ADR modules part 1

This is a restoration project I’ve been procrastinating around since about 2015. This will be quite some work, but then if one doesn’t start at least it will not happen at all. This will be a step by step project, and see how far we get with this Kaizen like approach.

So first a short intro of the four audio modules made by Audio & Design (Recording) Ltd, in the mid 70s based in Shinfield Green, Reading, Berkshire UK. The company still exists to this date and is still active in pro-audio and is now in a different location but still in Reading.

These modules can be dated to the mid 1970s, more precisely found the E900 Equaliser unit quality checklists to be dated 12 November 1976. There are two E900 equaliser modules, and two F760N Compressor/Limiters.

A picture of the now dismantled mixing desk with the four modules.

The mixing desk was in a quite dusty and sorry state. The desk was too space consuming and heavy to keep and many other parts were sold as separate items. The ADR modules and it’s power supply were stored in a box.

The first part to revive is the old 24 volt DC power supply.

It was a DIY jobby close enough to the original circuit from the ADR manuals! It’s just a transformer, full wave bridge rectifier, 2000uF electrolytic capacitor and a 7824 regulator. The manual shows a ’Fairchild 7824’, but our circuit uses a L7824, a more recent variant by SGS (merged into ST Microelectronics in 1987).

A picture of the DIY power supply under test (without load).

The electrolytic capacitors should be replaced because they have a limited lifetime. However it seems there is some life left in these specific ones.

Measured the ripple voltage over the buffer capacitor, about 40 mV peak-peak, which will surely increase with load, but not bad considering the age of the capacitor!

The output shows a nice DC flatline, oscilloscope in most sensitive setting. No hum whatsoever due to the 7824 regulator.

We’ll have to make an enclosure for the power supply, since there was no enclosure when it was removed from the mixing desk. Preferably it will need to shield any remaining 50Hz hum from the toroidal mains transformer and safely enclosing all parts exposing the mains voltage, and also needs some natural air flow to cool the 7824 regulator.

It might make sense to put it into an enclosure similar in size to one of the modules. This will be for a next episode of this series.