SynthGuy's Synthworld: A Quick Peek Under The Hood...The Moog 930 Powerful Supply!

Here's a look at the Moog 930 power supply, the supply that powers the larger 35 and 55 modular systems.

This module has been on my radar for a while as a prime candidate to be refurbished. It's the lifeblood (the beating heart, if you will) of the whole system, and aside from cleaning an intermittent pot decades ago, it's never had (or needed) any other service.

At 40+ years old, it's certainly more than ready to have the various electrolytic capacitors replaced. These type of capacitors don't last forever, and even though the ones used in this supply were very high quality devices, the time had come for them to be swapped out with nice new ones, and have the entire unit completely cleaned and refurbished. I've read that you can typically expect about 10-20 years of reliable service from most electrolytic caps, so these have done very well indeed!

The supply creates 4 different voltages...+12V, -6V, +15V, and -15V. Each voltage has its own separate card complete with adjustments to set output voltage, current limit, and overvoltage trip points.

Interestingly enough, the two 15 volt cards play almost no part in the Moog 55 operation. The only thing powered by the +15 volt supply is the trigger buffer card, which just draws a few milliamps of current, and the -15 volt card is not used anywhere in the system. These are both fairly robust cards too, with each one capable of supplying 1.5 amps of current. The only use for them I could find was at the accessory connector in the back, where you would connect a Moog ribbon controller, sample and hold controller, or drum controller, and none of these draw anywhere near that much current.

In the schematics for the CP3-A mixers they are shown as being connected to the 15 volt supplies, but in the cabinet wiring diagrams and in reality they are only connected to the +12 volt and -6 volt supplies. These supplies are even brought up to the top cabinets through the cabinet interconnect cables, but unused.

As you might imagine, I have big plans for these supplies in a future project...

...Yes, big plans indeed...Excellent.

Although Moog put their name on this, the individual supply cards were actually made by Powertec (apparently no longer in business) and are marked on the boards as model 1607. A nice bit of kit back then, and still pretty nice even today, especially after some much-needed maintenance.

This is what the cards looked like before being refurbished. Although the caps looked to be in pretty good shape physically (no cracks or bulges in the rubber end seals), measuring them showed that they were at less than 50% of their rated value. Not really surprising, given their age.

I replaced them using a combination of Sprague and Nichicon high reliability capacitors. This should carry the system at least several decades into the future. While I had the cards out for servicing, I replaced the large power transistor mica insulators with new thermally conductive pads to help them transfer heat to the large black anodized heatsinks more efficiently, and run cooler.

Each card had the old dried up heat sink grease underneath the card mounts removed and replaced with fresh thermally conductive paste to help transfer heat from each card's heatsink to the metal chassis for better cooling.

Here's what a card looks like after being cleaned and refurbished with new caps.

All connector points are cleaned by being lightly burnished to remove any corrosion with a small square of 3M Scotchbrite green scrub pad sprayed with a little Deoxit D-5, then wiped clean with a soft cloth. The same treatment is applied to the screws used for the connectors as well.

Here's the newly refurbished cards being re-installed back into the chassis, which has been previously cleaned inside and out with Windex to remove any old heat sink grease left behind.

Now it's starting to look powerful!

I created a duplicate AC input plug to power up the supply outside the Moog 55.

This will allow me to completely test and adjust the individual cards before the assembly is returned to service inside the modular system. All of the pins of the two Molex connectors (one is AC power in, the other is DC voltages out) are scrubbed with a small stiff nylon brush and D5, then cleaned with a soft clean cloth. This will remove any built up corrosion and make sure we get a solid electrical contact for all of the supplies.