Quad 405 Power Amplifier Service & Refresh

Readers will already know how much I like the Quad 405 and this inspired a customer to acquire some Quad gear for himself, including this lovely, original 405. In this article, I look at servicing a Quad 405.

Talk about great sounding amps, and this particular 405 is completely original, with its original capacitors and connectors. The owner wanted me to freshen her up and fit some RCA connectors, so that’s what I’ve done. Let’s look briefly at what’s involved in a Quad 405 service like this.

By the way, if you want to read or find out more about the Quad 405, check out my Quad 405 restoration article and my comprehensive review.

How I Freshen up a 405

I try to keep this simple, a freshen up is not a full restoration. A service like this focuses on reliability improvement and bang-per-buck sonic gains. I start by carefully inspecting the 405 and looking for any signs of parts failure or earlier poorly executed work.

img_7516 Quad 405 Power Amplifier Service & Refresh
Lovely, simple, elegant, retro – what’s not to love about a 100 watt per channel amp this small and cool-looking?!
img_7517 Quad 405 Power Amplifier Service & Refresh
That’s all there is inside. Less really is more with the 405 and these amps prove it. Centrally, there’s a large, high-quality potted transformer, to the left of which lie two decent-sized filter capacitors. These capacitors will stay in place for this service.
img_7518 Quad 405 Power Amplifier Service & Refresh
So English, this beautifully executed wiring loom really is a blast from the past. If only equipment were still made like this.
img_7519 Quad 405 Power Amplifier Service & Refresh
Note the crappy looking ERO caps, one covered in I don’t know what. These always fail, in this 405, two out of four measured leaky and had ridiculously high ESR.
The Thermal Interface

the next step involves removing both amplifier modules and cleaning all the old thermal compound from modules and heatsink.

img_7529 Quad 405 Power Amplifier Service & Refresh
Four screws hold each module in place, remove them and the connectors, and the module just pops out. All the old thermal paste must be removed.
img_7530 Quad 405 Power Amplifier Service & Refresh
This is how both surfaces should look before adding fresh silicone thermal paste to the module and reinstalling it. I always check the tightness of the heatsink fixing screws, before reinstalling the modules.

img_7531 Quad 405 Power Amplifier Service & Refresh

Refreshing the Modules

The modules themselves just needed a little freshening up in this amp. Depending on what my customer asks for, I’ll do more or less at this stage of the overhaul. In this case, the original ERO capacitors came out, two of which were absolutely dead.

I pay careful attention to fasteners. In this amp, some of the transistor mounting fasteners were not tight, and the module fasteners were barely finger tight. I remake the solder joints under each transistor after doing this, to remove stresses. Lastly, I clean each board, removing flux where necessary.

If the customer wants something more like a restoration, I’ll remove the output devices, possibly replace them, and remake the thermal interface with silicon thermal pads.

img_7522 Quad 405 Power Amplifier Service & Refresh

img_7524 Quad 405 Power Amplifier Service & Refresh
Close-up of a module. Note the Toshiba 2SD424 transistors, superb devices. The ERO caps are the cylindrical maroon coloured things. Also, note the original LM301 op-amps. I replaced these with TL071 which Quad used in all later iterations of the 405. The TL2071 is an ever better replacement and I’ve gone further with my own 405-2 and others where full restoration is requested.
img_7526 Quad 405 Power Amplifier Service & Refresh
Finished module with new, high-spec caps in place. I use super-low ESR, low impedance, high-temperature replacements in all my work.
img_7523 Quad 405 Power Amplifier Service & Refresh
The other module before …
img_7525 Quad 405 Power Amplifier Service & Refresh
… And now recapped
img_7527 Quad 405 Power Amplifier Service & Refresh
A better view of the new TL071 op-amp I’ve used as a replacement for the LM301. It’s important to use FET-input op-amps like the TL071, or TL2071 which is better again, in the 405, to avoid problems.
Installing RCA Connectors

Finally, I installed some RCA connectors on the back panel. Whilst not strictly necessary, the owner requested it here.

img_7532 Quad 405 Power Amplifier Service & Refresh
Finally, you can see the modules reinstalled in this shot, and the new panel-mount RCA sockets, waiting to be soldered to.
img_7535 Quad 405 Power Amplifier Service & Refresh
Here they are, connections soldered in place.
img_7534 Quad 405 Power Amplifier Service & Refresh
I now always connect the common ground connection to chassis ground, to avoid any possible DC offset at the outputs. This is not a big deal but helps the amp run cooler and sound better.
img_7533 Quad 405 Power Amplifier Service & Refresh
As you can see, with the RCA sockets installed like this, they basically look as though the amp came this way. It does take up the service holes used to remove screws holding the module in place, but with the correct screwdriver, access is still no problem.

6 thoughts on “Quad 405 Power Amplifier Service & Refresh”

  1. From where do you take the signal to the RCA connectors?
    From the DIN connector, or the module connector?

    1. Hi Fredrik, as mentioned in there somewhere, I remove the DIN from the circuit. In most cases it won’t be used again and the extra connections and wire are counterproductive.

Feel free to leave a comment and share your thoughts!