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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 21st Jun 2018, 5:58 pm   #1
TonyDuell
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Default Quad 405-2 stock problems?

I have read that the original Quad 405 had a couple of under-rated resistors that caused mayhem if they failed and that it was a good idea to replace them.

Does anyone know if the 405-2 has any similar problems. I have such an amplifier (it started life as a plain 405 but was upgraded by Quad) which needs very minor repairs. While I have it apart I wonder if there's anything I should replace.

I am not interested in 'audiophile' upgrades using exotic capacitors for no technical reason. Just things that have an electrical justification. And I do not intend to start a debate as to whether Quad amplifers are 'the best', whatever that may mean. I have one, I want to use it (partly for sentimental reasons...)
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Old 21st Jun 2018, 6:15 pm   #2
Craig Sawyers
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Default Re: Quad 405-2 stock problems?

Same problems with the 405-2. R7 and R8, 3.3k between the 50V rails and the 15V zeners D1 and D2 for the 741 op-amp. If/when one goes open circuit it kills lots and lots of silicon.

They dissipate 0.37W and the part is a 0.25W rating, mounted hard to the circuit board. Replace them with metal oxide 1W part, Farnell 1357889. And space them off the board to allow air to circulate.

The other thing that need attention, depending on the issue of the board, is C10. That can be directly in the heat plume from R30 and R31, which cooks the capacitor - but only on one channel! Anyway replace those too, and mount the replacements on the back of the board to prevent them suffering the same fate.

Craig
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Old 21st Jun 2018, 6:20 pm   #3
KeithsTV
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Default Re: Quad 405-2 stock problems?

The resistors in question are R7 & R8 (3k3) and looking at the service data it appears that the 405-2 uses the same underrated resistors so it may be best to replace them with higher wattage parts.

Keith
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Old 21st Jun 2018, 6:21 pm   #4
TonyDuell
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Default Re: Quad 405-2 stock problems?

Thanks. I remember reading that the problem was the supplies to the op-amp. I guess if one opens, the op-amp output crashes into the other supply rail and causes a lot of damage.

I wondered if the 405-2 had higher power rated resistors there from the factory, but I guess not. I will certainly replace them while I have it apart.
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Old 21st Jun 2018, 6:23 pm   #5
KeithsTV
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Default Re: Quad 405-2 stock problems?

I've had both fail, one in each of the channels but it proved the crowbars on the output worked preventing the speaker cones ending up on the other side of the room!

Keith
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Old 21st Jun 2018, 6:33 pm   #6
TonyDuell
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Default Re: Quad 405-2 stock problems?

I'll replace them, for sure! RS do suitable 1W resistors so I'll add some to my next order.
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Old 21st Jun 2018, 10:40 pm   #7
Craig Sawyers
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Default Re: Quad 405-2 stock problems?

Quote:
Originally Posted by KeithsTV View Post
I've had both fail, one in each of the channels but it proved the crowbars on the output worked preventing the speaker cones ending up on the other side of the room!
The problem is that if one resistor goes, the output transistors go to rail (one way or the other), and the crowbar operates. Then the output transistors and drivers blow because they have been shorted to ground. Then the crowbar blows because of the massive current. Then the fuse blows after everything else is toast. Because silicon always blows to protect the fuse.

But hey - I've seen electrolytics the wrong way round (manufacturing error) in a Quad 33, so they certainly were not error prone either in design or manufacture. But since the designers - Peter Walker and Mike Albinson - are both in the hi fi shop in the sky, we'll never be able to ask them about it.

Craig
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Old 21st Jun 2018, 11:22 pm   #8
KeithsTV
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Default Re: Quad 405-2 stock problems?

I was lucky when the first one failed as no silicon was harmed but when the second one failed it took out both fuses, both output transistors and the crowbar triac.

Keith
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Old 22nd Jun 2018, 10:26 am   #9
nutteronthebus
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Default Re: Quad 405-2 stock problems?

If you visit the DADA site they have a lot if information on the Quad 405/405-2 including a kit to upgrade them. http://www.dadaelectronics.eu/Home.htm you need a login but it is free

Dave
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