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Old 18th December 2012, 01:50 AM   #1
diyAudio Member
 
Join Date: Mar 2011
Location: Gisborne, New Zealand
Default Quad 405-2

Greetings from the country bringing you the Hobbit. I have a customer who has dropped in their amp and preamp

1x Quad 405-2 Amplifier. Using a Quad44 preamp. With the 405 completely disconnected from any input, using a 3 core IEC plug, I have a low level hum coming from both outputs. Both outputs have exactly the same level hum.

From reading this message board I have replaced C5 on both channels. I have found R2 on both boards have gotten very hot almost to the point where the resistors would have blown. Even chassis cover has burn marks. Have replaced both of these resistors, but no change.

From a distance, I can't hear the hum, but can hear it if I have my ear right up against the speaker. Personally, I would have expected a small hum. You can't hear it when music playing at all.

I have checked some of other electrolytic caps and checked transistors for any leaky readings. Nothing out of the ordinary found. I'm sure the problem is in some way linked to both R2's getting identically hot but not sure where to go from here. Any advice appreciated although may require pictures if possible. I see there has been quite a few threads on this unit with lots of people having hum problems. I thought I'd make a new thread incase anyone has newer information.

Thanks
Tony

Last edited by blindreaper; 18th December 2012 at 03:04 AM.
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Old 18th December 2012, 03:21 AM   #2
Tajzmaj is offline Tajzmaj  Slovenia
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Join Date: Mar 2011
Location: Ljubljana
Is amplifier still original? I mean all transistors and op amps. There are several possible things. One is of course oscillation. If there is some very ''modern'' op amp installed without decoupling capacitors than it could be the reason. Other possibility is loose connection from input connector ground to chassis. This will result high output offset (some 20mV instead of 1-5mV) and maybe even hum. Too fast output devices and drivers could also oscillate and some capacitors are necessary to add. Of course there is one more thing. Big 10.000uF filter cans are also probably in bad shape. Temperature is quite high in closed quad's enclosure.
My advice. Don't listen hum but first measure output offset and if possible check output with scope to confirm if there is any oscillation. If R2 kick the bucket you will get nasty oscillation at the output and it will fry tweeters instantly. Believe or try. If you need service manual send me an email.
Cheers, Taj
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