KEF PSW2500 active base unit repair

[Diver300]

I've been looking at one of these for a friend, and I found this video:-

Basically the +/- 15 V supplies are regulated from the +/- 55 V, and lots of heat is dissipated in the regulators and some series resistors, which get too hot and are damaged, burn the board or give poor solder joints.

The manufacturer has obviously realised that there's a problem, and the one that I'm working on is newer, has different resistor values, and they are mounted remotely on the heat sink.

I found the circuit diagram here:- masterelectronicsrepair.blogspot.com/2019/02/kef-psw-2500-powered-sub-woofer.html but the one I'm looking at isn't quite the same. Here is the circuit of its power supply that I have modified to match the PSW2500 that I have here:-

The 270 Ohm resistors are the white, square section ones that are mounted remotely.

The question I have is about the 2.2k resistors. They are 2W rated, and have discoloured, and some have come loose. Obviously the two that are before the 270 Ohm resistors are worse. They are right beside the 12000 uF capacitors.

What is the function of the 2.2 kOhm resistors? Could I just remove them?

BTW, before anyone asks, the main capacitors are bulging at both ends, and they feel like something is loose inside and they will be replaced. Oddly, the capacitance is about right. I can't measure their ESR.

 

[Nigel Goodwin]

I've been looking at one of these for a friend, and I found this video:-

Basically the +/- 15 V supplies are regulated from the +/- 55 V, and lots of heat is dissipated in the regulators and some series resistors, which get too hot and are damaged, burn the board or give poor solder joints.

The manufacturer has obviously realised that there's a problem, and the one that I'm working on is newer, has different resistor values, and they are mounted remotely on the heat sink.

I found the circuit diagram here:- masterelectronicsrepair.blogspot.com/2019/02/kef-psw-2500-powered-sub-woofer.html but the one I'm looking at isn't quite the same. Here is the circuit of its power supply that I have modified to match the PSW2500 that I have here:-



View attachment 125327
The 270 Ohm resistors are the white, square section ones that are mounted remotely.

The question I have is about the 2.2k resistors. They are 2W rated, and have discoloured, and some have come loose. Obviously the two that are before the 270 Ohm resistors are worse. They are right beside the 12000 uF capacitors.

What is the function of the 2.2 kOhm resistors? Could I just remove them?

BTW, before anyone asks, the main capacitors are bulging at both ends, and they feel like something is loose inside and they will be replaced. Oddly, the capacitance is about right. I can't measure their ESR.

The capacitance doesn't matter, it's ESR that does - VERY commonly the capacitance is within tolerance, but the ESR is absolutely shot, and the capacitor is completely useless. However, this mostly occurs with switch-mode supplies, I'm presuming this one is just a conventional transformer.

The power supply itself is horrible though, it's a REALLY, REALLY bad idea having large droppers like that, and as you've seen they WILL overheat and damage the board. It's also really poor not having capacitors across the input of the regulators, I thought 78/79xx's required those?.

As for the 2.2K's?, I can only imagine they are there to discharge the 12,000uF's at switch off.

 

[Diver300]

It is just a conventional transformer. It's a toroidal.

I'm looking to supply the +/- 15 V from an alternative supply, to avoid the dropper resistors. The actual droppers, the 270 Ohm ones are on the heatsink on long wires, so they aren't such a problem as in the video.

It's the parallel resistors that I was wanting to get rid of. I wasn't sure what the consequence would be of omitting the parallel resistors, or having higher resistance ones, would be. There's a relay that disconnects the speaker from the amplifier, and I wonder if the resistors are there to discharge the power supply quite quickly, so that the relay drops out quickly and there isn't a pop from the speaker if the power is turned off and back on again a bit quickly.

I'll bear in mind about the input capacitors on the regulators. I'll make sure some are there.

This shows the 7815 and 7915 on their aluminium heatsinks, in the top left. The edge of the toroidal transformer is at the bottom. The brown and green wires are for the white resistor that are in the bottom right of the photo, bolted to the heatsink. Those wires go from the former location near the regulators to the current location.

 

[Diver300]

Well the repair and modification is done. Here's what it looks like now, from the side. The top would be to the right of the picture. The big capacitors are vertically above the transformer. The plate that screws to the back of the cabinet is at the bottom of the photo.

The owner got some information from KEF. They won't repair this model any more, but suggested C52 (labelled 5 in the photo) was changed, along with the two big capacitors. It was a useful suggestion as C52 is nominally 47 μF and it measured less than 1 μF and appeared to be discoloured. The two big caps were going to get changed anyhow.

C52 smoothes the supply to the relay that isolates the speaker. The supply to that is half-wave rectified from the main transformer. Label 3 on the photo shows where there was one pair of the 2.2 kΩ, 2W resistors. You may be able to see the outlines of where they were on the circuit board. The other pair would have been behind C52 in the photo, between 5 and 4. Those resistors were running at well over half their rating, even when the amplifier was on standby. C52 had been cooked.

Anyhow, to stop it happening again, I arranged an alternative supply to the ±15 V regulators. The regulators are 7815 and 7915 and you can see their heatsinks at 4. I wound 2 lots of about 65 turns on the transformer (1) to give around 2 x 15.5 V rms with centre tap, and rectified and smoothed that with the components at 2 to give ±20 V which is wired to the regulators.

It all seems to work fine.

I still can't work out why the four 2.2 kΩ, 2W resistors were there. With the ±15 V regulators no longer fed from the big capacitors, the ±55 V supply decays quite slowly, maybe taking a minute to get to ±20 V. The resistors would have brought that down to 10 seconds or so, but it would have been less than a minute if the regulators were also taking power from the big capacitors. I can't see why the manufacturer was so keen to get it to turn off fast. It's a subwoofer. No-one ever turns off a subwoofer at the mains or the switch on the back to silence the sound, as it doesn't work. All the mid-range and higher frequencies will be running. If the sound has already been muted, I can't see any problem with the capacitors going down slowly.

Now that the ±15 V supply is separate, that decays in less than a second. It could be that the amplifier does something funny if the main power supply decays while the ±15 V supplies still work. However, the relay would turn off in well under as second, so I can't see what the amplifier could possibly do that would make a 10 second discharge important, especially when it was those resistors that we such major cause of unreliability.

The other question that I have is what would the effect of C52 being effectively missing? Here is the circuit.

The "V" at the top right is the connection to one of the transformer windings, so D10 is half-wave rectifying that. The IC's output is a transistor that connects the bottom of the relay to ground, or not.

The relay has inductance, which is why D9 is needed, and I would have expected it have enough inductance to work fine without C52. Anyone got any experience with situations like that?

I never tried the amplifier until I had changed C52 and the big capacitors, so I don't know what had actually stopped it working.