KEF PSW 2500 Relay buzzing and audio output buzz

The internet is awesome, I just followed John Bloomfield's advice above and my 10-year-old Kef 1000.2 sub-woofer is back in action!

I have experienced the problem of the output cutting out in a KEF PSW2500. I have also previously replaced C52 which is mentioned in previous posts and is the storage capacitor to power U13 that protects the speaker from direct current in case of fault. The problem of cut out at high volume levels occurs on my unit as follows: When the input level is high in bass content, regardless of the input level (VR1) setting on the back of the unit, the output has a loud pop and stops until power is recycled, or just pops loudly on peaks but will recover. If I remove one channel of the input the problem is much less apparent.

The previous C52 fix corrected the buzzing noise caused by the protection relay coming on and off when the capacitor was dry, but the new problem is slightly related as follows: The audio operational amplifiers (opamps) make up the filter and gain stages and are powered by a +/-15V supply composed of U14 and U15. If you measure the -15V output (this can be done without the speaker connected and using a VOM) you may see that its output will go to 0V during peaks or may lock at 0v while the +15V side is stable. If you do measure this, the result is that the output of the opamps will have DC on them instead of 0V, causing U13 to detect this and shut down the output to the speaker to avoid damage.

Cause: The cause of this problem is due to the lack of input capacitors in the original design, between the Vin pin and ground of the voltage regulator IC's which in some cases can lead to latch up of one of the regulator IC's; especially the negative regulator (U15, LM7915). This part is not defective, but just latching up due to instability in the design.

Fix: To fix this problem, you need to add some components as follows: For simplicity sake, I used the same 47uF/63V parts that are used in the C52 repair. You need to solder 2 pieces in parallel with Vin and ground of U15, making sure the minus side of the capacitor is to the input and the plus side is to ground. Secondly, you need to solder 1 piece to the Vin and ground of U14, where in this case the minus side will go to the ground and the plus side to the input. Note: If you get the polarity wrong, the capacitor will blow up due to overheating. The reason that you unbalance the capacitors, is so that you make sure the positive regulator comes on first and then the negative regulator second. This is well documented by the manufacturers of the parts so it is not new.

Since this instability may be due to parts ageing, you may also want to add 1 piece of capacitor at each side of the +/-15V connector on the preamp board side, which is labeled CN2. This will make sure the audio circuits still have a healthy filter on the supply to keep the audio nice and clean. Note that the ground is common between the supplies so CN2 is only 3 pins.

You can test the unit without the speaker connected first with some high level content while re-checking the 15V supplies. Both should be stable and the protection relay should only come on when powered up and stay engaged anytime there is music content. If that checks out, re-assemble the unit (remembering to connect the red and black speaker wires) and enjoy your new sounding sub-woofer.

If the chattering goes on continuously until you remove power or cover the photo-cell, then your contactor (relay) is wired wrong. If that is the case, read on.... The relay is probably a double-throw relay, and it is wired in such a way that turning off the photo cell allows the relay to open, but when it opens, it closes on the other contacts, which are wired to re-energize the coil, causing it to close again. It is, in effect, bouncing off of the secondary contacts. This is exactly how buzzers and old phone ringers operate.
Good luck.

Have just repaired a similar problem with a KEF 2500 - relay was chattering at 50 Hz rate, buzz in speaker output.

Problem was caused by overheated / dried out Electrolytic filter cap (C52 - 47uF 63V) on the relay coil supply feed.
The overheating is caused by adjacent power supply dropping resistors which run very hot due poor design of power supply.

A service manual for a similar model is available at following link, refer to power supply section:-
http://www.eserviceinfo.com/downloadsm/35375/kef_psw2000.html

To replace,
1) remove chassis from speaker cabinet (8 screws), disconnect red/black speaker wires from main PCB spade connectors.
2) Remove 4 screws attaching main pcb heatsink assembly from face panel - lift assembly from faceplate (a bit messy , much heartsink compound between mounting faces)
3) unplug the AC 3 pin connections adjacent to the large capacitors and the 3pin connector on end of pcb (near C52), then roll assembly over the top of the audio sub-board to access underside of pcb.
4) solder connections to C52 is accessible at edge of board without further dis-assembly.
Take extreme care replacing C52, as the circuit board is of poor quality, (usual domestic electronics quality) being single sided copper without plated through holes.
The board is already heat damaged by heat buildup from power supply resistors, so the track will most likely lift and crack when disturbed- to counter this problem link the +ve capacitor pigtail to an adjacent solder pad on the same track.
Assembly is reverse of above, take care to preserve/ensure minimum disturbance of heatsink compound.

send me a request to my email address [email protected] and I send you a fix, Axel