The Quad QPM1 ACTIVE speaker is based on the model 11L passive speaker as studio monitor.
https://www.avmentor.gr/reviews/quad_qpm1_0.htm
Symptom: one speaker burnt fuse and cannot power on.
Cause: The filter cap is dry out and shorted. 4700uF 50V
After replace the fuse and capacitors, the set works again but find another problem on both speaker during listen test. When RCA connection plug is inserted in RCA jack , large hum noise is heard. First suggestion is that the RCA wire is not shield well. After further trial and error for few weeks, identify the issue as below:
- RCA socket ground is isolated on back plate but is NOT connected to pcb ground.
- The RCA/XLR input pcb ground shield is connected to back panel directly and the same point is connected to the main amp pcb ground near the signal input socket.
- The power transformer is not shielded from the input and main pcbs.
Fixs:
- Recap all capacitors
- Connect the RCA socket ground to input pcb ground at XLR ground point.
- Move the ground wire to the main filter capacitor ground.
- Connect input pcb ground shield to ground point of the main pcb.
Result: the hum from the tweeter and bass unit reduce at least 3 fold compare to original connection. Also there is no more noise or oscillation using RCA connection wire.
