I've got a 520F that is 100% perfect on Channel 2, but Channel 1 has a persistent and annoying buzz, even with no input connected. Passes audio fine with no distortion.
Also farts/motorboats for a second when powering off.
Rail voltages seem ok.
Things I have tried, that didn't work:
Re-seated all internal connectors (worked on a different amp!)
Replaced C9 and C11
Replaced all output devices
Any ideas of what to look at next?
Thanks
Also farts/motorboats for a second when powering off.
Rail voltages seem ok.
Things I have tried, that didn't work:
Re-seated all internal connectors (worked on a different amp!)
Replaced C9 and C11
Replaced all output devices
Any ideas of what to look at next?
Thanks
If the buzz is at twice mains frequency, check the smoothing capacitors. Otherwise there is probably an unbalance in the common ground circuit.
Replacing the smoothing capacitors fixed the buzz.
Then C100 exploded after a few hours...
D8 and D9 are measuring suspiciously (but not dead short).
Everything on the PSU board seems ok (except for poor old C100).
Does that make sense to anyone?
Then C100 exploded after a few hours...
D8 and D9 are measuring suspiciously (but not dead short).
Everything on the PSU board seems ok (except for poor old C100).
Does that make sense to anyone?
C100 is one of your new smoothing caps, if we're on the same page here. It shouldn't explode unless you got the polarity wrong and/or the voltage rating is insufficient. Also, the thing to do when powering up after replacements is check the voltages on the supply rails to ensure they are balanced at roughly +/- 50V, (I'm guessing that voltage because there is no mention of voltages on the schematic). Replacements would probably need to be ≥ 63V rating to have a safety margin. The choices are fewer now, so even 80V rating may be necessary. Note John Snell's comment about the buzz wrt the common ground circuit (his terminology)
Balanced supplies are important, particularly with Quad gear because they insist on using each secondary winding on the transformer in a separate power supply for each channel, making it a dual mono arrangement of a sort. As the amplifiers still require balanced rails, the central earth point is set by a 'virtual earth' generator comprised of transistors T100,T101 and some resistors in the channel shown. If these are snuffed, your capacitors may get an unbalanced share of voltage and one may have had the much larger share, exceeding its rating - you know the rest. The problem may have been there before you touched it but I'd check the other channel voltages in case there is a nasty problem lurking there too.
Balanced supplies are important, particularly with Quad gear because they insist on using each secondary winding on the transformer in a separate power supply for each channel, making it a dual mono arrangement of a sort. As the amplifiers still require balanced rails, the central earth point is set by a 'virtual earth' generator comprised of transistors T100,T101 and some resistors in the channel shown. If these are snuffed, your capacitors may get an unbalanced share of voltage and one may have had the much larger share, exceeding its rating - you know the rest. The problem may have been there before you touched it but I'd check the other channel voltages in case there is a nasty problem lurking there too.
Last edited:
- Status
- Not open for further replies.