Repairs | Modifications
Restorations | Customizations
AKG BX 20 Repair
The AKG BX 20 is one of the elaborated independent stereo moving coil spring reverbs worldwide.
From its launch in the late 60's many prime sound productions used it because this
offered the glorious depth and colour of spring reverb without the limitations usually known with such devices.
To obtain a superior frequency density each reverb spring is divided into 12 segments of differing diameter,
with irregularily reversed windings and irregular etch marks.
The work inside is done by 4 tiny torsion coil pairs.
One of them acts as motor, the other one as sensor, picking up the reverb signal from the opposite driver coil
and at the same time acting as motion feedback sensor for the own driver coil.
These coils have to be tiny enough to be partly inserted into the reverberation spring itself
and lighter than 50 mg to respond precisely with a soft roll-
These coils have to be wound of copper wire, thinner than human hair.
A number of AKG BX 20 devices are known to have developed broken coils over time.
I can offer repairs and some spare parts now!
How to proceed (detailed procedure available on request):
Removal of defective Supension/Coil Assembly by owner or local technician
Shipping to me (small size, fits into a DVD case)
Repair of defective Supension/Coil Assembly incl. replacement of coil pair here in Berlin
Shipping back to owner (small size, fits into a DVD case)
Replacement of repaired Supension/Coil Assembly by owner or local technician.
Two strong AlNiCo permanent magnetic circuits are fixed onto that structure, one on top,
one on bottom.
Each magnetic circuit has 2 focussed air gaps, each belonging to one audio channel.
This is the place where the spring suspensions end.
New BX 20 coil pair before final coating
Success rate 100% ! Until today (February 2018) all made by me coil subassemblies have survived !
Successful repairs include:
mounted there by local technician)
mounted there by local technician Waltzing Bear Audio
AKG BX20 #2783 (2 coil subassemblies for Alex Malgarinos-
AKG BX20E #4047/A for Daniel Sonntag, Berlin (3 coil subassemblies, complete service).
AKG BX20E #4445/A for vintagetools, Mörfelden (1 coil subassembly)