Monday, September 18, 2017

AKG BX20e Fixer Upper

AKG BX20 fun!

I happened across a used AKG BX20e on Reverb a little while ago. The price seemed reasonable and it wasn't too far away. Actually, it was in Echo Park, which is quite excellently ironic as the BX20 is an old school reverb unit from the 70's. :-)

It was being sold as is with only one of the 2 channels functioning correctly. I love a good challenge, especially in the realm of old studio gear, so I paid for it and drove out to pick it up.


Here it is all strapped in and ready to go. Must have been quite a sight going down the freeway sticking way out of my car. 

Anyway, I got it home safely and proceeded to immediately rip it apart.


This is the top of the spring assembly. Those are big ass magnets. Alnico I believe.


Above you can see the gap in the magnets, which is where the spring drive and receive coils are suspended. They are tiny!!
They are also one of the more common failure points on these units, at least according to the research I've done. 


The first several minutes were spent simply trying to wrap my head around what the heck is going on with this crazy contraption.


Here, to the left, you can see the cardboard tube that the spring assembly lives inside.


The foam discs are there to keep the springs from moving around too wildly during transport. The ones on this unit were badly degraded and would crumble away when I touched them.


The springs themselves are crazy things. They are bent and crimped into an irregular shape in order to aid in the diffusion and complexity of the reverb's character.


Here you can see how they are held suspended and actually bent in a manner that allowed AKG to put rather long springs in a relatively small space. There are 2 springs that bend twice each so the unit has 2 channels. They are also designed to each have a different reverb character.


Here's where I was starting the process of sorting out what was wrong with the unit.


Here's what the foam discs looked like after I removed them as well as I could.


We laser cut new foam discs and here you can see them installed.


This is the multi pin connector that connects the reverb tank to the amplifiers. I started looking for continuity between the spring ends and this connector just to rule out bad solder joints or wires. I was crossing my fingers that those tiny coils were all still functional. 

I guess I got lucky, it turns out there were 6 wires that were not cleanly connected to the spring termination points and had failed somewhere between the connector and the spring ends.


So I ran new wire from the connector into the reverb tank.


6 pieces in total.


2 went to the top end and 4 to the bottom end.



Continuity problem solved it was time to hook it all back up and see if it worked.


If you're curious, here's my trouble shooting drawing. Might seem crazy, but it made sense to me.


The power light turns on!!


But oops. Something was hooked up wrong. Not sure it was me or someone before me, but there was a feedback loop so one of the amps was being fed right back into itself. Not good. Burned up a couple of transistors on one of the amps. You can see them here about halfway down the right end of the amp card. 

Here's the amp circuit schematic. I've written in pencil the part numbers for the transistors that needed to be replaced. I found original OEM parts on ebay and ordered them up. 
The parts came in a couple weeks later and I got them installed, reversed the bad connection, and put it all back together. The unit fired up and now it works flawlessly. 

The best part is it sounds amazing!


One last photo. This one shows the signal path for the springs. How the engineers came up with this scheme I'll never know. But it's works wonderfully well and I'm delighted to have such a killer piece of vintage gear now living happily in the corner of my studio.

BTW, if you want a really amazing plugin version of this reverb I can't recommend the Universal Audio BX20 emulation enough. Their plugin is the main reason I ever started looking for a real BX20. I did some A/B testing and the difference is so subtle it's almost insignificant. The plugin is more flexible for sure, but there's just an undeniably serious cool factor in having one of these beasts in the corner, quickly available in my sessions as a plugin insert. Love it!


1 comment:

  1. How cool! I'm the guy who helped bring the BX 20 plugin to the platform. Glad your coils were still intact. I still have a bad amp card, it has self-oscillation. I had to swap the good card between channels for the capture! Gotta get it fixed one of these days.

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