This vintage Sansui amplifier was putting a weird popping sound on the speakers when the volume or bass was turned up.
Putting a 1kHz test tone into the amp and connecting the speaker outputs to a dummy load and an oscilloscope showed a nasty glitch on the positive part of the audio on both left and right outputs every 20ms when the volume was increased beyond a certain level.
20ms (milliseconds) is equal to 50Hz which is the frequency of the UK mains supply. That’s a big clue as to where the fault is as nothing in this amplifier generates a 50Hz signal.
I repair hifi and audio equipment. Please contact me for a repair quote if you need something repaired.
A look at the main amplifier power rails with an oscilloscope showed the positive supply having nasty ripple on it when the volume was increased. Turning the volume up too high caused the positive power rail to totally collapse!
This was due to a faulty smoothing capacitor.
As the amp was old I replaced both of the big main smoothing capacitors and a number of smaller ones associated with the amplifiers other power supplies.
Not an easy job, the capacitors were held in with the worlds strongest glue that had to be cut away with a scalpel. Trying to pull the unsoldered capacitors out was causing some nasty bending of the circuit board due to the glue!
It looked like a number of electrolytic capacitors in the audio path had recently been replaced so I didn’t replace those.
There’s a fine line between repairing an amplifier and carrying out an unplanned recap / restoration at the same time. Recapping sounds like a simple task but in reality it can add hours to the job as the capacitors have to be selected, priced up, ordered and fitted.
The output power transistor bias was bang on which is a little unusual for an old amp unless it’s been repaired recently, which I suspect it had.
Both the left and right speaker outputs on the oscilloscope showed them to be virtually identical when putting different frequency sine waves through it at various volume levels so the electronics appeared to be in good shape.
I’ve uploaded a video about this repair to my @orangevalleysystems YouTube channel. This is a popular video gaining 1000 views in just four days. YT videos fizzle out after about a week due to the algorithm so every view counts!
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