Schematic needed for popping H&K zenAmp Combo!

[pacAir]

Well, I spent some "quality time" with the beast over the weekend. The schematics Yorkville provided were for a newer version of a newer head and the layout and design of THIS combo model (and a year or two older) didn't exactly match but seemed to be about 85% similar.

I did find a partially functioning 7915 regulator which I replaced but the problem didn't go away. After isolating sections of the amp I determined that the explosive DC pop was coming out of the DIGITAL board. If you plugged a 1/4" plug into the channel return loop jack (which disconnects the front end and digital board) the rest of the amp was normal. I had previously ruled out the input stage so the only thing left was the digital board... the last place I looked!

I figure the owner has 4 realistic options:

1. Dump the programming and return the amp to factory stock in the hopes that this problem is a memory corruption and reprogramming will get rid of it.
2. Pay big $$ to get a replacement digital board (I have to contact Yorkville about that one to find out how much $$).
3. Look for a basket-case chassis that has a good digital board.
4. Use the amp but ALWAYS turn down the Master volume control to "0" before turning the amp off (the amps seems to otherwise work fine)!

I did not want to list "modification" of the amp for a Turn-Off circuit to add to the MUTE circuitry, but one design difference I found was in the mute circuit. The one in this amp is a turn-on mute (which isn't totally silent) and is pretty basic (3 components besides the actually mute FETs) while the one in the schematic has about 8-10 components including a transistor switch and Zener diode circuit!

Steve

[J M Fahey]

Then *clearly* it's still an unresolved problem, and they hide it with a more elaborate mute. Oh well.

[pacAir]

Then *clearly* it's still an unresolved problem, and they hide it with a more elaborate mute. Oh well.

That's what I thought at first. Since the owner claims it didn't used to do this something else is going on that I am not aware of. The only inexpensive thing to try is to restore the digital board to factory default (or whatever passes for a reset/memory wipe) and see if that fixes it. It is interesting that the mute driver circuit got more complex! Perhaps we discovered the reason why!

Steve

[pacAir]

I got a message from Yorkville (H&K USA Distributor). H&K has the digital board in stock and the end-user price is $950! It is a 2+ month wait if the part hitches ride on the same boat as the next H&K shipment to the USA or $100 to put it on a plane. The owner says he will get in the habit of using the Master volume control as a "Standby" switch. I pointed out that replacing a worn-out Master Volume pot is a lot cheaper than fixing the underlying problem! Dumping the programming wasn't well received either as all the user-programmed presets cannot be downloaded to a computer for archiving.

End of story, amp back in service and customer is happy with the work-around "solution" to his problem.

Thanks for all the suggestions!


Steve