Rebuild Of An Audio Mixer Power Supply

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 original regulator wiring
rewired regulators with 0.1uF decoupling capacitors

(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.

TRANSFORMER BUZZ

rubber grommets glued underneath the transformer to reduce acoustic buzz

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.

MISCELLANEOUS ISSUES

the inside of the PSU after rebuild

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.

output connections soldered for reliability

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.

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8 Comments

  1. Nice work. I acquired a Delta 200 myself a week or so ago with a blown CPS150. Needed lots of work. Higher rated bridge rectifiers, R32 resistor mod, replaced blown R1 and R2. Hardwired the totally burned up voltage selector switch and board, then grommeted the transformer. Tweaked PR1 2 and 3 and its working fine again. Haven’t added the caps to the heat sink regulators but will consider it. Great post.

  2. Hi Dan, Big Thanks for this great post, I ve bought CPS150 power supply & want to modify it.
    Just simple Q : 0.1uF decoupling capacitors is connecting to which grounding point ? I see black wire’s going to touch circuit on somewhere but i don’t know whith earth point is the best. thanks for yr help !

  3. Hi… Nice work on the PSU there… I have recently bought a second hand soundcraft desk that has a CPS 150. I think it has some issues… For starters the voltage Leds dim out and come back on a few times at first start up and the desk makes low end oscillation noises.. I’m thinking the PSU is the main problem because these issues fall away as things get warm. The desk has some dodgy pots,but these I can do.. The PSU? I’m well out of my depth…. Would you consider taking a job like this on?. …
    Cheers…

  4. I don’t know if you’re aware, but Tim Jones at Studio Systems doesn’t seem to renovate many Soundtracs desks these days, but he does manufacture new power supplies for most analogue desks. A few years back I went to see a Soundtracs Solitaire that had had a new PSU built by Tim. He sometimes builds them within the case of the old Soundtracs psu case, as this one was. I asked the seller if I could have a look inside. Tim’s PSUs use switched mode power supplies to provide the rails (usually +17, -17V and 48V), with one unit per supply. I know a Spirit Studio user who has had one of the new SMPS (switched-mode power supply) supplied in the ‘Blue Dog’ case. I think Tim mentioned on a thread that he used Meanwell SMPS. What are your thoughts on this? Tim swears by this approach, saying the noise floor is much better than the original manufacturers’ PSUs.

    Like I said on another comment, I’ve just found your site and musings so I’ll have a look at your other stuff.
    Al

    • I know Tim quite well, he has sometimes recommended me to European clients to do work on SOundtracs. I haven’t used his PSUs but I hear they are very good, and he knows what he is doing!

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