quad 303 Crackling

tony_ives

Member
2008-02-28 4:36 pm
Hi Folks

Has anyone any experience of a quad 303 power amp developing a fault where it crackles like mad!
I use one for monitoring in my little studio and have done so for 20 years or so! I have changed the electrolytics in it, some 13 years ago!
It is a very useful amp as it provides a true audio representation of my mixes, which will then sound equally as balenced on others' systems. Exactly what I need.

The fault sounds is as if the mains fuse is loose or plugged in to dodgy adaptor, that sort of thing. I have obviously checked out such things. It is common to both channels. I don't use the 33 preamp.

Thanks

Tony
 

tony_ives

Member
2008-02-28 4:36 pm
Hi Burbeck

Thanks for that. Yeah spent about 15 years in the repair trade and have tried freezer, nothing seems to be worse or better for a squirt so to speak!

It's definately power supply as i can see it on the scope. When I disconnect red and Blue wires 67V and ground from the PSU, Rail looks good. I will load it later on today to make sure, if all is well I will have a gander at the little PSU board.

Thanks again for your reply. I intend to publish my findings to help anyone else who may experience this later on
 

bigpanda

Member
2004-06-19 11:30 am
HKSAR
Hi,

If any of you guys happen to look for a replacement board for it, you can email me.

Slightly modified to use PNP-NPN for output and to-126 for drivers.
 

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tony_ives

Member
2008-02-28 4:36 pm
Quad 303 Crackle

Hi Folks. I hope this proves useful to someone...............
I was pretty certain that this fault was power supply related, as it affected both channels. After a dose of freezer and a good visual inspection. I had to fire the scope up (had to find it first!)

The DC rails showed the crackle quite clearly, noise and dips on the trace as opposed to a nice flat line. I have a schematic so was able to disconnect the feed derrived from the caps and the bridge, from the PSU board, enabling me then to check transformer, main electrolytics and Bridge in one foul swoop so to speak. Remember to ground your scope from the negative side of the caps, blue wire in my case. This waveform looked good to me. I figured that as the amp crackled like crazy even at idle with no speaker load, then it was a good bet the fault would lie further on. ie PSU board or Tranny.

I checked on ebay and there's a guy on there called Ivan (his ebay name is "Gazember")who upgrades Quads regularly. More importantly for me he sells off the "pulls" (Old PSU boards and Trannies) Sure enough £24.00 and a week or so later I have board and Trannies in my hand. I went for the PSU transistor first. Figuring this was what took the most abuse. The old one measured ok (never a good sign) and of course there was no difference, however replacing the little PSU board did the trick and she's now happily back where she belongs. singing like she always did.
Tony