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

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Old 24th Jan 2011, 9:29 am   #1
dsergeant
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Default Quad 44 Control Unit

This Quad 44 control unit suffers from a low level rustle, like some sort of electrolytic problem but it may be the volume control track. I cannot seem to nail it down, and it does seem to be worse after power on, tying in with the capacitor theory.

Although I have circuits in the user manual and have found a service manual, these both show the circuits for the earlier versions. Mine is s/no 24221, a very late one, and the tone control board where I suspect the problem lies is type 12784-1, rather than the earlier 12512-9, and it is very different. The volume control is a custom unit in rectangular black case which solders directly to the tone control board rather than flying leads on the earlier ones - if it is the volume control I cannot see it being available.

Does anybody by chance have the circuit of this later 12784-1 pcb, or any suggestions as to what is causing the problem.

Thanks
Dave
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Old 24th Jan 2011, 10:43 am   #2
arjoll
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Default Re: Quad 44 Control Unit

I know nothing about Quad - are there any op-amps in the 44?

The reason I ask - I had similar problems in one channel of an old Teac mixer - sounded like DC on the fader, but I couldn't measure any on there. Recapping didn't fix it, and changing the fader didn't fix it, but eventually I decided to change one of the dual op-amps and the problem just disappeared.
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Old 24th Jan 2011, 6:08 pm   #3
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Default Re: Quad 44 Control Unit

The board in question has four TL072 op-amps on it. It is possible that one of them is duff so I might try to see the noise on my scope. It would be a lot easier of course if I had the right circuit.... Thanks for the suggestion.

Dave
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Old 24th Jan 2011, 6:25 pm   #4
KeithsTV
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Default Re: Quad 44 Control Unit

This should cover your 44.

I've had 2 problems with mine, one was dry joints on the through board connections on the motherboard and the other was with a poor connection on the cable connecting the motherboard to the tone control board.

Keith
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Old 24th Jan 2011, 6:34 pm   #5
percival007
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Default Re: Quad 44 Control Unit

Is the 44 similar to the 34?

If so, this thread may help.

http://www.vintage-radio.net/forum/s...ad.php?t=46783

James.
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Old 25th Jan 2011, 8:56 am   #6
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Default Re: Quad 44 Control Unit

Dada Electronics can be trusted for decent advice, spares and tweaks, although the later 44 was better than the earlier one. They also have a "Revision Blogspot" which may be of help in service and troubleshooting.

At least Quad still have a service department in Huntingdon, although their repairs tend to be of the get-you-going type, rather than full refurbishments.
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Old 25th Jan 2011, 6:31 pm   #7
dsergeant
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Default Re: Quad 44 Control Unit

Many thanks KeithsTV for the circuits, they are indeed the correct ones. I did have one leaking electrolytic which had corroded the board, replaced that and cleaned it up. I suspect one of the other 100uF caps is the cause of the problems so will change the lot, hopefully tomorrow.

Dave
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Old 27th Jan 2011, 8:53 am   #8
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Default Re: Quad 44 Control Unit

Leaky caps affect the Quad 34 too...
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Old 31st Jan 2011, 4:07 pm   #9
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Default Re: Quad 44 Control Unit

I have finally managed to sort out this Quad 44. I had originally cleaned up the mess from the leaking caps, but obviously not enough. It took a bit of detective work, with the help of the Keith's circuits and isolating stages one by one to show that it came from the middle TL072 - IC702 on his circuits. When I removed that I found there was a small amount of gunge on its pins. A thorough clean of the chip and the pcb under it sorted out the problem. No crackles or the like, good nice audio.

Thanks for the help. I am sure the owner will be pleased.

Dave
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Old 31st Jan 2011, 7:48 pm   #10
Trevor
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Default Re: Quad 44 Control Unit

I went off quad with the 44 control unit ! They were expensive and when you consider all you are listening to is a TL072 op amp at about 1 I rather got diselusioned with them
regards Trev
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