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MFC6070 replacement PCBs

The MFC6070 reverb driver in the MK2 Synthi A and VCS3 is getting increasingly rare and expensive.
The reverb circuit using it is the less good sounding despite very common.
It's fragile and a frequent cause of reverb issues.

Rather than pay unjustified price for such a crappy part, why not replace it with the better sounding and more reliable MK1 or MK3 driver ? The three circuits are very similar and the MFC6070 can be replaced with a small PCB without any modification of the original traces.

The MFC234 replaces the MFC6070 with the PA234 or GEL234F1 (same part renamed) used in the MK1 units. GE ceased production of this early audio amp IC around 1970, it is thus hard to find and expensive today. Many see it as the most desirable output amps and reverb driver in a Synthi or VCS3. It's the essential part of the nice MK1 sound.

The MFC741 replaces the MFC6070 with a 741 and a AC153K/AC176K germanium transistors pair, easier to source and cheaper than the PA234. This is the MK3 reverb circuit used in the Synthi A and VCS3 since the late 70s and still today.


         

The installation is straightforward and doesn't cause any damage to the original PCB :
  1. remove the 2 resistors and 3V6 zener diode above the MFC6070
  2. replace the 3V6 zener diode with an insulated wire
  3. remove the polystyrene cap to the right of the MFC6070
  4. remove the MFC6070
  5. solder a piece of component leg in the holes of the MFC6070 emplacement, the top right one can remain empty, it's not used and a corner of the AC153K obstructs the hole on the daughter PCB
  6. thread the MFC741 or MFC234 populated PCB onto the component legs
  7. solder and cut the exceeding pieces of component legs

MFC234 PCB - free worldwide untracked shipping

MFC741 PCB - free worldwide untracked shipping