AMB Laboratories DIY Audio

[Yoga Flame]

Finished the M^3 assembly and passed steps 1 through 6**. Using a regulated supply at 23.6VDC, which I had previously been using with my Mini^3.

I'm now at step 7: "Measure the DC voltage across the R5G resistor. Adjust the BIASOPG trimpot until you get around 0.4V to 0.5V."

I get a reading of 0.7V no matter how far I turn the BIASOPG in either direction. Any suggestions on what I got wrong?
Thanks!




** In step 2, "there should be no shorts". Is it bad that my resistance reading between opamp socket pins 4 & 7 is about 620Ω / 120kΩ (depending on which way they are measured)? Or am I just making sure that the resistance is not 0?

[amb]

I'm now at step 7: "Measure the DC voltage across the R5G resistor. Adjust the BIASOPG trimpot until you get around 0.4V to 0.5V."

I get a reading of 0.7V no matter how far I turn the BIASOPG in either direction. Any suggestions on what I got wrong?

Are you measuring across R5G as indicated, or did you measure relative to IG? Is the BIASOP trimpot the correct value and installed in the correct orientation? The 2N5486 JFETs are in the right way?

** In step 2, "there should be no shorts". Is it bad that my resistance reading between opamp socket pins 4 & 7 is about 620Ω / 120kΩ (depending on which way they are measured)? Or am I just making sure that the resistance is not 0?

As long as it's not "low resistance" (i.e., less than tens of ohms) then it's fine.

[Yoga Flame]

Thank you! I now see that all my Q3s are the wrong way around. I hope nothing is fried as a result.

My trimpots are installed the same way as pictured on the M^3 overview page. (http://www.amb.org/audio/mmm/mmm_assy.jpg) But now that you mention it mine do appear to possibly be backwards as well. I'm using this for my BIASOP: http://search.digikey.com/scripts/DkSea ... q=90241422
And the side with the markings is facing away from me currently, such that the pins are 1 2 3 from left to right. I guess I should not have assumed that all trimpots have the shaft on the same side.

[amb]

Thank you! I now see that all my Q3s are the wrong way around. I hope nothing is fried as a result.

Probably not.

My trimpots are installed the same way as pictured on the M^3 overview page. (http://www.amb.org/audio/mmm/mmm_assy.jpg) But now that you mention it mine do appear to possibly be backwards as well. I'm using this for my BIASOP: http://search.digikey.com/scripts/DkSea ... q=90241422
And the side with the markings is facing away from me currently, such that the pins are 1 2 3 from left to right. I guess I should not have assumed that all trimpots have the shaft on the same side.

If your trimpots are mounted as shown in that pic, then it's the correct orientation.

[Yoga Flame]

Thanks again, amb.

I think I've probably destroyed my 2N5486 JFETs while trying to desolder them. Man, that's a tight space. My cutters aren't even small enough to get in and clip the leads. I'll just get some new ones with my next order from Newark.

[Yoga Flame]

Ended up ripping off the JFET heads with my pliers so I could properly desolder the legs. Replaced them with new ones of course.
I am happy to report that everything works now and it sounds amazing. Now for the case work.

[MisterX]

+1 for the blob of solder technique?