OSA MP-1A OPAMP

I have a OSA MP-1A thats making a static noise intermentently. the opamp I'm sure is the problem (bulging out the side). can I just swap out with a api 2520 or is the OSA opamp different? are schematics avalible? 

Is it the same foot print as the 2520? pin out?

yes its the same foot print, I have a jensen je110k on the input end which I beleive is a 1:8 ratio. just wondering if I'm gonna run into any problems! or if any one out there had a similar issue. thanks 3nity. arty

Is it the sea foam green potted opamp? If so, that is nothing more than a 5534 with a pair of output transistors. Pretty much the same as the BB2521 kit that I have. Any 2520 or 1731 will work just fine in there. The bulging look is probably just a piss pour potting job.

great info jeff, the last 2 sessions I had, the mp-1a was working fine with my u87, and then static and poping out of nowhere, looks like I'll be ordering a couple of the gar2520's thanks. arty

the opamp is just black, not the sea foam green, is there a difference between the black and green?

dude24man said:

the opamp is just black, not the sea foam green, is there a difference between the black and green?

Click to expand...

I am not sure but doubt it.

I had one of these or similar on my bench just a few weeks ago.  The amp was showing the same thing.  Looked like the heat was melting whatever it's potted in.  Turns out though that the amp was fine and it was just a bum switch, I think the pad.  Might want to just roll through that mechanical stuff and see if it's ok.

I replaced the osa opamp with a melville ny 2520, the static/poping is gone and is sounding pretty rad! now if I can find another jensen je110k, all will be will!!!

This is an old post, but does anybody has a schematic or OSA MP-1?

Thanks!

kmozart said:

This is an old post, but does anybody has a schematic or OSA MP-1?

Thanks!

Click to expand...

I had to repair one of these OSA 500 series preamps in the past, it had a Melcor 1731 opamp that was making noise, it got fixed when the opamp was replaced.

I don't remember the models now but is the MP-1 the one with Lundahl transformers?

OSA went bust some years ago so is might be difficult to get any schematics, but these circuits are really simple with minimal components so you can easily trace them

Whoops said:

I had to repair one of these OSA 500 series preamps in the past, it had a Melcor 1731 opamp that was making noise, it got fixed when the opamp was replaced.

I don't remember the models now but is the MP-1 the one with Lundahl transformers?

OSA went bust some years ago so is might be difficult to get any schematics, but these circuits are really simple with minimal components so you can easily trace them

Click to expand...

I changed to op amp did not fix it. I’m not that savvy with the electronics, but if I have a schematic I can follow will make it easier for me.

Thank you!

kmozart said:

I changed to op amp did not fix it. I’m not that savvy with the electronics, but if I have a schematic I can follow will make it easier for me.

Thank you!

Click to expand...

Take some pics of the circuit, both sides of pcb and components and show us.

I'm pretty sure that besides the OPAMP the rest of the components are minimal and pretty simple.

Replace all electrolytic caps.
And re-touch all solder joints, static noise can be a colder solder joint or broken joint

Here is the front

I have two bad units, one is not working, the other one has very low out compared to the other units (4 more) I have working. Feeding it with -50db 1k tone, the good ones output +12 db turned all the way up. One of the bad ones outputs only -1 db.

Here is the back:

Exercise all the cheap switches. (pad/phase/phantom/inst/ etc.)

chops said:

Exercise all the cheap switches. (pad/phase/phantom/inst/ etc.)

Click to expand...

Thank you.
I did, measured them with continuity tester, they are ok.

kmozart said:

I have two bad units, one is not working, the other one has very low out compared to the other units (4 more) I have working. Feeding it with -50db 1k tone, the good ones output +12 db turned all the way up. One of the bad ones outputs only -1 db.

Here is the back:

Click to expand...

The soldering job seems a bit dirty and there's also a lot of links there, I don't know if those were to correct some mistakes in the pcb traces.
I would retouch all the solder joints and clean the pcb traces and joints with iso alcohol.
Also replace all electrolytic capacitors there only seems to be 3.
Measure also all diodes with the multimeter to see if any failed.

Besides the opamp itself, Lytic caps and cold solder joints I'm not seeing much that could have failed there

By "opamp" ist probably meant the big green sub-pcb with components on it and not the grey thing with "NE5534N" printed on it.  Changing the electrolytics is wasted time - they are only on the power supply rails and won't cure the problem.  And yes, they are bypassed with styroflex capacitors - how ingenious....

analogguru said:

By "opamp" ist probably meant the big green sub-pcb with components on it

Click to expand...

By Opamp I meant the Discreet opamp, 2520 or  Melcor 1731, not the
IC.

Although changing the IC is cheap and easy, like changing the lytic caps also