3A, no DC on output, but heat sink and power transistors get hot to the touch fairly quick. I measure 480mV DC across R18, for a current of 1.45A. My first question - at idle what should the current be? I am assuming a couple milliamps through each emitter resistor. Adjusting the trimpot has very minor affect, except when turned all the way counterclockwise - then the heatsink and power transistors stay cool and there is only 0.5mV across R18. Question 2 - does this mean my substitute transistor is not functioning properly (turning power transistors on too hard, e.g.)? Question 3 - what would you do next, try a better sub for Q13, for example PN4250A? Any help would be appreciated.

[telecaster]

Power Amp schematic
sunn-sr-series-power-amp-schematic.pdf

[The Dude]

Here's a link to an amp manual that looks to have the same power amp circuit. It details the bias procedure.

https://www.prowessamplifiers.com/sc...00_11_x_17.pdf

[telecaster]

Thanks The Dude - that bias procedure answers my first question, .5-.7mV DC across R18 (0.33 ohms) yields 1.5-2mA. The only bias setting that keeps the outputs from overheating is fully clockwise on R42 - tomorrow I'll measure the voltage across R18 at that setting.

[telecaster]

Tried to adjust bias R42 with no luck. Full CCW keeps the outputs and heatsink cool, but there is no voltage across emitter resistors, i.e. no current flow. Turning R42 up CW jumps the emitter resistor voltage to 300-400mV throughout the rest of the sweep and everything gets hot. The AC mains current jumps up similarly with the emitter resistor current. My theory is Q13 is not working properly, do I replace it with a better hfe match?

[The Dude]

Have you checked R42?

[loudthud]

It seems from your description that Q13's collector is not connected to the circuit or Q13 has failed. A base-collector, base-emitter diode test should show the fault.

[telecaster]

The Dude - I checked R42, I believe it's OK. It measures 3.7K top to wiper fully CCW and tapers smoothly down to 26 ohms fully CW, bottom to wiper always 26 ohms. There are a number of other resistors essentially in parallel.
loud thud - I verified all Q13 connections were made and correct, and I successfully diode checked it with my multimeter.

I did find that R16, originally 220 ohms 5%, had been replaced with a 240 ohm resistor which is ~10% off in value. Not sure if this is related but I'm going to change back to 220 ohms.

[x-pro]

If R42 and Q13 and all the resistors around it are good and you still can't adjust the quiescent current level smoothly, you need to change the circuit a bit. Thereby improving it. It will also have protection against accidental loss of contact in the slider resistor R42, which will save the output transistors from high current and their failure.

If even in this case there is no smooth Bias change, R40 should be of a smaller nominal value, by a factor of 2. In other words, a resistor selection is required.