ATC SCM12 repair

Original state measurement and findings

I got pair of SCM12 to my hands with the description and complaints that stereo image was not stable and was often moved to one side. So I started to measure........

On the graph below, blue trace is FR of one original speaker, the red trace shows the other one. Ok this was easy, the cause of the complaints found. Just out of curiosity I reversed polarity of the tweeter and got green trace. Not good match anyway so I started to go deeper.

All measurements done from 90cm and gating 4ms.

Horizontal off axis measurements 0-90deg of the speaker with blue axial response. Quite good. The impact of front baffle edge is clear here - very good diffraction generator, and also broad peak at 600Hz would deserve to flatten out.

D26NC-55-06 were original tweeters, I got also older pair of the same tweeters and also new pair of D26NC-56-06. According to datasheets, 56 version has slightly different FR, but impedance is very close to 55 version. Further measurements proved they are interchangeable. Impedance measurement of all tweeters is below, it was clear that new pair (blue and purple) would be used.

Just a picture of crossovers, Bennic MKP capacitors, air cored coils,...

Measurements after repair

Final FR measurement of both loudspeakers, this is quite good match. Impedance of both loudspeakers on the right.

Just to show full picture, this is DI and power response graph based on horizontal measurements 0-90deg, for reference angle 0deg.

Horizontal measurements 0-90deg for both speakers.