QUAD 405 (original version) with no treble on one channel

Hi all !
I need your help/advice, I have the following problem with my Quad 405 (original one) since yesterday evening: the sound is messy, treble are not soft, they are "brittle".
The amplifier have been "refreshed" using Dada Electronics kit (many) years ago, the power supply is a dual power one from Net-Audio (UK). I use the amplifier daily (used for the TV). Preamp (44) is not the issue as when connected a 405 clone I had built years ago (Hungarian kit) I do not have this sound issue (despite this clone doesn't sound as as the original one), so I use the "clone" as long the other one is not repaired.
As I do not have a scope beinglimited to voltmeter/ohmmeter, I did check if there were any offset voltage on the outputs: nothing wrong (both <0.01 V)
Anyone having an idea where it can come from ?
Thanks in advance,
Michel.
 
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Welcome to diyAudio :)

Something like you describe is virtually impossible to diagnose at a distance. If both channels are the same then it is unlikely to be a problem with amp itself although I suppose a problem with the power supply (common to both channels) could conceivably cause a problem.

A scope is the go-to diagnostic tool. You have to get a handle on what is happening and the scope would show if there was any low level high frequency oscillation for example, something that can cause such as you describe.
 
Not sure the 2 channels are involved, I will check that tomorrow (I should have done that before unplugging it). The PSU is showing + and - 55 VDC on the board terminals. Usually, when the PSU ave "dead" capacitors it "rumbles" in the loudspeakers, and that's not the case.
What is strange is that there was no warning, sound was OK and suddenly bad ....
I'll update as soon I have "fresh" information.
Thanks,
Michel
 
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Interesting. Swap your speaker leads around and see if the fault remains on the same speaker (speaker fault) or transfers to the other speaker (amp fault).

If the amp is faulty then switch off and swap the inputs around to prove the preamp is OK.

(Faulty caps in the signal path of an amp usually give lack of bass.... see what those tests show)
 
The pre-amp and loudspeakers are not faulty, as with the "clone" I do not have the issue. Nevertheless, same input gave "good" sound on one board, and "bad" sound on the other, so faulty board is now identified. The faulty board lacks on treble, like response is limited in frequency. That was not the case this morning, trebel were not good but still there, now there're not (faulty component now fully dead !).
 
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Very strange. So when you say 'faulty board' you mean one of the 405 channel boards. We must be in no doubt about that.

That was not the case this morning, trebel were not good but still there, now there're not (faulty component now fully dead !).
You mean no treble at all now, not a fully dead channel.

Honest answer is I don't know and can't really think of anything obvious that would cause lack of treble. It really needs checking with a scope to see what is going on.

I'll look in later :)
 
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I've looked at the circuit and also played with the simulation of a 405 and I can't see a viable failure mode that would cause just a loss of HF. The 1nF across the base/emitter of the VAS stage is the only one I can see that comes remotely close and for this to be at fault would mean it has to increase in value significantly. Simulating significant leakage on the small signal caps does not give this fault.

The pre-amp and loudspeakers are not faulty, as with the "clone" I do not have the issue.
I'm going to say repeat that test of swapping speaker leads and this time do it 'hot', in other words while the problem is present.
 
Board have been removed from case. I've checked the chemical capacitors with my multimeter (there is a C measurement on it) :
On component side:
  • 100 µF 25V
  • 100 µF 16V
  • 47 µF 63V
On Cu side:
  • 220 µF 25V
  • 220 µF 25V
With connected capacitors, nothing look wrong, I have readings for all of them.
But I not sure this is valid, I have to unsolder one lead on each of them and re-check.
About swapping "hot" the loudspeakers, it doesn't change anything.
It it did, that would not be logical as both speakers are working fine using either the "good" channel or the 405 Clone (used as long as the original one is not repaired).
Again thanks for your help.
Michel.
 
I just made a (silly) test: I put the "good" board on the "bad" board location, and tested: problem is the same, in addition I noticed that the heat sink became rather warm after the few minutes of the test.
So I now suspect that the problem is coming either from the power supply or from the loudspeaker protecting circuit.
But using the louspeaker wire of the "good" location on the "bad" location does't change anything so it now point the power supply.
I will check that later on (I hate touching wires with residual voltage ......
 
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About swapping "hot" the loudspeakers, it doesn't change anything.
But did you actually try it ?

It it did, that would not be logical as both speakers are working fine using either the "good" channel or the 405 Clone (used as long as the original one is not repaired).
It may not seem logical but it has merit. Just the very act of unplugging and reconnecting may lead you to some discovery. Speakers can have intermittent issues like a dead tweeter that can appear in 'random' tests like these.

I just made a (silly) test: I put the "good" board on the "bad" board location, and tested: problem is the same, in addition I noticed that the heat sink became rather warm after the few minutes of the test.

I would suggest you use just one speaker to eliminate any possibility of a problem and use that in turn on each channel when testing. It removes one variable. Also use just a single input between channels, that will remove another variable.

If your 'clone' uses speaker relays (which a genuine 405 does not) then speaker relays can be problematic but they usually cause low and distorted sound at low volume due to poor contact resistance and typically if you turn the volume up loud it temporarily 'fixes' the problem as the higher speaker current forces a way through any tarnish/contamination layer.
 
I did swap the same loudspeaker "hot", and as I mentioned it doesn't change anything. I reviewed the power supply, now it is OK BUT it seems that inverting board have now harmed the "good" one. Volume is now misearable on both channels (moving the same loudspeaker connection from one channel to the other) and it's getting warm rather quickly (within minutes).
BTW, the 405clone I am using have no relay for the loudspeakers. These are connected direct to the boards (on the clone, on the original 405 it goes throgh the protection circuit).
I guess that now cheapest will be to get clone boards from China, that will cost less than needed investigation + components .....
 
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The protection on the original 405 (and it was only on later versions) is based on a Triac which is a rather rather brutal but effective method of protecting the speakers. It works by shorting the audio output to ground in the event of a DC offset and allowing the supply fuses to then blow. The audio does not 'pass through' this protection on its way to the speaker and so it can not affect the audio as such.

It sounds like you have a lot going on with this but ultimately its all pretty conventional. You need proper diagnostic test equipment to look at what is happening rather than haphazardly swapping parts around. That often doesn't end well I'm afraid. You disturb as little as possible while 'gathering evidence' is the general rule of fault finding.

As this thread is about the 405 clones and yours is about a faulty original Quad I'll move this and earlier posts to a new thread.
 
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