A12 Problems

[JackofallTrades]

Hello All,

I've just finished my A12 build and I am having a problem. I don't get any output signal when using a condenser or dynamic mic. Also, the leds on my condenser mic don't light up. Both mics work fine on my N72, J99, and C84.

Here are my troubleshooting steps thus far:

1. Doublechecked component locations and soldering quality.
2. Doublechecked the DC voltages on the U1 sockets (all ok).
3. Doublechecked jumpers on J3 and J4 (2 and 3 are jumped).
4. Doublechecked Jumper on J5.
5. No jumpers installed on J1, J2 and J6.
6. Verified J7 voltages (all ok).
7. Verified 48 volts present on pins 2 and 3 of Conn 1.
8. 0 Ohm resistors are installed in place of L1 and L2.
9. Verified continuity through the output transformer windings.
10. Verified continuity between pins 2 and 3 of conn 2 with phase switch in both positions.


Anyone have any ideas?

Thanks!

[tele_player]

What's the resistance between pins 1 and 4 on the input transformer? Between pins 2 and 3? What's the part number on the input transformer?

[JackofallTrades]

Thanks for the reply.

Pin 1 to 4 is 35.7 Ohms.
Pin 2 to 3 is Open.

The transformer is a Cinemag CMMI-8PCA.

Thanks,
Mike

[tele_player]

I'm intrigued by the phantom power. You've got 48v, but no LEDS lit on the mic? Try jumpering 1-2 on J3 to see if this changes.
You've got to make sure the pots get grounded to the chassis through the front panel holes. Scrape away some paint there.

[JackofallTrades]

OK. My A12 works now, but I'm not really sure how I fixed it.

I moved the J3 jumper to pins 1 and 2. I tried this and everything worked. My mic LEDs light up, and the preamp worked.

I moved the J3 jumper back to pins 2 and 3. I tried this, and everything works! I'm not sure if there was a bad conection on the jumper, but I've never seen that before.

On a side note, I had reamed the holes in the front cover with a round file during inital assembly so there should have always been a good ground there.

Thanks for your exellent advice!

Mike

[tele_player]

The 'no LEDS' symptom, along with the known presence of +48, suggested there wasn't a path to ground for the phantom power. I don't know if that should matter to a dynamic mic connected to pins 2 and 3 of the XLR - but I'm glad it's working now.

[tpryan]

You may have an intermittent connection in the power connector. Pin 6 is CGND, an open on that pin could cause the symptom you're seeing when J3 is jumpered across pins 2 and 3. Make sure your crimps are tight and the terminals are properly seated in the housing.