Seventh Circle Audio

by **Nu-tra** » Sun Jun 29, 2008 9:53 pm

I have completed an A12. It is the first time the phantom power has not worked. This is a newer revision. Rev 1.6.1

All the other a12's I have built are older revs

A dynamic works fine but no juice for condencers any ideas where to start?

Thanks

by **tpryan** » Mon Jun 30, 2008 8:09 am

What voltage do you measure at (either end of) R33?

by **Nu-tra** » Mon Jun 30, 2008 11:29 am

47.4 volts on R33. I checked the legs on the transistors and the right leg on Q1 shows 46.6 but nothing on the other legs and nothing on Q2. You think those are bad? R35 is showing 0.642v

by **Nu-tra** » Mon Jun 30, 2008 12:02 pm

Nu-tra \$m[1]:47.4 volts on R33. I checked the legs on the transistors and the right leg on Q1 shows 46.6 but nothing on the other legs and nothing much on Q2. You think those are bad? R35 is showing 0.642v

by **tpryan** » Tue Jul 01, 2008 8:57 am

What voltages do you measure at R35 (both ends) with the phantom switch ON?

What voltages do you measure at R35 (both ends) with the phantom switch OFF?

by **Nu-tra** » Tue Jul 01, 2008 9:46 am

Ok so on r35 I get 46.6v on one side and 0.642 on the other with the phantom on

I get zeros on both sides with the phantom off.

by **tpryan** » Tue Jul 01, 2008 10:10 am

Are you sure you have an MPSA56 at Q2? If you do, there's something wrong with it.

by **Nu-tra** » Tue Jul 01, 2008 1:00 pm

Positive. What should I do? Order it? So Q1 sounds like it's ok? One leg on q1 has 47v when I check it.

by **tpryan** » Tue Jul 01, 2008 4:32 pm

What are all the voltages at Q1 with phantom switch ON?

by **Nu-tra** » Tue Jul 01, 2008 6:17 pm

if you are looking at the flat area where the part number is on the transistor in Q1 the right leg is at 46.6 the middle leg is 0.642 and the left leg shows nothing.

by **Nu-tra** » Thu Jul 03, 2008 8:31 am

I tried ordering there part and there is a high minimum $$$. Can you sell me one unless you think I need a Q1 as well but it shows 46.6 volts on one leg.

by **tpryan** » Thu Jul 03, 2008 9:18 am

Try snipping the C leg of Q2. If it's not clear which leg that is, here's the datasheet: http://www.fairchildsemi.com/ds/MP/MPSA56.pdf Let us know what happens.

by **Nu-tra** » Thu Jul 03, 2008 11:49 am

I snipped C leg and all is well.

by **Nu-tra** » Thu Jul 03, 2008 11:52 am

Is that it? I don't to replace q2 do I?

by **tpryan** » Thu Jul 03, 2008 1:36 pm

Q2 protects the BE junction of Q1 from being reverse biased and holds the phantom output close to ground when phantom is off. Leaving it out makes the design less robust.

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