Quad 405-2 switch on THUMPPP! - UK Vintage Radio Repair and Restoration Discussion Forum

stevehertz · 17th Dec 2015, 12:48 pm

The last few times I switched on my Quad 405-2 it had a very bad switch on thump, so much so that I feared for my speakers. May I say that I serviced the amp a few years ago after it developed problems that were the result of naff capacitors, so I replaced all of the offending Cs and others that were advised to be changed too. I've also got a kit of parts to effect the reduced gain mod as well, but not got around to fitting it yet.

So, this switch on thump, it was never this bad? Any ideas as it seems to be something that has 'developed' rather than been there for all time.

I may add, for better or worse, I use the setup's preamp, a Quad 44's linked mains switching system to switch on most of the associated components; CD, tuner in one go; all at the same time. But again, however that may affect 'the thump' (or not be good practise) it didn't used to be anywhere near so bad. The preamp was set to tuner, so maybe the Sony tuner has started to deliver a DC output at switch on.. dunno.

dave walsh · 17th Dec 2015, 1:13 pm

Mine's been in "retirement" for a while Steve-heavy aren't they? It started doing that and I read [somewhere] that it could be one of the very large blue electrolytics [web photo illustration] but I haven't opened it up. Counter-intuitively, it was suggested that the [somewhat over-engineered] protection circuits in this design can actually accelerate a problem in some situations... with a cascade effect

. I think there's a dedicated mod kit to sort that. You've obviously got the ability to carry out work yourself but I decided to avoid disaster until I got mine over to someone suitably experienced. Another roundtuit, nothing has happened since!

I'm sure there's more about this problem and the "fix" in threads on here and certainly in audio forums but you've probaly seen those. Don't take what I've said as necessarily accurate but I did get the sense that it is possible for something catastrophic to happen unless the full mod was done. I will be very interested to see how you go on with yours.

Dave W

fetteler · 17th Dec 2015, 5:14 pm

Quote:

Originally Posted by stevehertz

I may add, for better or worse, I use the setup's preamp, a Quad 44's linked mains switching system to switch on most of the associated components; CD, tuner in one go; all at the same time. But again, however that may affect 'the thump' (or not be good practise) it didn't used to be anywhere near so bad. The preamp was set to tuner, so maybe the Sony tuner has started to deliver a DC output at switch on.. dunno.

You need to try the 405 on it's own to see if it really has a problem or if the culprit lies elsewhere

If it is the 405 do you hear the problem on one or both channels?

Other than that, if all the caps are recent then check the Zenners and associated dropper resistors which provide +-15V to the op amps. The resistors are under-specified for power and sometimes fail...

Steve.

stevehertz · 17th Dec 2015, 6:31 pm

Thanks Dave, Steve for your input and suggestions.

Actually, I can't really see how the problem can be originating from anything else other than either the Quad 44 or the 405-2, as the inputs on the 44 will be capacitor coupled I'd think? So surely even if - say - the tuner was giving out a DC level at switch on, you wouldn't expect it to affect the amp? Come to that, I need to check if the 405-2 is capacitor coupled on its input, because if it is, then that means that the problem has to be within the 405-2 itself?

Edit; just checked, the 405-2 input is capacitor coupled at its input. So, amp experts, does that confirm that the thump - that I presume is a momentary (or even standing) DC level at its output - is coming from within the amp itself?

coil1234 · 18th Dec 2015, 1:04 am

Hello,
I had a similar problem, with one channel on a 405 (BBC AM 8/16). I tried recapping and replacing and replacing the supply Zener and associated series resistor. I finally eliminated it by replacing one of the small signal transistors towards the front end.
Unfortunately I'm working away at present so don't have access to my notes.
IIRC it may have been towards the +ve supply rail? Possibly a constant current source?

Regards
Mike A

timewave · 18th Dec 2015, 10:48 pm

Turn the volume control to zero and disconnect the speakers and measure the DC across the output terminals at switch on. Leave the voltmeter on for about 20 minutes and check the volts periodically.
You should be seeing less than 15mV, preferably 0V after that time. You will see an initial voltage spike if there isn't a circuit to delay loudspeaker connection on this amp. If there is a delay circuit then maybe it isn't working. If the standing DC speaker volts are as mentioned above check, the delay circuit.
If the standing volts are higher then they need to be reduced. I'm assuming this is a DC coupled amp and there should be a preset type pot to do this.

John G4IJD

stevehertz · 19th Jan 2016, 11:53 am

Typical me, I've not got around to doing any proper analytical testing yet, but I have been using my hi-fi a lot more lately and the thump at switch on has 'gone'. Could it be that after a long period of not using it, the main smoothing caps discharge and take a large inrush current that somehow results in the thump? Equally, does it mean that those caps are on the wain or something?

17th Dec 2015, 12:48 pm	#1
stevehertz Dekatron Join Date: Sep 2011 Location: Rugeley, Staffordshire, UK. Posts: 8,835	Quad 405-2 switch on THUMPPP! The last few times I switched on my Quad 405-2 it had a very bad switch on thump, so much so that I feared for my speakers. May I say that I serviced the amp a few years ago after it developed problems that were the result of naff capacitors, so I replaced all of the offending Cs and others that were advised to be changed too. I've also got a kit of parts to effect the reduced gain mod as well, but not got around to fitting it yet. So, this switch on thump, it was never this bad? Any ideas as it seems to be something that has 'developed' rather than been there for all time. I may add, for better or worse, I use the setup's preamp, a Quad 44's linked mains switching system to switch on most of the associated components; CD, tuner in one go; all at the same time. But again, however that may affect 'the thump' (or not be good practise) it didn't used to be anywhere near so bad. The preamp was set to tuner, so maybe the Sony tuner has started to deliver a DC output at switch on.. dunno. __________________ A digital radio is the latest thing, but a vintage wireless is forever..

17th Dec 2015, 1:13 pm	#2
dave walsh Dekatron Join Date: Jan 2006 Location: Ramsbottom (Nr Bury) Lancs or Bexhill (Nr Hastings) Sussex. Posts: 5,817	Re: Quad 405-2 switch on THUMPPP! Mine's been in "retirement" for a while Steve-heavy aren't they? It started doing that and I read [somewhere] that it could be one of the very large blue electrolytics [web photo illustration] but I haven't opened it up. Counter-intuitively, it was suggested that the [somewhat over-engineered] protection circuits in this design can actually accelerate a problem in some situations... with a cascade effect. I think there's a dedicated mod kit to sort that. You've obviously got the ability to carry out work yourself but I decided to avoid disaster until I got mine over to someone suitably experienced. Another roundtuit, nothing has happened since! I'm sure there's more about this problem and the "fix" in threads on here and certainly in audio forums but you've probaly seen those. Don't take what I've said as necessarily accurate but I did get the sense that it is possible for something catastrophic to happen unless the full mod was done. I will be very interested to see how you go on with yours. Dave W Last edited by dave walsh; 17th Dec 2015 at 1:25 pm.

17th Dec 2015, 6:31 pm	#4
stevehertz Dekatron Join Date: Sep 2011 Location: Rugeley, Staffordshire, UK. Posts: 8,835	Re: Quad 405-2 switch on THUMPPP! Thanks Dave, Steve for your input and suggestions. Actually, I can't really see how the problem can be originating from anything else other than either the Quad 44 or the 405-2, as the inputs on the 44 will be capacitor coupled I'd think? So surely even if - say - the tuner was giving out a DC level at switch on, you wouldn't expect it to affect the amp? Come to that, I need to check if the 405-2 is capacitor coupled on its input, because if it is, then that means that the problem has to be within the 405-2 itself? Edit; just checked, the 405-2 input is capacitor coupled at its input. So, amp experts, does that confirm that the thump - that I presume is a momentary (or even standing) DC level at its output - is coming from within the amp itself? __________________ A digital radio is the latest thing, but a vintage wireless is forever..

18th Dec 2015, 1:04 am	#5
coil1234 Pentode Join Date: Jan 2006 Location: Almeria, Spain Posts: 221	Re: Quad 405-2 switch on THUMPPP! Hello, I had a similar problem, with one channel on a 405 (BBC AM 8/16). I tried recapping and replacing and replacing the supply Zener and associated series resistor. I finally eliminated it by replacing one of the small signal transistors towards the front end. Unfortunately I'm working away at present so don't have access to my notes. IIRC it may have been towards the +ve supply rail? Possibly a constant current source? Regards Mike A

18th Dec 2015, 10:48 pm	#6
timewave Pentode Join Date: Apr 2006 Location: Clitheroe, Lancashire, UK. Posts: 173	Re: Quad 405-2 switch on THUMPPP! Turn the volume control to zero and disconnect the speakers and measure the DC across the output terminals at switch on. Leave the voltmeter on for about 20 minutes and check the volts periodically. You should be seeing less than 15mV, preferably 0V after that time. You will see an initial voltage spike if there isn't a circuit to delay loudspeaker connection on this amp. If there is a delay circuit then maybe it isn't working. If the standing DC speaker volts are as mentioned above check, the delay circuit. If the standing volts are higher then they need to be reduced. I'm assuming this is a DC coupled amp and there should be a preset type pot to do this. John G4IJD __________________ Old radio engineers never die, they just go intermit...mit...mit...mit...

19th Jan 2016, 11:53 am	#7
stevehertz Dekatron Join Date: Sep 2011 Location: Rugeley, Staffordshire, UK. Posts: 8,835	Re: Quad 405-2 switch on THUMPPP! Typical me, I've not got around to doing any proper analytical testing yet, but I have been using my hi-fi a lot more lately and the thump at switch on has 'gone'. Could it be that after a long period of not using it, the main smoothing caps discharge and take a large inrush current that somehow results in the thump? Equally, does it mean that those caps are on the wain or something? __________________ A digital radio is the latest thing, but a vintage wireless is forever..