How to repair and make as new a vintage synthesizer

It makes sense to change all electrolytic and tantalum capacitors which have a limited life.


Examples:


An Oberheim DMX was dead. After changing the tantalum capacitor that was connected to a start control pin of the CPU it came to life.


An EKOsynth could not get to 440Hz. After some electrolytic capacitors were changed it did. After all electrolytic capacitors were replaced the filter envelope attack control was still not responding. After the only 2 tantalum capacitors were replaced it come fantastically back.

The worst enemy of electronics

Dust

AKG D130 microphone with Pleiades filters or Pleiades V4 or V5 preamplifiers

What a microphone!

Dynamic (moving coil) mics are really underestimated. And omnidirectional ones have something so natural...(Simplicity bringing excellence).

Setup:  (the symbol "-" means connected to...)

D130 - Pleiades (in line XLR) filter - Step up transformer (in line XLR) - Pleiades V4 (1 stage EF183 battery powered pre preamplifier) - Realistic Disco Mixer (mic in) - Sennheiser HD580 headphones

Not so much low cut was needed as omnidirectional mics do not have a proximity effect.

Nevertheless a Pleiades filter of 100 to 250Hz did a great job of restoring presence in voice (Fletcher Manson and voice effort curves) with the added advantage of removing the muddy room acoustics and annoying background noise.


So the inductance connected in parallel to the mic out was of the order of 210mH to 140mH. These were measured with a digital Escort ELC-120 L meter by measuring pins 2 and 3 after disconnecting the mic. The reading taken is the parallel combination of the Pleiades filter and the primary inductance of the transformer used. Interesting combination is a Canford impedance converter (400mH) in parallel with a Pleiades filter of 200Hz (166mH).  Other natural combination was a 200mH primary inductance step up transformer and a Pleiades filter of 100Hz (333mH).

Bravo to AKG

One of the best microphones ever made.

Neutrik modules with switches and color code for variable Pleiades microphone filters

Many Neutrick modules can be connected together.

Inside them some 073 Magnetc Nanoperm cores.

the Neutrik KMX modules colored according to each 073 cutoff frequency ie how many turns are wound on each 073.

Every 073 inductor in parallel with pin 2 and 3 through the Neutrik SM2/2 switch.

So we have a long XLR adapter with rainbow series colors according to frequency.

4 filters would give 16 combinations (binary code).

Example

0000      0Hz

0001     100Hz

0010     200Hz

0010     300Hz

0100     400Hz

Etc when Pleiades filters of 100Hz, 200Hz, 400Hz, 800Hz are used

Cleaning microphone XLR output connector

It is very easy.


Using ear cotton buds moistened with alcohol.

Variable Pleiades microphone filter box

Various low cut, high cut, mid reduce or mid boost etc can be implemented as follows. There are useful for restoring psychoacoustic presence due to Fletcher Munson and voice effort curves.


Everything is connected in parallel to signal ie on pin 2 and 3 of mic output.


Multi tap inductor or various switched inductors in series with a variable resistor and the whole lot in parallel, for high pass.


Same but with switched capacitors for low pass. This is parallel with 2 and 3 too.


Switched capacitors in series with switched or multi tap inductors in series with a variable potentiometer resistor. The resultant in parallel again. This is for reducing a certain band of frequencies and making a mic with a peak to become flat.


Switched capacitors in parallel with switched or multi tap inductor, a series variable resistor and the resultant in parallel with signal. This can be useful for increasing a middle band of frequencies to restore psychoacoustic presence.

Color code specification for Pleiades XLR in line proximity compensating filters

Inside the Neutrik XLR modular adapters we have a Magnetec 073 Nanoperm core. This core is wound as inductor and connected in parallel to signal ie to pins 2 and 3. This is to low cut (or high pass) as needed to compensate for the unnatural increase in bass at various mic distances. (A directional mic at 1in as we can see on most microphone frequency response charts starts to rise in sensitivity below even 1KHz!)


Specifications of Pleiades inline filter color code.


Black : deep deep bass.     ?

Red     : only the deep bass is cut.   700mH

Orange : bass cut.     300mH

Yellow : bass and mid bass cut.  140mH

Yellow-Green : natural.     70mH

Green : natural and bright.    47mH

Blue.    : bright.                25mH

Violet    : very bright.        14mH

White.   : extremely bright.  eg 7mH


Inductance values are indicative. A white filter can make a ribbon mic sound amazing at 1in while electromagnetic damping from the Pleiades filter protects to ribbon from blasts of air pops etc. for a Grampian ribbon microphone 3 mH gives very interesting and natural results. (The high frequency mic content shines too.). For best results this is implemented as a Pleiades 1:4 step up transformer to make up gain, the 073 core fitted inside the XLR adaptor.


Similar codes can be used for Capacitor in parallel filters to cut high frequencies and give presence.


Reference : (Fletcher Munson curves and voice effort curves as explained in the Hollywood paper in earlier euroelectron posts):
Sound Picture Recording and Reproducing Characteristics - D. P. Loye and K. F. Morgan
Journal of Society of Motion Pictures Engineers
(Presented at the 1930 Spring Meeting at Hollywood, California)