A12 cinemag potential issue

[gitsnamps]

hello,

i find myself with an a12 that will not pass audio. all voltages work out right according to the lasts few steps of the build.

after going over everything again and then finally reading this post:

http://www.seventhcirclestudios.com/SCA ... 3&start=15

i believe that the cinemag may be the reason for the behavior of the pre. resistance measured between pins 1 and 4, and pins 5 and 8 are as follows:
1 to 4 - 36.4 Ohms
5 to 8 - 77.9KOhms

the 5 to 8 on T1 reading is the only reading that doesn't match your troubleshooting suggestions. (post was from 2009, maybe different board rev.)

i have another working a12 that i assembled that reads all the proper troubleshooting voltages. again, according to the post linked above.

anyways, any help is appreciated. these kits are great. this is the first snag i've ran into.

thanks,

joe santo

[gitsnamps]

i figured i'd up date the thread.

i have contacted Tim via email. this is where i am at so far:


From Tim:
Does one op-amp get noticeably warmer than the other?

What happens if you swap op-amps between modules?

Best regards,

Tim Ryan


Me:
Tim,

no abnormal temperatures with either op amp inserted into either pre. i tested all combinations for about 30 mins. no heat difference with op amps.

Joe


Tim:
OK

On the bad module, do you hear increasing noise with increasing gain? Does it hum or hiss?

What resistance do you measure between pins 2 and 3 of the input XLR?

Best regards,

Tim Ryan


Me:
ok, so some new results.

i'm getting 36 OHMs at pins 2 and 3 on input XLR.

i am getting hissing while turning up gain stages. mainly past 6.

i actually am passing audio on the 12th gain stage. i'm using an octava 012 condenser. it's very low. it also has some distorting sounds when speaking into the mic.

thank tim for responding and working with me directly. this is my 6th module, and this has been my only snag. i have 2 n72, 2 c84 and, now, 2 a12. almost fulllllll!!!

happy holidays.

joe

[kdg665]

I am having an issue just like this. This is my 5th SCA build (all successful), this time around I am doing two A12's. The first one built fine and had no issue, sounds awesome. The second one however just barely passes audio and only with the gain turned all the way up. Unit passes all calibration tests in the build doc.

I see the same behavior as gitsnamps on the non functioning unit,
Resistance between XLR_IN Pin 2 to Pin 3 is 36.6
Resistance between T1 pin 1 and pin 4 is 36.3
Resistance between T1 pin 5 and pin 8 is 73.5K

On the functioning unit, the same measurement
Resistance between XLR_IN Pin 2 to Pin 3 is 36.4
Resistance between T1 pin 1 and pin 4 is 36.0
Resistance between T1 pin 5 and pin 8 is 2.94K

[tpryan]

It sounds like an open secondary. Send an e-mail to rma@seventhcircleaudio.com for return instructions.