phantom problems with api312

So, yesterday i had two channels of perfectly working fabio bauman api312.

Then, after trying out some different melcor mods, and screwing up, I dont anymore.
Please help me!

I put the cards back in the box, and connected all the wires from the psu.
Fired up both channels to test the mods. (i dont have the millmax pins, so i have to desolder the opamps, and take the whole card out of the box everytime i change anything).


Then I fired them up, and soon noticed that one of the wires from the psu had jumped out on one of the channels (i think it was +48v). Then the wire jumped even further by itself and touched something it shouldnt have, and i think there was some kind of short. I even heard a loud pop.

Now one channel doesnt work like it should.

When i connect a mxl603s condenser mic and turn on the phantom, i get signal, but there is alot of noise (alot). A "Hiss" like noise.

I think its some issue with the phantom power because:

Dynamic mics still work fine with the preamp.

And when i disable the phantom power switch, the condenser mics work fine for a couple of seconds (no noise), until the power runs out or so to say.

Could it be that i blew the phantom power electrolytic (designated CPP1 on fabios pcbs)?

The other channel works just fine btw.
Another strange thing is that my behringer ecm8000 electrets work just fine.

Please tell me i only blew the electrolytic cap, and not something expensive!! :green:

Thank you for any help solving this problem.
/Jonas

ok, since noone would help me i took matters in my own hands...which is probably for the best anway, might learn something from it even!

I found some problems in my psu,
mainly that the phantom power voltage is about 76!!! volts... A bit high ehh?

Plus, one of the caps for the +5 supply exploded, but i replaced it, and now it seems to work fine.

So im gonna get a new lm317t regulator tomorrow, and see if that will solve everything.

Still doesnt explain why one of the 312 channels worked fine, when the other didnt though.

Any ideas?

/Jonas

Hi Jonas

I found some problems in my psu, mainly that the phantom power voltage is about 76!!! volts... A bit high ehh?

Click to expand...

Sounds like a shorted 48v regulator. Output shorted to input.

Still doesnt explain why one of the 312 channels worked fine, when the other didnt though.

Click to expand...

The 312's have diodes on the supply rails, so the other opamp would have been protected.

Sounds like one (or more) of your transistors has blown if you heard a pop...

Peter

Thanks!

would the damaged channel still be able to pass a good signal from a dynamic mic if one of the transistors had been blown? Cause thats what it did when i last tested it...

(And condenser mics still worked "ok", only very noisy).

Perhaps i should test it with a chip opamp, and see if everything works ok after i get the psu up and running again?

/Jonas

I fixed it!

Replaced the broken lm317t regulator, and changed the 240r resistor for the phantom power to 220r (to get 48v, i got 44 before). Now the psu is working fine again.

Now the noise was gone in the preamp, but the transients sounded wierd and distorted. Almost no headroom in the pre.

Tested with a chip opamp to see if everything else was ok in the pre... Everything was working fine.

So I remebered that ive had the same problem before with the melcor opamp...

So i did the same thing as last time and replaced the bd136 transistor (the same component that burned up just shortly after the problems started appearing the first time), and then everything started working again!
Great!

Btw, i also compared the chipopamp (OPA604), to the melcor. And i definately preferred the melcor. More character, and to my ears more depth.
The OPA604 sounded pretty good also though, a bit "flatter" if you will.

/Jonas

Hey Jonkan, noticed you had a post up about a 48v problem you had with a fabio psu and just thought you might be bale to help me as im having the same problem.. Im getting 68v across my phantom power.Would i be right saying that the lm317 reg is meant to be on the positive rail and the 48v and the lm337 on the negative? I think this is what might be causing my problem but im not sure..

Cheers

Sounds like a shorted 317 regulator in the 48V section.

The 48V is taken from both secondary windings over two capacitors (C11, C12),
rectified and the negative rail is connected to the 0 of the bipolar part.