Paradigm PDR10 Rattle and Hum Faults

So a rather old Paradigm PDR10 sub woofer I bought over ten years ago from Richer Sounds Sevenoaks had developed some faults and needed servicing. It would have been too easy to just throw it out but after a few hours effort it’s working again sounding even better that before.

Warning: Never operate you sub-woofer with the amplifier removed – there are exposed mains voltage connectors inside and the amplifier has a heavy transformer that can topple over!

You may too have one of these subs that needs similar treatment so here’s what to expect and what may need doing.

First the rattle, most obvious on movies such as Star Wars Revenge of the Sith on the opening sequence. The whole front baffle vibrates and rattles as if it’s going to fall off. There’s no obvious fixings from the front grille. A quick Google finds several questions asking how to remove the front grille followed by pictures of broken baffle and the answers – it’s not designed to be removed!

Disconnect all cables, place the sub on its front and remove the black chipboard screws holding the power amplifier in place. Lift the black metal plate holding the transformer and power amplifier carefully out removing the red and black speaker cables and the plug connecting the front power indicator. Remove the white wadding that surrounds the speaker – it simply lifts out as it’s not glued in place – more on that later.

Next remote the speaker held in place with longer black chipboard screws. My speaker showed no signs of foam rot on the cone surround that could have caused the rattle – see my other post if your sub has a damaged surround Paradigm PDR10 Foam Rot Replacement Sub Woofer Fix. So I put the speaker out of the way of metal tools and feet and looked more closely at the cabinet.

Inside my PDR10 there was no glue seams along the front baffle board or two of the cabinet sides – see photos. There was however glue on the other two cabinet sides. It looked a Friday afternoon botched assembly where they forgot to apply all the glue but PDR10s may all be like this. I made up three bracing blocks from some pieces of hardwood I had in the workshop. I made a cardboard disk to put in place of the speaker to avoid getting glue on the grille material. Then I found the PVA wood glue – a fairly runny consistency – and applied liberally all around the inside edges in the hope it would seep between the front baffle and sides. You can see the result.

Although fairly warm at 22 C the glue still looked white after an hour. The inside had little air movement so I added a small PC fan blowing down into the port to help dry the PVA. Next day the glue has turned clear and set. Hopefully the baffle would no longer move.

Paradigm PDR10 showing glue dry inside cabinet

Next the hum and the circuit board. First problem I noticed that two resistors R81 (2K7Ω) and R82 (5K1Ω) had fried. The PCB was scorched around the resistors and the solder joints looked suspect. The resistors are connected in series to make a 7.6KΩ dropper from the 240v mains for I guess the standby power supply so they are always hot if the sub is left plugged in. Reminded me of the old valve radios with hot running resistors although these were only hot when the radio is on!

Paradigm PDR10 Burnt Resistors and PCB

To desolder the resistors it’s easiest to remove the metal side panel. This is held in place by two screws from the back panel and a square headed security screw from the side panel that holds the heatsink in place. The square headed security screw needs a special driver bit. A small flat bladed screw driver may work across diagonal if you have one that fits.

I found an 8KΩ wire wound resistor to replace the two smaller wattage resistors that had cooked. This avoided using two of the solder connections that had suffered overheating. A quick calc shows this resistor could dissipate over 6 watts if it’s dropping 220v so it’s no surprise it cooks the PCB if it’s left on all the time. Will check the voltage drop next time I open the sub. It’s not a good design and would hope newer versions of the PDR10 use a small transformer or non resistive supply instead for standby power.

Paradigm PDR10 Replacement Wirewound Resistor

Hum is most likely due to faulty smoothing caps and I noticed one of the two large black electrolytics showed a domed top compared to the other. The suspect cap was removed with help of a solder sucker or you could try copper braid. Helps to heat one side and gently rock the cap to pull each pin gently as you apply heat from soldering iron. Have to be careful not to press too hard and break the PCB tracks. The faulty capacitor is marked 5600µF at 50v 85C. It’s a 30mm diameter electrolytic and I only had a 4700µF 100v one that fitted temporarily.

Reassembly of the metal plate just needed the addition of a small stick on rubber disk on the end of the PCB mount where the top had disintegrated due to the heat from the resistors. This I hoped would avoid more rattles as I did not have a replacement plastic PCB mount.

Before replacing the power amplifier I turned attention to the wadding that looked added as an after thought. I found some more sound deadening foam and cut it to fit top and bottom and around the port tube. Gluing in place with Copydex. The remaining wadding was cut to fit either side nearest the front baffle and again glued to the inner cabinet. I left the area above the power amplifier PCB to avoid heat from the resistor getting near any wadding.

With the glue dry and components replaced the speaker and power amplifier can be assembled – the PCB shows the red and black speaker connections and the indicator lead clicks into place. I put a couple of fixings in place and applied power for a few minutes to see how hot the new resistor got. It seems okay with no discolouring so I reassembled and fired it up. Perfect, no rattle or hum just nice solid bass shake. So I’ll order two replacement caps when I get time.

Update – Replaced both reservoir caps with10,000µF 63v electrolytic and sounding excellent.