This mixer power supply belongs to a Soundcraft Delta 200 desk which I restored. It is pretty old. On first inspection the output voltages were correct, the only problem was that the mains transformer made an annoying buzzing sound. Looking more closely, it had other issues.
This post shows the kind of things I pay attention to when restoring or rebuilding old equipment. The devil really is in the details with this kind of work.
INSTABILITY OF AUDIO SUPPLY RAILS
Although the rails measured fine with a voltmeter, and could be adjusted for +/-17V with no problem, examination with a scope showed that there was some oscillation at the outputs. What you can see below is about 1V pk-pk scillation, in sync with the AC mains – the regulator is just starting to oscillate as the unregulated supply dips during each mains cycle. This would never be detectable without a scope, and of course it can degrade audio quality quite seriously.
What was needed was the addition of some capacitors very close to the legs of the regulators. Because they are mounted on the rear panel heatsink, and the way the unit is designed (the -17V regulator is actually a positive one used upside-down), this was a bit of a fiddle. Mounting the caps on the circuit board was a little easier, but not as effective – there was still some low level oscillation, visible as about 20-30mV of “fuzz” looking the rail with the scope. Pics of the regulators before and after rewiring:
(The capacitors I used are 0.1uF ceramics, rated at 50V or so – fairly standard for this task.)
The sleeving on those very old soldered connections is hard and brittle, and turns out to be hiding a solder joint which is cracked. This is why I prefer unsleeved connections, unless connections are so close together that a short circuit is possible (which they never should be, really). Sleeves can hide all kinds of problems. A joint like this will become an intermittent connection, which is worse than having the wire just fall off.
Once the regulators are rewired with the capacitors soldered right on their legs, which is where they need to, there is a huge improvement. Oscillation is completely cured. The rails are stable to better than 1mV, the measurement limit of the scope. This is how things should be.
The old transformer is a laminated type, and due to age it has started vibrating. The vibration is getting amplified by the case of the unit. Very annoying – this is a fanless supply, so you should be able to park it in the control room and hear nothing. The solution is to create a damper between the base of the transformer and the chassis of the unit. The transformer still vibrates but it is now almost impossible to hear.
This is the inside of the unit after rebuild. An overheated resistor (which still worked despite being burnt black) in the 48V supply has been replaced with an equivalent of higher power rating (actually three in series). One of the smoothing caps looked a suspect, the top was bubbling up, indication of failure. Although it seemed to function OK, both were replaced.
I replaced the connector for the output wiring with soldered joints. I mistrust any connection that is not gold-plated (or “gas-tight” like wire wrap). They always oxidise. These ones carry the full current – about 1A in this case, more for a larger desk. Oxidisation causes a resistance that can vary up to about an ohm (it could be more) – that means that the ”regulated“ output might vary by up to a volt. Again, this would degrade headroom in the desk, and maybe cause defects in the audio itself. A few minutes with a soldering iron makes sure that this cannot happen. Good soldered joints last for decades!
All of this adds up to about a day of work, but at the end – the PSU will work perfectly for years to come.