Seventh Circle Audio

It looks like the last two N72s that I received last week are the old PCBs (with C5,C22,CC7,C28,C31,C32 and J2 on the corner). I built these up with the only assembly sheets that I can find and they don't work. I checked the schematics and the input transformer wiring looks completely different. I am about to start desoldering and moving the transformer wiring (basically reverse engineering). This is a PITA.

Does anyone have older assembly instructions? Older schematics?

Why are they shipping old revision PCBs with new instructions?

UPDATE: 10/19/2010
These are not old revision PCBs, they are the latest. I was just not matching them up in the assembly photos, so I was confused. Tim cleared this up for me as usual.

So , I buzzed out the PCB and found this so far. It looks like T1 BLU 5 and T1 VIO 8 are reversed on the PCB I am using. This would just flip the phase, but it is different from my first couple of N72s I built 4 months ago (at least according to the schematic). This doesn't explain why I am not getting a very clean signal out. It's down, i am guessing, 60-70 db compared to my other N72s. The signal is weak with a lot of noise. I am using a 1khz tone plug on the input. My previously built N72s check out fine. I set the V at R40 to 3.75 (it's screened on the board there "3.75V"). I am getting 22.7V at Q6 and 24V at C16. Unfortunately my scope is out on loan right now, so i can't really check PPVs.

Oh, I tried the input impedance at both 1200 and 300. No difference.

You need to stop what you're doing before you screw up the PC board. The optional transformer has been removed and optional RF filter caps added, but the circuit is exactly the same otherwise. If your modules aren't passing a signal, the first thing to do is verify your transistor voltages are correct. Then, measure DC resistance at between pins 2 and 3 at the input and output XLR. Let us know what you find.

Which transformer is optional, the Cinemag?
Is an LM317 compatible either LT1086?
The schematic shows LT1086 and the BOM shows (and you supplied) a LM317. Just curious on that one. I checked the BCE voltages, but didn't write them down. I will do it gain and document them for next post.

CBE respective

Q1 - 3.98, 2.54, 1.97
Q2 - 13.20, 3.98, 3.37
Q3 - 22.02, 13.23, 12.58
Q4 - 4.86, .89, .34
Q5 - 22.73, 4.86, 4.29
Q6 - 22.73, 4.28, 3.77

Dc Resistance
XLR IN: 39.2
XLR OUT: 47.9

tpryan \$m[1]:You need to stop what you're doing before you screw up the PC board. .

With all due respect, I am just trying to troubleshoot my problems. The pictures of the PCB in your assembly instructions don't match the PCBs I recently received. I have built 2 N72s for myself previously and 6 for a fellow engineer with not one single problem. I don't see how taking an ohmmeter and following traces that are visually hidden beneath components would "screw up the PC board". That said, I trust the quality of your engineering immensely. I just am seeing some discrepancies on the documentation side at the moment. Your pre-amps speak for themselves sonically...

We just don't need another person jumping to wrong conclusions and ripping parts out of the board for no good reason. Your voltages look OK as do the resistances. Are you absolutely sure the cabling between preamp output and monitor input is good? You can relax regarding the rev 3.2 circuit boards (newer, not older). We just finished another nine N72s today and they all work fine.

Getting a scope tonight or tomorrow. Will start looking at the transformers for a start and get back to you.

Ok. Got a scope. First, I checked a known good unit. With a .177 Vrms 1kHz tone on the input I got these readings:

Known good unit (1200 ohm input impedance)
T1 - Pin 2 - .50 Vpp clean sine wave
T1 - Pin 7 - .35 Vpp clean sine wave

Suspect unit (1200 ohm input impedance):
T1 - Pin 2 - .51 Vpp clean sine wave
T1 - Pin 7 - dead (some inherent noise)

I checked the DC resistance between T1 pins 10 & 7 (xfmr on board):

Known good:
4k ohms

Suspect unit:
immeasurable (open?)

Should I remove the xfmr and check it. If I do what is the preferred out-of-circuit testing method?

You have soldered a jumper between pins 8 and 9, right?

Got them both fixed. I did have the T1 Pin 8-9 jumpers. On one I re-flowed all of the pads on SW1 and it started singing. That was a first.

The second was a little trickier and I finally ended up removing the input transformer. It tested good, so I removed the leads and started over with new leads, new solder and it worked. It was probably pin 8-9 with a bad solder joint, but I couldn't get to it mounted on the PCB. I guess I could have just added another jumper, but I wanted to do it cleanly.

Thanks for your help. I guess I was due for a hiccup. Glad I could figure it out though. I am 8 for 10 out-of-the-panavise now for N72s. Just had a couple of zombies on these last two. BTW, just purchased another chassis and 4 more modules on eBay and will be building up an A12 array next...

i'm putting together a 3.2 N72 and i have the same Q1-Q6 values listed earlier, but my resistance on my XLRs is way off.

the XLR input reads 12.4 Ohms
The XLR output has no connection between 2 and 3

my jumper between pins 8 and 9 is solid