Quad 405 problem

:(Hi guys

I have a lovely old Quad 405/405 Mrk 2 amps which I had biamped with an Audiolab 8000C I wanted to upgrade so bought 2 of their upgrade kits containing capcitors and new opamps, etc, the Quad 405/2 works like a dream, although not technically minded, if shown where and how to solder and replace its not a problem. Dare i say that this problem is nothing to do with Net AUdio. The guy has been very helpful with me and very patient. But on first power up. It wailed very loudly in the right speaker, coughed , spluttered and died. So I sent the whole boards back this time to Dave at Net Audio, and he fixed for a minimum price. I got the boards back yesterday and fitted them back into the chassis connected to the heatsink, all correct and seated properly The boards were checked by Dave and so no problems there.
He has told me to check the ohm readings with a multimeter to the negative and positive teminals on the speakers. The right side reads, 3.12 ohms, and the left side 0.9ohms. The DC voltage on either side left and right, into the boards from the transformer is 105 volts, so no probs. But Since the wailing was down to my wrong placement of some Capacitors i knew that the first time it wailed it was my fault. But I think to add into the equation, that the right sided speaker terminals are shorting themselves?? . Might that account for the different readings , left and right. It is an old 405 with those spring clip terminals, so if a short peice of wire has somehow jammed inside the housing snd is causing a problem, would this reading of 3.12 ohms, be an indication of that?

I hope i have explained my self well enough as i dont really know anything about amps above soldering and replacing parts, so be gentle

Thanks for any comments, Daves away for the weekend so im pretty much on my own with this!

If your boards have been checked they are probably fine, your speakers dont sound right though you may have fried one of them 0.9 ohm sounds very low, almost short circuit.

Is the Zoebel (i think i spelt that right) network in place at the output of your amp?

Try and test your amp through some cheap speakers, or even better get a scope and use a chunky resistor.

I just realised u meant that the speaker outputs were giving those readings.

Well the amp should have been off when you made them with speakers disconnected.

You should have measured a very high resistance or infinate on your meter from output to ground, +VE rail and -VE rail.
If it was a few ohms then certainly your output transistors are short circuit between collector and emitter.

If it is short circuit only to ground then check form misplaced wires and check the resistance between the actual supply rails to each other and then to ground (keep the probes measuring for a few seconds to allow your caps to charge).

Hope this time its more helpful :)

On the back of the spring-loaded terminals is a small PCB containing the DC output protection circuit for each channal. If you disconnect the output connector from the pcb and check there is no DC offset preasent at the output of the amplifier you can then connect the speaker directly to the amplifier output. This will eliminate any problems with the output protection PCB.


if you really like your speakers and you want to keep them working even in case the amplifier should ever fail again, I believe it is a better idea to fix the clamp board. As 405man said, the tiny pcb is fitted directly at the speaker sockets (However, this is only correct when speaking of a 405; most later pcb versions especially of the 405-2 had this circuit fitted on the amp board directly).Your measuring results sound as if one TRIAC is faulty (must be TIC2800... or equivalent). Simply replace this item in order to get the "crowbar circuit" working again.

best regards, audiomatic