Api 512c weird behavior

hi guys,
i seem to have a problem with a mid-2000 api 512c preamp. 

It works but it seems like it's missing some gain,  I cannot get the converters to clip with a dynamic mic, although the preamp is distorting and the meter in the red. The sound is anemic and  lacking bass, high seems semi-normal appart from distortion.

Also very strange, when i press the polarity switch the sound gets louder a little ( maybe 3db ) and the bass seems better.  Clearly night and day.

I searched for this problem online and a few people reported the same behaviour but haven't found a solution online.  Most people say it's a psychoacoustic effect but i can asure you it is not. Clear sound difference.

What could the problem be? Would a deffective 2520 opamp do that?
A bad contact somewhere? The polarity switch is switch a relay, bad relay contacts degrading sound? Maybe a bad capacitor?

Let me know if you've seen similar behavior please. 

Thanks,
John.

take measurements... feed it a fixed signal and measure the output.  take a frequency sweep and sweep the preamp from 20-22K and see what it is doing.

the polarity should have no effect on gain when by itself, but if used with other sources at the same time, it might effect gain because  flipping the polarity may phase correct and issue that would otherwise be canceling out. 

Thanks Pucho, i will try to make measurements with tones tonight to get exact picture.

But the polarity switch clearly affects the sound, frequency and level. 

I'm suspecting the tiny relay (panasonics tn2-12v) might have dirty contacts, because the switch had very low usage.


Is it possible a 2520 opamp only half-fails?  because it still passes signal, just cannot drive very much.

I'm pretty sure this 2520 is from the "bad batch " from the 2000 period cause i had other opamps fail in a 525 from the same period.


Another details, this 512c cannot drive the 600 ohm input of an api 550b eq or la-2a, no sound. Only works straight in the interface with low volume. Not sure if it is normal behavior tho.

none off that is normal behavior.

if you really want to dig in,  the 2520 should be on sockets so you should be able to swap it out with a known good one from another unit to see what is what.

This is a balanced output off a transformer, right? Have you connected both legs of the transformer? Your description is that of one leg being open.

Anything that comes across my bench with a 2520 of *any* vintage or batch the first step is swap the 2520.

Thanks for the advices guys.

i have made progress.  I will write here if people ever need the info.

I swapped opamps, eliminated variables and tested various things.

Good news is the opamp is good and the problem isn't in the 512c.

I swapped the module around the lunchbox and it was working perfectly in another slots.

I haven't tested all the slots yet but the same problem happened in slot 1 and 2.  ( low gain, phase switch affect sound etc). very weird.

Very strange, i'm not familiar with what the lunchbox contains but i'm guessing nothing appart from the psu and traces, contacts etc.

So maybe dirty contacts on the connectors?  I will keep investigating.

Very weird, it works, just attenuated, not "gated" and distorting.

So at least i got more clues now and i will research if some people had that problem.  It is the first 6 chan lunchbox.

john

diggy fresh said:

I swapped the module around the lunchbox and it was working perfectly in another slots.

I haven't tested all the slots yet but the same problem happened in slot 1 and 2.  ( low gain, phase switch affect sound etc). very weird.

Click to expand...

So the problem seems to be related to the 500 series rack.

Well you have input connections and output connections. And then + and - 15V , 0V and +48V
Your problem seems to be related to the output connections, Audio + , Audio - and Audio ground

Check audio cables that connect to slot 1 and 2
clean the rack connector contacts, clean the XLR contacts
Measure continuity between the Edac connector and the XLRs

I had a client recently who had a 512C mic pre that they swore was not working right.  in this case it was not passing phantom. turns out the module was fine and their rack had an issue, a bent pin was not making contact  in the edge card connector to the module preventing it from getting  phantom. was a PIA to replace the edge card.

Thanks guys,

Yes the problem is indeed in the rack.


Swapping around in different slots i noticed the modules don't quite sit well in the back connector, they barely make contact and don't go all the way. The backplane is too far back..

I read on the net it happened to a batch of lunchboxes. It looks like i have been unlucky and bought that stuff when Api had a bad batch of opamps and problems with lunchbox tolerance.

The modules had never moved from their slots, maybe the contacts got dirty for not sitting deep enough and not having enough pressure on them.

I think i read Api had issued a fix with some washers or shims.

I'll try cleaning the contacts for now and i'll look into if it's doable to hack a shim or something.

diggy fresh said:

I think i read Api had issued a fix with some washers or shims.

i'll look into if it's doable to hack a shim or something.

Click to expand...

Pretty sure you can easily fix it that way