A12B Initial Power Up and Testing...

[keevhren]

Hey there!

Just got a pair of SCA A12B's in the mail and was about to install them in my rack to finish out the last two empty slots. I was reading over the A12 assembly guide pdf for the initial power up and testing but seeing as the A12B has the modular Op Amp it seems that I don't have R25 and R29 in the trimmer pot form so was wondering if there's anything I need to adjust upon installing and powering up the a12b?

Also with the following steps, what do I do instead?

Connect the PS03 to J7. Simply wire the power supply connectors together in a 1:1 fashion. That is, PS03 J2, pin 1 to A12 J7, pin 1, pin 2 to pin 2, etc. Pin 1 is toward the front, pin 6 toward the rear. Set your DMM to measure DC voltages of 20V or greater and turn on the power. Connect the negative meter probe to U1, pin 4, and measure the voltage at U1, pin 3. You should see about +14V.
Keep the negative probe at pin 4, and measure the voltage at U1, pin 5. You should see about -14V. If these voltages are way off, you have problems. Possible things to check are incorrectly installed diodes D2 – D7, backwards caps C4, C5, C10 and C13, or shorts around U1, U2, and U3.
With your DMM still set to read DC voltages of 20V or greater, adjust R25 and R29 for the proper working voltage for the op-amp you intend to use. The original API 2520 will be happiest with about +/-16V, though you can get away with a little more. If you’re using an SC25, +/-18V will provide a maximum output level of just about 30dBu.
Remove power and allow the voltages at U1 to drop to 0V. Insert the op-amp in the socket and apply power. With your DMM set to the lowest voltage scale, measure the voltage between U1, pin 4 and the output at pin 6. Adjust R16 until this voltage drops to 0V. Wait for the op-amp to come up to working temperature, about 15 minutes or so, and adjust again. The offset voltage will drift slightly with temperature, but this will not be a problem with the great majority of op-amps.


What is pin 4 on U1? I thought the LM337 (U1) only had the three pins...

Any help would be much appreciated! Thanks for some killer sounding pres. Have a great week!

[tpryan]

There's nothing to adjust - just plug them in.

[keevhren]

thanks!