Best of luck. Cheers, Martin. Resurrecting an old thread, but thought better than starting a new one! My PSW appears to have died last week. Green light comes on, but simply no audio output. I am presuming something has died in the amp circuits. Is it worth replacing the amp with one of these - BSBP Or just replace the whole thing with one of these - Gemini Thanks for your opinions. Gemini - a different class of construction.
Click to expand Frank Member. See my sig. I repair a lot of these. A LOT cheaper then Kef! CaptainCoombes Novice Member. I'm trying to fault find my PSW It powers on, green led, red led. The relay clicks. I can feel the speaker vibrating but no sound. You can leave the main toroid transformer unconnected for now. Make sure that the back of the PCB is not shorting to anything, power the unit up, and test the unregulated DC with a voltmeter.
At least 20V, in any case. Check again with a scope. Unregulated ripple will be about 1V pk-pk. Regulated supplies should be free of signs of oscillation, of course. I also tested some of the electrolytics — many of them seemed to be going a bit high in ESR. I did replace the two at the output of the 15V regulators though.
If I was being really thorough, I would replace them all. You can now refit the PCB to the frame. I assume you already know how to thermally bond the power transistors to the heatsink compound, and so on. Connect everything back up, power up, and check the amp works OK, first unloaded, and then into a dummy load , if you have one. Even with all these mods done, the unit still has a problem — lack of airflow.
Here is a simple way to improve that. This modification really could not be any simpler. When refitting the panel to the box, use spacers of about 3mm on each of the eight fixing screws.
This creates a small air gap around the edge of the panel, which allows hot air to escape the enclosure and cool air to be drawn in. I used Farnell The first KEF subwoofer that I came across was the PSW, in which the output transistors are mounted on a large fan cooled heatsink. This is a redesign of the PSW and , and runs a lot cooler. Every electronic device generates heat, and a power amp needs to get rid of a fair amount of it. Making sure that things do not get too hot is a big deal.
When an output transistor or a voltage regulator for example cannot lose heat quickly enough, its temperature rises, and eventually it will fail. How hot is too hot? Every device has, as one its design parameters, a maximum internal temperature. The designer can estimate the maximum allowable air temperature inside the case, which will produce that temperature inside the device. This depends on the thermal resistance of the device and the power being dissipated. There is another reason to keep the internal air temperature in the case as low as possible.
Other components, especially electrolytic capacitors, which need to be slightly wet inside to work, degrade over time, and much more severely at high temperature.
Above is the unmodified power amp board. The bad capacitor has been removed, not yet replaced — you can see the circle on the board, by the white connector. You can also see that the connector is damaged by heat — it is a bit brown and part of the plastic has flaked away. Luckily, most electronic hardware is made of flame retardent material. It is harder to see on the photo, but the green capacitor next to connector, and the back of the board itself, are all brown.
Like most modern audio amps, these boxes have two pairs of internal DC power rails. All fine so far. This was a bad idea. So, the designer had to use dropper resistors — R and 2. These are four of the six that we replace with this mod. Below is a schematic of the droppers, also showing the voltages that I measured. Capacitors omitted for simplicity. Using droppers like this is a bad idea, even without the overheating it causes : because the voltage on the input side of the regulator now depends on the current it is drawing.
Of course, as long as the voltage remains high enough at all times, it is not, strictly speaking, a problem. But, a sudden spike of current would easily cause the input voltage to drop too low. We can now figure out the current through each path in the circuit, and the power dissipated by each component.
In fig 13, current is shown in blue and power in red. The 4 resistors and the two regulators which have 13 and 17V across them are burning over 5W between them. That 5W of heat heats the air. Moving two of them off the board will have helped this a bit. That accounts for four of the six resistors.
They each have a value of 1k8, which means that they each dissipate a further 1. So that means that the 6 resistors and the regulators dissipate over 8W between them. In a sealed box with no airflow! What should the designers have done?
Well, the moment they decided to use linear regulators to get 15V from 50V, there was a problem. Both the problems are in the power supply section of the subwoofer, the first concerns the absence of switching off the amplifier when it is in stand-by and the second one is an excessive heating of some resistances. During the standby all the amplifier parts is still powered and only the loudspeaker is disconnected from the amplifier.
This problem cannot be solved, also adding a second transformer for the driver stage and other services many parts of the amplifier are always powered during the standby so the solution is remember to turn off the switch after use if you do not want to risk your house can go fire. The main transformer with about 35VV AC is used to create the 50VV DC to power the final section of the amplifier and for the driver stage and other service a lower voltage is obtained by linear regulators like and These linear regulators cannot accept an input voltage greater than 35V so the KEF designer have created a resistive divider to reduce the 50V.
A resistive divider create much heat and the 2W resistors used cannot support this for long time H
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