Crown DSi 4000 repair

October 16th, 2016

The symptom is can power on, display show LOW RAIL but no sound.  LED on main board not flash.  +rail at heat sink at 89V (normal at +30V with no signal) and -rail at heatsink -30V (normal).  The cause of defect is due to probe around fan control circuit and suddenly power off and then show defect symptom above.  The fan noise is really too loud for home use and but if not set the fan on, the drop down resistors are too hot and also the +/- 15 regulator is also hot and cause the pcb to be brownish color.

The failure is due to U17 LM358 defective (body very hot) and thus replace a similar op-amp M5238.  Reboot the set and it is working again.  The +rail voltage will only drop to +30V after the set is fully boot up.  Tested for 2 hrs output and confirm the set is fully functional again.  See some AP measurement.

Note that with no input, the output is quite noisy, may be due to the switch mode supply.  Also at 1kHz, the harmonics distortion is quite a lot.  S/N is about 95dB, while the spec quoted for 100dB.  Thus this amp does not sound very quiet at background but good for high power drive for PA and Cinema use.


ProAc speaker repair

October 14th, 2016

You can see from the photo the tweeter has damaged foam but it does not affect the function!  The the bass unit, the coil is open circuit and all 4 units (brought two more spare) are due to the corroded connection at the flex wire to the paper cone.  Thus this is not internal open circuit and thus I try to reconnect them using a wire as shown.  After repair, the driver can be used without any issue!

JBL 4642A + Crown DSi 4000 tear down photos

October 14th, 2016

Got these all at very good price back in March 2016!  The amp and the subwoofer is used in one of a 70 seat cinema locally by government for film screening.  All equipment are on auction due to relocation.  Hope to hear more!

JBL 8340A tear down

October 14th, 2016

Got 4 of these real cheap!

Crown CT8150 tear down photos

October 14th, 2016

Got this B-stock only US$400 cheap from Harman on-line shop.  Sound great for this amp!  125W x 8 channels.  Good for 7.1, bi-amp, tri-amp use for home theatre or home audio etc…

No fan and this is really high tech!  1000W for a 1U box!  This amp is good for studio use for quiet operation.  Only issue is that it has no network connection and cannot bridge the output channels.