My SV-1 73’s right channel was about 6dB down on both the line outs and the XLRs.
Flat across the whole range, no distortion, didn’t change with patch, volume knob
or EQ - just quieter. Same sound, lower level.
Every thread I could find on this stops at “it’s capacitors” without ever naming
one, so here’s the actual designator.
IT WAS C78 - a 10uF 16V SMD electrolytic on the main board, one of two cans sitting
directly underneath IC19. C79 is its left-channel twin. It hadn’t lost capacitance;
its ESR had gone up, so it was acting as roughly a 4k resistor in series with the
audio.
THE BIT WORTH KNOWING EVEN IF YOUR FAULT IS SOMETHING ELSE
A coupling cap that dries out and loses capacitance makes things THIN - the bass
rolls off and the top stays put. A cap whose ESR has climbed makes things QUIET at
every frequency equally, with no tone change at all.
So: level down + tone unchanged = something resistive has appeared in series. That
one distinction is what pointed at the capacitor instead of at a board.
THERE SEEM TO BE TWO DIFFERENT CAP FAULTS THAT LOOK IDENTICAL FROM THE KEYBOARD
A) Output board, near the XLR jacks. Caused by phantom power getting into the
outputs. Caps visibly bulged, usually more than one, often distorted or cutting
in and out.
B) Main board, C78/C79 under IC19. Just age. Nothing visible whatsoever. Clean,
flat, undistorted level drop.
If you already replaced the bulged ones by the jacks and it didn’t fix it, you’ve
probably got B. That seems to be why these threads keep ending inconclusively.
HOW TO CONFIRM B IN TEN MINUTES
Power it up, hold a sustained note, scope on AC coupling, ground clip on the chassis
and LEFT THERE. Touch the probe to one pad of C78, then the other pad.
A healthy coupling cap drops essentially nothing across itself. Mine dropped half.
Then do exactly the same across C79 as a control. If BOTH caps show a drop, your
technique is off, not your capacitors - check the ground clip before believing it.
Measuring across one part beats comparing left against right, because both readings
come off the same probe seconds apart, so drift cancels. Mine wandered 300-500mV
otherwise.
TWO GOTCHAS THAT COST ME TIME
-
The service manual has two board layout drawings and only ONE is marked “TOP
side”. R166 and R170 are on the underside. C78 and C79 are on top. Read the wrong
drawing and you’ll hunt for parts that aren’t on the face you’re looking at.
-
The LEFT-hand can is the RIGHT audio channel. And the two caps are fitted
mirrored, positive terminals facing each other. Don’t assume both go in the same
way round.
PREVENTION, WHICH MATTERS MORE THAN THE REPAIR
Never send phantom power to the SV-1’s XLR outputs. 48V reverse-biases and
over-volts the output coupling caps and takes the output stage with them. Use the
line/TRS inputs, switch phantom off on those channels, or put a passive DI in
between. Phantom is often switched in banks of eight rather than per channel, so
check what you’re actually arming.
This is one instrument, found by measurement and confirmed by the repair working.
I’m not claiming every quiet SV-1 is this same part. But it’s a pennies component
and ten minutes of measuring, and worth ruling out before you buy a board on a
guess.
Fair warning: it’s a surface-mount cap on a dense board. The part is cheap, the
removal isn’t a beginner job - hot air, or patient rocking on alternate terminals.
Pads lift.
Full writeup with an annotated photo of the board and the signal path:
Will see if I can find someone local who can complete the electronics side of things. Will probably have a go at dismantling and checking the capacitors myself as I’ve found a lot of guidance online