new a12

[art]

I have a a12 I just built and am at step 21 in the doc. I have the
chassis ch01 with the ps02, voltages on ps02 j2 are:

pin 1 +27.06 vdc
pin 3 -27.12 vdc
pin 5 +50.90 vdc

pin 5 seems a bit hot as the doc states it should be +48 vdc.
a12 step 21 in the doc I measured u1 pin 4 gnd to pin 3 and
then pin 4 gnd to pin 5. I get the following:

pin 3 +19.79 vdc
pin 5 - 19.73 vdc

again, the doc says it should be 14 vdc and -14 vdc respectively.

is this OK or do I have a problem?

lastly I have the sc25 op amp the a12 doc step 22 says to adjust
r25 and r29 for the proper working voltage for the op amp you
intend to use. what is the optimal voltage for the sc25 ?

do I just turn each pot slowly till I get the right voltage, is
one pot for + and one for - voltages?

[tpryan]

It sounds like you're doing everything right. Answers below:

I have a a12 I just built and am at step 21 in the doc. I have the
chassis ch01 with the ps02, voltages on ps02 j2 are:

pin 1 +27.06 vdc
pin 3 -27.12 vdc
pin 5 +50.90 vdc

pin 5 seems a bit hot as the doc states it should be +48 vdc.

The phantom power spec is 48V +/-4V, so you're fine.

a12 step 21 in the doc I measured u1 pin 4 gnd to pin 3 and
then pin 4 gnd to pin 3. I get the following:

pin 3 +19.79 vdc
pin 5 - 19.73 vdc

again, the doc says it should be 14 vdc and -14 vdc respectively.

is this OK or do I have a problem?

No problem, the minimum voltage is about +/-14V. You must have skipped step 20.

lastly I have the sc25 op amp the a12 doc step 22 says to adjust
r25 and r29 pro the proper working voltage for the op amp you
intend to use. what is the optimal voltage for the sc25 ?

+/-18V to +/-20V is optimal.

do I just turn each pot slowly till I get the right voltage, is
one pot for + and one for - voltages?

Yep. R25 adjust the positive voltage, R29 adjusts the negative voltage.

[tele_player]

Did you build the power supply? I'd check the resistors on the +48 regulator, R10 should be 8.87K, R11 should be 237 ohms. 50 probably won't cause a problem, but it's really easy to just get it right.

Yes, you adjust R25 and R29 to set the + and - voltages at the op amp. I use 18v with the SC25.

[art]

Allrighty then, I set the voltages to +18.0 vdc and -18.0 vdc exactly
using r25 and r29 ...

thanks fer the help!! now to cal the next a12 then the 2 n72's
with the scope. i'm sure i'll be posting again when I test the
n72's...

[art]

I did not build the ps02, it came already installed in the ch01.
All I did was finish wiring it up to the switch, light etc...

if 50.90 vdc on pin 5 is ok then I'm not going to fuss with it
unless it is super easy to adjust (meaning no parts removal)

it was hard enough to get that kep nut tight on the ground lug
with that divider there and the ps02 already installed, I am not
going to take it apart.

[tpryan]

The phantom supply is a little high because there's probably a TL783 regulator fitted instead of a 317HVT. I've used them in the last 20 or so supplies I've built since they're more rugged that the 317 and can withstand being shorted to ground with 70V on the input. Unfortunately, they also have a higher minimum current requirement. The output will settle very near to 48V once a mic is connected.

A better solution is on the new PS02 boards which just started shipping this week. They use an ordinary LM317 with a clamping circuit that protects the regulator if the user accidentally shorts it to ground with a meter probe. I'll post the updated schematics as soon as I can.