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Old 2nd Jan 2017, 12:59 am   #1
AndyW NZ
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Default Quad II, sudden distorted and low output

Happy New Year everyone,

Just looking for some advice on troubleshooting one of my Quad IIs please.

The output has suddenly dropped to a very low level, with considerable distortion. Swapping valves with those from the healthy QII doesn't help.

Coupling caps have been replaced a couple of years ago. HT is very high (459V) when measured between L1 and the GZ32.

Any suggestions on what might cause these problems? Obviously something's very wrong for the HT to be so high, but I'm reluctant to leave it on for an extended period in case more problems occur without at least checking to see if anyone's seen this before.

Thanks for any suggestions,

Andy
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Old 2nd Jan 2017, 2:16 am   #2
Techman
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Default Re: Quad II, sudden distorted and low output

This should be a very easy fault to diagnose.

The output valves are obviously not passing any current, so you need to find out why.

The cathode resistor may be open circuit / gone high in value - easy fix.

There could be an open circuit within the output transformer - not so good.

Whatever the fault, some static resistance checks with the power off should soon track down the fault.

WARNING - before making any static resistance checks, check for any stored voltage within the HT smoothing capacitors as this type of fault can sometimes cause them to retain a high charge for some time after disconnecting from the mains!
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Old 2nd Jan 2017, 2:39 am   #3
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Default Re: Quad II, sudden distorted and low output

As an addition to the above -

I don't have the circuit diagram of this amp in front of me but I'm guessing that L1 is in the HT line between the smoothing capacitors, so any open circuit, or more likely a high resistance, as you're getting some distorted output, along the HT feed line could also be causing this fault.

As said, careful and methodical resistance checks should soon find the fault, but make sure those smoothers are discharged and the unit is disconnected from the mains.
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Old 2nd Jan 2017, 5:38 am   #4
AndyW NZ
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Default Re: Quad II, sudden distorted and low output

Hello Techman,

Thanks for the advice. Spot on - it was the cathode resistor, and not the output transformer, thank goodness. Usually if there's two possibilities it's the expensive one, so I feel lucky!

Odd though, no sign of discolouration or anything else around the cathode resistor. Marked Erg, 180R, 5%. Quick temp replacement from the spares box and we're up and going.

Thanks again for your help, good to be going again.

Andy
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Old 2nd Jan 2017, 7:33 am   #5
Audiophile_AU
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Default Re: Quad II, sudden distorted and low output

Hi AndyW NZ,
I used to have Quad IIs, and the output valves cathode resistor is a weak point, in that the original wattage rating of it is barely adequate, and it runs incredibly hot. Maybe adequate for the UK's chilly climate, but elsewhere....
It is s'posed to have 26V across it, dissipating theoretically 3.75 watts in a resistor rated at 3 watt- hmmm. I suggest replacing it with at least a 10W (wirewound) resistor.
Enjoy the sound
David
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Old 2nd Jan 2017, 10:01 am   #6
AndyW NZ
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Default Re: Quad II, sudden distorted and low output

Thanks David, I'll get on to RS and get something well rated. Right now in the Australian summer it feels like everything's running hot, not just the resistors....
cheers

Andy
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Old 2nd Jan 2017, 11:08 am   #7
nutteronthebus
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Default Re: Quad II, sudden distorted and low output

HI here is the cct diagram for a Quad II hope they help in the future


Dave
Attached Files
File Type: pdf Quad+II+Mods+Chris+Beeching+92.pdf (919.4 KB, 68 views)
File Type: pdf Service+Manual+Quad+II-40a.pdf (1.63 MB, 58 views)
File Type: pdf QII+Classic+User+Manual.pdf (252.0 KB, 41 views)
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Old 3rd Jan 2017, 6:19 am   #8
AndyW NZ
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Default Re: Quad II, sudden distorted and low output

Thanks Dave, much appreciated.
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Old 3rd Jan 2017, 12:53 pm   #9
frankmcvey
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Default Re: Quad II, sudden distorted and low output

H'mmm - might not be a good idea to overspecify the common cathode resistor by too much, because if it ain't the weak link in the current chain any more, then the smoothing choke or the OPTs might well take its place next time!

Keith Snook, Quad guru, mentions the cathode resistor problem and suggest a 5 or 7 watt replacement on this page.

Cheers,

Frank
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Old 4th Jan 2017, 2:53 am   #10
AndyW NZ
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Default Re: Quad II, sudden distorted and low output

All good points. I've ordered 5W ww replacements and I'll see how hot they get....

Thanks for your help everyone, good learning experience!
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