Bad Hum, dry joints and capacitors Service - Ashdown MAG
An Ashdown MAG 300 amplifier with a reported loud intermittent hum fault on the speaker regardless of volume control. Initially the first point of call is to measure the speaker terminals for any DC appearing on the output.
Any significant DC on the speaker output will generally make a hum. This amp all seemed clear. A great little workshop tool is a small insulated screwdriver, lightly tapping on the circuit board often reveals all kinds of issues, microphonic capacitors, bad joints, broken legs.
Suspecting a storage capacitor issue because of a) the lack of DC on the output, b) significant hum the power board was unscrewed.
The below picture reveals a fairly large dry joint causing half the rail DC supply to not being smoothed by the capacitor superimposing a full wave rectifier output on the supply rails.
The solution to repairing a dry joint is often half hearted by DIY repairs.
Often people who have a soldering iron just heat up the joint and add a tad more solder. This is ok however it still keeps the impurities in the solder joint and before long the fault will be back. The correct way to repair a dry joint is to remove the existing solder fully either using braid or a solder sucker. Tin any component leads with fresh solder with cored flux and reflow the joint with fresh solder leaving a small shiny valley. A good solder joint is shiny and looks neat, not a blob of matt crystallised metal.
A word of caution, ensure the capacitors are fully discharged before attempting any repair. Because they’ve not been connected in the circuit there may be residual charge that has not discharged!!
If you’ve like this guide but not confident to repair it yourself and close to Harlow, Essex or Hertfordshire then we can do this repair and service your amplifier at the same time!