Repairing a Speakercraft MZC-66

[johnhoef]

Macattak, I hope you are still with us here. It's been a while! I'm dealing with the exact same issue you dealt with. In my case, two of the amp boards have two caps burned,  literally. They are C6 and C7. These are small SMD style low value ceramic caps. I have no idea of their value. Also, the circuit board traces are burned. A schematic would be VERY useful in determining how to repair this. Would you happen to have a schematic you can share with me? Or at a minimum, do you know the value of these two caps? Here's a photo:

[johnhoef]

I did some poking round with my meter, as well as looking around with a strong magnifier. It looks like C6 and C7 are two capacitors connected in series,  across the plus and minus 27 volt rail. Measuring the capacitance across these two pins, I find roughly the same readings on one of the good boards, compared to the damaged board, with the two capacitors removed. I'll guess these two caps are a very small value, and being across the two rails, probably not that important if I make a guess as to their value. Probably in the double digit picofarad range...say, 20pf?

[johnhoef]

I looked up the spec sheet for the amp chip. They have an example circuit with 100nf caps across the power supply,  so looks like these tiny caps could be .1mfd each?

[PA0PBZ]

I looked up the spec sheet for the amp chip. They have an example circuit with 100nf caps across the power supply,  so looks like these tiny caps could be .1mfd each?

Could very well be the case, and it will not hurt to try. Still wondering how they were destroyed though, is your supply rail still in tolerance?

[johnhoef]

What happened was one or more of the capacitors actually shorted causing a high current flow on the power supply traces. Those tiny caps were simply in the way, and burned up along with the burning traces! Recapping solved all problems with all six power amps. So, I strongly advise a total recap of these power amps! The .1mfd caps work fine as replacement of the burned caps, and as advised in the spec sheet, .1mfd is the correct value for these caps. I also replaced a 450mfd bulging cap on the power supply board. What a hassle.....I had to dismount the power transformer to gain access to the bottom of that board! I have the unit reassembled, but have no way to test the entire unit, so hoping all will work well, once the customer installs it.

[johnhoef]

Just thought I would add a few more suggestions for any of you that are planning on performing this repair. The power amps will not work unless there is 5volts on pin 6 of the amp chip. This is the "mode". As I recall, it's the red wire of the 5 wire connector on each board. I rigged a two resistor network, and connected it across the +28 volt supply. Then just used a clip lead to connect the resulting 5v to the mode pin on each board. Do unplug the 6 conductor cable that supplies the mode logic so you're not feeding this 5v into the logic circuitry!!
  According to the amp chip specs, the "osc", pin 7 needs to be dealt with. They recommend using a 30k resistor connected from pin 7 to the negative rail. Speakercraft left this pin floating! So, I also left it floating, and the power amps worked fine that way. But, in  my opinion, this is poor design, not to follow the manufacturer recommendations!! The thought occurred to me to add these resistors, but decided to leave it alone.

[MacAttak]

Glad you solved your problem, and sorry I couldn't assist.

I dismantled my whole workshop a few years ago (gave all my gear away to a local makerspace). I just didn't have room for it anymore after moving to a new place, and my eyes aren't as good as they used to be. So I decided to hang up my hat on electronics.

I also left that Speakercraft amp in the home that I sold, so don't have it (or the specs) any longer to refer to.