Original api 550 problem

[bieckmusic]

Hello everyone,

I haven't had time for DIY in a while.  But I just bought an original api 550.  It was said to have worked when shipped but is not working.  I had a tech take a 5 min glance at it...  He couldn't see anything, just told me to check a few solder joints.

I'd like to get it working, though I could return it and get my money back...  But I paid 900 for it, and I'd love to fix it.

When I received it the transformer and the main pcb card were loose.  The screws where loose, probably from ups vibrations.

Anyway, I don't get signal, except I get a slight bit of high frequency signal with no low end.  I only get this little signal when the eq is bypassed and the MF know is boosting.  Otherwise the unit is silent, not even a hum.

I checked for voltage on the two doa's (both + and - and com) were ok...

I've tried multiple 500 rack enclosures, etc, checked the cables...

Any suggestions of where to look.  This is a complicated eq.  I've been using a 550 a schematic to check against and it looks pretty similar.

thanks!
Greg

 

[qmp audio]

do you have some known working 2520s to test with?

 

[bieckmusic]

Well I've pulled out the 2520's and they both work fine in my access api 312 pres.

It looks like something is draining current...

I pulled a few suspicious transistors...

Does anyone know the modern equivalents of these transistors:

 

[bieckmusic]

Also this one

 

[Walrus]

The 2N5087 is still available. However, I have been successfully using BC550C and BC560C's to repair 550 EQ's.

 

[bieckmusic]

Hi Kevin, thanks for the response!!

Do you use the bc550c as a straight replacement for the TIS97 without any modifications?

By chance do you have a schematic for the 550 I can't find one anywhere??  I'd like to replace the electrolytic caps and diodes while I'm working on it...

Thanks again for your help!! Greg

 

[Walrus]

Yes, straight replacements, no modifications.
No, I only have various versions of the 550A schematics.

Have you seen this....
http://www.technicalaudio.com/pdf/API_Automated_Processes/API_550_EQ/API_550_Equalizer_Versions.pdf

 

[Whoops]

Can you post more pics of your Unit?

 

[Whoops]

I checked my files and uploaded the 550 docs and schematic to the Technical Docs section,
check it here:
http://groupdiy.com/index.php?topic=44738.20

Do you have a "Signal Probe" ? if not you should make one, it's easy enough:
http://www.diystompboxes.com/pedals/debug.html
http://diy-fever.com/misc/audio-probe/
https://www.youtube.com/watch?v=jWvIfDSxbIk

So what you should do is connect the module to the proper voltages, check if the right voltages are reaching the dedicated pins on the 2520.
Then inject signal at the input, it could be music from the ipod.
Then following the schematic from the input, probe the circuit while listening before and after each component, there will be a point where you will not have any signal, then thats the component or part of the circuit to suspect.

Do this before blindly starting to replace original transistors or parts.

 

[bieckmusic]

Hi yes thanks for the tip, already done that I have a 500 test rig and a vintage signal tracer that's very cool.  I get audio at the input of the first 2520 and nothing on the output side of it.  Something is shorted and pulling down the voltage on the 2520 I get +16 but the -16 is way off...
Checked the diodes and the transistors with a diode checker.  These two transistors measure off.

Both 2520s work great I've tested them

 

[bieckmusic]

Thank you so much for posting the schematic, appreciate it!!!

 

[Whoops]

Hi yes thanks for the tip, already done that I have a 500 test rig and a vintage signal tracer that's very cool.  I get audio at the input of the first 2520 and nothing on the output side of it.  Something is shorted and pulling down the voltage on the 2520 I get +16 but the -16 is way off...
Checked the diodes and the transistors with a diode checker.  These two transistors measure off.

Both 2520s work great I've tested them

Do you have general purpose transistors you could quickly replace for the 2 you are measuring off?
even if you dont have the perfect replacements you can use them for the sake of testing

 

[Walrus]

Whoops, according to the Paul Wolff/John Klett data I linked to, the 550 original only had 3 frequency selections on the top and low bands, and 5 in the mid.
However the schematic you posted has 5 bands on top and low, so therefore is a 550A isn't it?

 

[bieckmusic]

I think your correct Walrus, that document is titled 550a on the front page.  I've searched pretty hard and checked with some techs I know and can't track down an original Api 550 schematic anywhere.

It is an amazing sounding eq, I just used 3 of them on a session.  Can't wait to figure mine out... 

 

[bieckmusic]

Ok, everything is working perfectly.  There was a short in the transistor section.  Man this thing sounds incredible... Everyone thanks for the help