quad 520 power amp fault help

aardvark

aardvark

aardvark

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leeds
So, i've had this amp for 20 years and i bought it second hand then, so its old.
Now, it always made the speakers thump a little when turned off, but i moved recently and now when i turn off the right channel is more a crackle fart when i turn off.
Still plays music fine.

i contacted quad and they say send to them, which i don't really want to do because:
a - its 12.5kg and will cost a fortune to send (and likely fix)
b - they have a 12 week turn around anyway

what do people think?
i opened it up and can't see anything dodgy.
i'll probably look for a local place to maybe have a look at it if i can find one, but if any one knows what causes this kind of problem i would appreciate it.
 
The cause of the thump is the main filter capacitors discharging. All amps do it if the outputs remain connected to the speakers. Most amps have a muting circuit to stop turn on/off thump. If you leave music playing and turn the amp off, it should play for a few seconds before getting quieter and distorted just before going silent.

This thread on DIYaudio suggests the main filter capacitors could well be the cause, though it would be wise to check all of the power supply components and all of the electrolytic capacitors. (test them if you can rather than assume no leaks/bulges means ok) The problem is, those BHC capacitors in the 520f are 5 pin which is practically non existent. I have the smaller Quad 240 which uses normal 2 pin snap in caps which is handy. There is a good bit of information contained here regarding this amp.
 
well, i think i might have found the culprit.

I took the cover off to take some photos to send to someone and there is a blown fuse on the right channel. Its just above the big capacitors in the picture.
Not sure what it does - i'm surprised it still worked. Maybe it is just to connect to the capacitors so the right channel wasn't using them?

I'll get another fuse and see what happens!

C1PvcgJ
 
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If a fuse has blown that typically means that something has failed/shorted and the current has blown the fuse. It could be a cap that's failed and shorting for example.

There's a few 'quad' people in the diy audio forum as Koi has suggested. If you're not used to high power/current then I would take the hit of sending to a quad specialist/quad rather than the local place.

It does sound like something needs attention and that it may need matching to get the right sound.
 
Its the normal 520, not the f version, although i'm not sure what the difference is.

I do find it odd that i have a blown fuse and both channels are still working though (and its definitely in stereo).
Its an T2A fuse though, not an FS100.

i know the 520's are notorious for being modified - certainly the xlr pins are connected in a non-standard way.
 
FS100 is just the component number in the schematic, usually there would be a label on the PCB to tell you it's that. (Fuse 100)

T2A is simply a time delay 2 amp rated fuse.

I have the schematic for the older 520 non f and that shows quite a different PSU arrangement. The secondaries are fused on the centre taps and the one side in addition to the main supply fuse. Fuse numbers in the schematic are FS1A, FS1B, FS2, FS3A, FS3B. FS1A & 1B are the secondary supply fuses. FS2 is the main supply fuse. FS3A & 3B are the centre taps.
 
aardvark said:
i've checked the manual and the T2A fuses are the FS3A/B fuses.

whats a centre tap?
Click to expand...
It’s essentially the half way point in the secondary winding of the transformer. It gets used as ground as it should sit pretty much at 0V. This fuse going will simply affect the grounding on that channel, which may be why it still works. It’s obviously got another path to ground.
 
Kei said:
It’s essentially the half way point in the secondary winding of the transformer. It gets used as ground as it should sit pretty much at 0V. This fuse going will simply affect the grounding on that channel, which may be why it still works. It’s obviously got another path to ground.
Click to expand...

thanks
 
there is one thing i forgot about, but when i moved i didn't have any sound in the right channel - i traced it to broken solder in the speaker cable connection (because its xlr i soldered my own speaker cables together).

i guess that could have shorted the fuse maybe?
 
Certainly could short and blow the fuse.

Most amps don't pass the final signal through fuses - it can degrade the sound quality. Instead a path is setup so that a short will cause high current flow that will blow the path down that path. This could be all the way down to the power supply - typically to cut off the smoothing decoupling capacitors. Unfortunately if a crowbar isn't present then you will simply pass the current through the existing amp until the fuse blows. There's a saying - the electronics are there to protect the fuse.

For safety you never fuse the ground otherwise the power has only one way to ground - another route either through the circuit or worse - you.
 
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