Zenamp h&k blower output fuses

[georage]

Thanks for letting me in. Im having issues with a hughes and kettner zenamp head. It is blowing the 4 amp fuse that powers the output transistors. Mj11015 and 16. It blew both and has 2 sets as it is a stereo amp. The mj11015 is a darlington pair with bias resistors in a button package. both transistors shorted out on the left and blew the fuse. i replaced the transistors and now the fuse blew again. i have not torn down the amp yet to see which channel has blown this time. any experience with this amp would be appreciated. thanks.

[The Dude]

Here's the service manual. Do not hook up any speakers until the amp is repaired and it is verified that there is no DC on the output. Use or build a light bulb limiter to test for shorts before applying full power to the amp. If the left channel outputs shorted, I'd also be looking at/checking Q11, R22, R39, R38, and R41. You should adjust bias on the repaired channel when repairs are complete and can use the unrepaired channel as a reference for bias voltage.

hughes_kettner_zen_amp_service_manual.pdf

[georage]

Thanks for the manual. That will be a big help. It is not exactly accurate, but its amazing that it even exists.

[georage]

Ok. So i got back into the zenamp head. The left channel has gone bad again and blew the fuse. Exactly the same as when the amp came into the workshop. So it was not just a bad output transistor...too bad. The darlington pair q12 has shorted again from emitter to collector leaving r22 as the only resistance from +33v to output L.

[georage]

I should add that,r41,r39, and r22 all read good and that r38 is a diode.

[georage]

The output transistors on this amp are mj11015 and mj11016. They are a darlington pair. Can anyone speculate on what could cause them to short? i thought maybe that the rail voltage was to high, but the transformwr is putting out a solid 64 volts and 34 from phase to ground. the bridge rectifier tests good and the filter caps arent a dead short.

[Jazz P Bass]

A sure way to 'blow' the outputs is to have them both come on at the same time.

[Enzo]

. Can anyone speculate on what could cause them to short? i thought maybe that the rail voltage was to high,

I can speculate all day. the main power supply really has no way to get high, other than a few volts if the mains are running high. The low voltage supplies can have a regulatot fail and higher voltage hits the preamp.

A zillion things cause output failure. Cracked solder, some problem with the load, including bad cords, speaker wiring, someone plugging the wrong thing into the speaker jack. And yes even bad components.

[georage]

Well. I replaced the 2 darlington output transistors and the one driving them, q11, 12 and 13 and powered up the power board without any preamp boards attached and everything is cool...just like before. I get about 1 volt on the base of q12 and 13 and 45 volts + and - on the collectors and about .05v on the emitters. The transistor that "blew" or shorted last time runs about 1 degree hotter than the others and sometimes gives voltages that are about 10 percent off of being perfectly identical to the other 4 transistors....other than that...it is sitting pretty with 0 volts apparent at the speaker outs and 0 volts between the pairs of emitters.

[georage]

I am about to put it back together and blow it up again......is there anything else i can check before reassembly? I wont be able to get access to the power board once the preamp and other boards are assembled.

[The Dude]

Have you checked bias? If the amp won't bias correctly or is way off, it would indicate there is still a problem with the amp.

[georage]

Idk how to bias a mj11015. 1vdc on the base seems to be normal....but that is without the preamp plugged in

[The Dude]

Since there's no stated bias procedure in the schematic above, I would measure the voltage drop across the emitter resistors on the repaired channel and compare to the unrepaired channel. Adjust to match. It will/should be mV DC. If there is something still amiss in the repaired channel, bias will likely be way off or not adjustable. It's good insurance to check before completely reassembling the amp.

[georage]

Alright, thanks for the tip, dude. I checked the voltage drop across the .47ohm emitter resistors of the good channel, they read .004v each....on the bad channel, they read .003 and .005.....i adjusted a pot on the board and got the bad channel to read .005 on both resistors.

[georage]

Its all back together and it played well with a 8ohm speaker...i didnt try a 4...i just dont want it to pop again.....its just going back home as an idk...kinda sad...anyway....i have one more chance to test things before the chassis goes back in the box....the rail voltage on this 2006 zenamp is 45 volts dc...the 2001 schematic shows 33volts...it may just be that the amp is designed to run on low voltage, because the transformer has taps for 220, 115, and 100 volts. My lines voltage is a good 120-123vac. That could be it...but the data sheet for darlington transistors mj11015 shows 60 volts max collector-emitter voltage...so it should be ok. My running base voltage is 1.1vdc and the transistors get to 113 degrees C during a light volume with stereo music playing. Its sounds great actually but very quiet on 8 ohms.