Compton Sonatina Maintenace

[jonnymcc]

Hi All.

Long story short, I've recently taken an interest in Organs; we'll I've always had an interest, but I've managed to get my hands on a couple of electronic and repaired them.

The latest one I looked at was a Compton Sonatina at a local church. I was totally shocked when i opened up the back; I've only had the cover off electronic organs so far; so the belts and pulley threw me a little!!

Anyway; the fault was crackling which seemed to be caused by poor contact at some of the tone generators earthing straps. A gentle clean up seemed to sort it out.

Couple of questions I'm hoping someone can help me with.

1. How on earth do the electrostatic tone generators work?????? I see a mention of a 500v output on the power supply so i assume they need some high voltage!
2. I feel that these earth points could do with a little dab of some sort of lubricant. I'm thinking petroleum jelly? Any advice?
3. Anything else I should do with regards to the tone generators?

This organ is fitted with a transistorised amp; I've seen mention of valve versions. There is a bit of hiss in the amp but to be honest you cant really hear it with the belt noise! Any other maintenance tips for this model?

Where would one get a belt should one ever be required?????

Any advice greatly appreciated.

Regards,

Jonny

[Lucien Nunes]

Hi Jonny

Welcome to the world of electrostatic tone generation!

1. For an explanation of the whole tone generation system see: http://www.electrokinetica.org/d8/2/index.php and the following pages. Sonatina generators will be single sided unlike the one shown dismantled but the principle is the same.

2. Faulty contact at the earth brushes can indeed cause crackling, as can debris inside the generators, and it is usually random and variable. Regular, rapid 'machine gun' sounds usually result from damaged coating inside, loose or warped parts or maladjustment of the rotor clearance. The reason for the earth brush is as follows: The rotor is AC coupled for signal, relying on capacitance to pick up the waveform and transfer it to the amp, but it needs a reference to ground at DC otherwise it would tend to pick up stray charge and drift to random voltages, causing background sing and noise. Mounted in the rotor is a tiny high-value resistor made of a pencil line joining the working part of the coating to the hub / shaft, through which the rotor sees its earth reference. In the early generators there was no earth continuity through the bearings and the brush was needed to complete the circuit. In later ones with screening on the outside, the bearings are theoretically earthed but the carbon brush makes better contact. They do not normally need lubricating and doing so may degrade the contact - if they squeal it may indicate that too deep a hollow has worn in the carbon. For replacements, belts and any other parts I recommend the helpful and knowledgeable Robert Cook at comptonorgans.com.

3. Lubricate generator, idler, trem and motor bearings with two drops Anderol 465 every once in a blue moon. That is the original recommended stuff, I have it in stock here. Make sure the belt runs true without touching the pulley flanges. Clean off any dirt buildup on the pulleys. Avoid opening or dismantling the generators or interfering with their shaft adjustment unless a problem is suspected. E.g. don't unscrew pulleys just to take a look!

Let us know how you get on!

[jonnymcc]

Brilliant, thanks for the wonderful information. I'm always pleasantly surprised by hour genuinely helpful the organ community are .

You brought up another point in your response; the belt does indeed have a bit of a wobble and does touch the side of the biggest pulley each lap. I take it replacement is the solution; to my thinking the belt must be out of shape?

With regards to the earth straps, any sort of cleaning I could do to stop the crackle? Even a wee drop off solvent out contact cleaner say on the end of the spindle?

Thanks, I'll get some that oil and give the old lady a wee drink!

Thanks again ,

Jonny