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Old 12th March 2015, 06:22 PM   #1
diyAudio Member
Join Date: Oct 2014
Default Quad II hums

After 5 months of waiting, I managed to grab a Quad amp locally and after recaping this unit - dismay! This amp causes a hum without any external units or cd players connected to it. It does play properly but the hum is extremely annoying. In fact the humming starts immediately as soon as the Quad 22 control unit is switched on without anything else connected except the speaker. I have changed all the valves but to no avail. If I connect my good amplifier instead the problem goes away.
Close inspection of the problematic amplifier reveals the following:
1. For some strange reason, the last owner had put a 0.22uF capacitor in place of the 0.1uF cap C1. This cap had been removed while recapping.
2. Most strange of all, the voltage selection wiring for 220V and 240V had been purposely removed from the voltage selector to the transformer by someone! Only the 200V primary voltage selector was left wired in. Upon comparing the mains transformer winding resistances with a good one it became clear why: Instead of this winding resistance increasing with the supposedly 220V and 240V positions, it does the unexplicable because the resistance drops dramatically! Maybe the transformer in the can is not an original and the 220V and 240V positions are something else? The markings on the transformer are handwritten and instead of the black tar I see that a yellow wax like substance has been used to pot it!
I have not done anything else to it apart from changing the capacitors as one of them had leaked and the others were out of spec. The resistors seem to compare favourably with my good amp.
Any thoughts or suggestions before I remove the tranformer and put it in the amp that has an overheating one? At least this one seems warm to the touch when left playing for 30 minutes.
Thanks and regards,
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Old 12th March 2015, 06:55 PM   #2
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Join Date: Dec 2010
Location: Amesbury, MA
Does the hum stay when the EF86's are removed?
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Old 12th March 2015, 08:08 PM   #3
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Join Date: Dec 2014
Location: London
Originally Posted by Alnasir View Post
. In fact the humming starts immediately as soon as the Quad 22 control unit is switched on . . .
does this mean, with the 22 unit switched off or disconnected you have no hum?
if that's the case, it sounds like an earth loop.
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Old 12th March 2015, 08:44 PM   #4
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Join Date: Oct 2014
Thanks for your replies!
Test1. Without the EF86s the hum remains but is lower in volume!
Test2. Put the EF86s back in. With the Yellow 6 Pin plug that carries the earthing from Quad 22 controller removed the hum continues.
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Old 13th March 2015, 10:41 AM   #5
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Join Date: Dec 2014
Location: London
Without EF86s I guess the input stage is out, however, if turning 22 introduces the hum without the first stage, I guess it points the finger at powersupply, perhaps 22 is drawing too much current.
-Is the hum still there, without 22 unit connected, but with EF86 in?
(if no, then investigate the 22 unit)
- is the hum there with only output tubes in, no input tubes - no 22?
(if yes, investigate powersupply and balance in output stage)

Just guessing though!
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Old 13th March 2015, 09:47 PM   #6
cerrem is offline cerrem  United States
diyAudio Member
Join Date: Jun 2005
Location: San Diego, CA
Can you determine if it is 60 Hz or 120 Hz hum ???
If it is mains frequency....the HUM would typically be due to a imbalance in the AC with respect to center tap.... I have had bad tube sockets cause HUM issues that took a while to locate...since PIN 3 and 2 are next to each other...if the socket was damaged at one time, there could be leakage between the pins.. This is where you will see a 120Hz supper-imposed on the 60Hz heater frequency....
Measure your AC ripple on the filter caps to see if they are doing their job...
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Old 14th March 2015, 12:38 AM   #7
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Join Date: Oct 2014
Hello and thanks for your suggestions!
Today, I changed the mains input transformer from the hummimg amp to the good but overheating tranformer amp: and the hum passed on to "good" amp! I let it play for 30 minutes just to make sure that the overheating problem had gone. Then the most amazing thing happened when I changed the speaker: normally I do all my testing on my vintage Scott speakers: I connected my Quad ESL57s and the hum disapeared! Perhaps the earthing on the Quad ESL57s has something to do with this! Then I connected one Scott speaker to one of the amps and an ESL57 to the other amp only to get the hum back. Any thoughts on this?
Anyway, for the first time I had the chance to listen to a full Quad combination. I will keep the 3rd amp for parts as the choke and audio tranformers are quite expensive.
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