AKG BX20 Spring Reverb - Restoration Project

So I found an AKG BX20 with 3 broken coils.  I picked it up cheap and after reading every horror story have decided to give it a go.  I love a challenge and have somewhat limited electronics skills.  I'll document the journey; good and bad. 

The plan: 

- Got someone to pick it up in Nashville.  It's currently traveling back to Michigan in the back of a truck (on it's side, terrible I know).  We packed the tube full of blankets to keep it from banging around. I figured that since every coil was essentially broken, we were not risking too much. 

- Once on the bench, we will take photos and publish

- I'm working with Chris Nash at Audio Repair Berlin to manufacture the coils.  We will put our order in and wait for delivery.

- Recap the amps

- Clean the thing up

- Reassemble

- Debug (forever)

- Get frustrated

- Debug

- Beg Chris for some help

- Fix it finally

- Make cool records

Wish me luck. 
bLo

A collegue of mine tried to do the same.
His BX20 ended as a rabbit shed...  ;D

Sound great when they work properly.  Only thing to rival a plate IMHO.

Some interesting pics on this site:

https://analogaudiorepair.com/2013/05/03/akg-bx20-spring-reverb/

Gene

Here she is, fresh in from a Nashville boneyard. 

https://www.instagram.com/p/BhSoWOEAfOW/?taken-by=willissoundandrecording

Greats reverbs. (When they work...)

My BX20e only had one bad coil.  I replaced it with one of Christoph's from audio repair Berlin. It was not easy, but it worked and sounds great.  The  litz wire to and from those coils is soooo tiny.  If you look at it wrong, it'll break.  You can barely see it with the naked eye.

Anyway, good luck!  They do sound awesome. 

chops said:

My BX20e only had one bad coil.  I replaced it with one of Christoph's from audio repair Berlin. It was not easy, but it worked and sounds great.  The  litz wire to and from those coils is soooo tiny.  If you look at it wrong, it'll break.  You can barely see it with the naked eye.

Anyway, good luck!  They do sound awesome.

Click to expand...

I have a BX10 with one broken coil and was considering using one of his to replace. Did you find it difficult to repair / replace?

outoftune said:

I have a BX10 with one broken coil and was considering using one of his to replace. Did you find it difficult to repair / replace?

Click to expand...

Yes!  I ended up removing my old fried coil and sending it to Christoph in Berlin. He attached the impossibly small lutz wires from the coil to the TO socket.  Then he wrote very clear instructions for raettachment of the coil inside the spring. Then I reconnected the audio wiring and all was well.  The original coils measure around 189 -193ohm, and his measure 187ohm. Most importantly, it sounds great. 

Nice. Congrats.

So, I have figured out that only one coil is bad.  I've recapped both amp cards and replaced the power caps. 

I have not figured out how to remove the coils yet.  The springs look welded.  I've ask Christoph for a procedure on how to remove.  How did you guys disconnect them? 

Christoph can send you detailed instructions, but in a nut shell:

You may remove the coil pair by heating the straight reverb spring end up with a soldering iron. Use a toothpick to impale the outer coil first, then pull gently.
You will be able to get the coils out without damaging anything, it worked here beautifully.