Quad 520F buzz (no input)

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18-Nov-2017 12:50 PM

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Posts: 6

I've got a Quad 520F with a buzz on one channel (with nothing connected to the input). It still passes audio. Other channel works perfectly.

It appears there has been a previous failure on the channel and some parts have been replaced, notably there is an excess of thermal paste around some of the transistors which seems suspect to me. Nudging one of the 40872s sometimes makes the buzz louder, but so far nothing I've tried makes the buzz quieter, including checking tracks and re-soldering some joints. 

The legs of the 40872s pass through holes in the heatsink with insulated boots on, could a wayward blob of thermal paste be the culprit here?

Any ideas of what to look at/test next?

Thanks


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20-Nov-2017 03:37 AM

EJP

EJP

Posts: 1525

This will be as simple as a loose connection of the earth braid of the signal connection between the PCB and the input sockets.

The silicon paste is non-conductive and not a concern. Some of those power transistor cases have over 50VDC on them: do not touch!
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20-Nov-2017 10:22 AM

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Posts: 6

Already tried that I'm afraid, worked on another amp. 

It is looking like a ground related issue however. 

And yes, fortunately I know enough about what I'm doing to not get killed(!)

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21-Nov-2017 12:26 AM

FBatista

FBatista

Posts: 119

Greetings, 

I'd need to know if it's possible to order a toroidal transformer for a 606 MK2.

Please reply with cost and shipment cost to the sender of one of the emails I've been sending to Dada Electronics Europe (info@dadaelectronics.eu
), regarding this issue.

Best,

FBatista


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21-Nov-2017 12:55 AM

EJP

EJP

Posts: 1525

Please start your own thread with an appropriate title. You're lucky I even saw this.
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21-Nov-2017 09:11 AM

Joost Plugge

Joost Plugge

Posts: 962

Check R40 and C14, they connect the circuit ground to the chassis, if one of them is open circuit there will be hum. 

Joost Plugge

DaDa Engineering

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21-Nov-2017 10:14 AM

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Posts: 6

Thanks, will test those

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26-Feb-2018 08:25 PM

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Posts: 6

So R40 and C14 seem to be fine. 
However, this now appears to be a more severe problem than first thought. 
On the affected (noisy) channel R16 and R17 are running extremely hot, and eventually the 3A fuse in my (test) mains cable blew.

Help!

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11-Jul-2018 02:47 PM

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Posts: 6

The buzz was caused by the power supply capacitors, replaced and now sounding and measuring great. 

Lesson learned - always replace all electrolytic capacitors first!

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