Fascination is the key to quality

Repairs on a bellow chest and other organ parts English text Nederlandse tekst

May 2016


The organ of the church of Our Lady of the Snows (Onze-Lieve-Vrouw ter Sneeuw) in Borgerhout (a suburb of Antwerp, Belgium), of which I am organist together with titulary Johnny Verbeken, was built in 1958 by Jos Stevens. It is a 40 stop instrument, utilizing two types of windchests: bellow chests and direct chests. The direct chests are mainly used for the stops that have extensions (units) and for the prospect pipes. The other windchests are so-called bellow chests.

On 10 May 2016 I executed two thrilling – at least for me – repairs on the bellow chests. Furthermore, I repaired two less complex defects in the action. Both for myself and the church council I made a lot of photographs and I like to share my experiences via this web page, consistent with my motto "fascination is the key to quality".

What is a bellow chest?!

A bellow chest is a kind of a windchest. Windchests make up the heart of the organ: they are the large wooden boxes on which the pipes are placed. Inside are pallets (or ventils or valves), that determine which pipe gets 'wind' (the organ term for compressed air) and hence 'speaks'. In history several kinds of windchests have been devised to let pipes speak as accurate as possible.

A bellow chest is a kind of a stop channel chest (or stop chamber chest) that is not very common. (Actually, I don't know what it is called in English, this seems to be the most logical term but any advice is welcome.) Jos Stevens built this kind of chest in the nineteenfifties and as usual every organ builder had his own variant. With a stop channel chest, all pipes of one stop are placed on a hollow space, called channel or chamber, that is only under pressure if the stop has been drawn. The wind-ducts to the feet of the pipes are normally closed by felt discs or pallets that are glued onto small V-shaped bellows. The photographs below will clarify this, I guess. These bellows are glued upright onto the wall of the stop channel. The bellow is under continuous wind pressure, and therefore the duct to the pipe is closed. If a key is pressed, a electromagnet opens a cone or disc valve and the compressed air inside all bellows belonging to that key can escape. The bellows do therefore collapse and if a stop is drawn, the compressed air within the stop channel of that stop can enter the duct to the pipe foot, letting it speak.

Repair of two defects in the chests of the Great organ

Since several months, the Great department of the organ of the church of Our Lady of the Snows has two defects that have to do with the workings of the bellow chests. The fourth D of the higher pitched stops of the Great keeps sounding sometimes if the key is no longer pressed. The second defect is the second E of the Trumpet of the Great sounds always as soon as the stop is drawn. These defects are of rather complex nature and therefore also rather thrilling.

In the first place, the cause of the problem must be identified and that is only possible if you understand the inner workings of the bellow chest well. It all comes down to that the defect is not what you would think it is. If the higher pitched stops keep sounding, the problem is not the bellows of these stops. By considering the inner workings and devising clever test the solution can be most possibly found.

In the second place, working on the heart of this kind of organ is not without risk, because it was malfunctioning just because of a tiny wind leak. A large board must be screwed from the bottom of the windchest and this board is normally connecting tens of ducts of compressed wind and must be mounted completely leak free for the organ to keep functioning properly. With pneumatic chests, the tiniest wind leak can already cause malfunctions that are difficult to understand and traced down. The cause can only be confirmed after the bellow has been cut loose from the chest (although one could also test it by stopping a duct by hand). And in the end I must of course be able to repair the bellow …

Now for the images. Below you see the underside of the lower windchest of the Great organ, which is for the smaller pipes (descant). We see in front the magnets and behind them the windchest bottom. These 'channel boards' are fixed with many screws, to prevent tiny leaks from the one duct to the other. At the bottom of the photo you see the concrete tiles that are on one of the regulator bellows of the Great organ. It is also clear that the lack of room doesn't make this an easy repair.


Here is the unscrewed windchest bottom board. This rather heavy board is fixed with 29 large screws. Inside are 12 channels that are part of the tone action for the pipes belonging to 12 keys. Normally these channels are under continuous wind pressure. All the visible black traces are caused by (dirt in) the air and near the throats they point to slight leaks. But OK, screwing a large board like this one air tight to the windchest is not an easy job.

bodem balgjeslade bodem balgjeslade detail

If a key is pressed, the magnet opens a valve and the air inside the channels and bellows can escape and the bellows are compressed. There is one bellow below each pipe, as described earlier. The channels running from front to rear in the second photo below, are the stops channels and these are filled with compressed air as soon as a stop has been drawn. If a bellow collapses, the felt pallet glued onto it no longer blocks the throat and the air from the stop channel can go trough the duct to the pipe above it.

The black traces on the white sheep leather, that serves to make the connection of the chest bottom to the body of the windchest air tight, also point to small leaks, causing the dirt in the air to stick to the leather.

balgjeslade open balgjeslade open

In the pictures below the bellows, the ducts that inflate them or let them collapse by the surrounding air in the stop channel, and the felt and leather pallet that is glued onto the moving board of the bellow can be clearly seen. These pallets block the duct that conducts the air to the pipe above it.

de balgjes in de balgjeslade

After I had reasoned which bellow would be the culprit, I removed it by cutting it loose with a chisel. The outside of the bellow didn't show damage or leaks. Indeed, now the bellow has been removed, it was clear that it leaks.

balgjeslade waaruit een balgje is verwijderd het uit de lade gestoken balgje

Yes! The leather on one side was worn away. By the way, the spring gives the bellow, independent of the air pressure inside and outside, a preference for the inflated, non-collapsed state.

het uit de lade gestoken balgje

The thin pneumatic sheep leather has been cut from the bellow. There are two cardboard boards that give the bellow its shape. You can clearly see how the bellow is constructed.

de belering is van het lekke balgje gestoken

From a new piece of sheep skin a new leathering is cut.

nieuwe belering

Old and new above each other.

nieuwe belering

After a little tinkering, this is what the newly releathered bellow looks like.

het balgje is opnieuw beleerd het balgje is opnieuw beleerd

It was glued into the windchest and the chest bottom was fixed with the 29 screws again. And guess what? It works!! The malfunction has been cured and no test could be executed that points to a remaining defect. Furthermore, the whole windchest and key action functions perfect again.

The Trumpet clearly shows a different error. The continuous sounding of one pipe cannot be caused by a leaking bellow, think about that! This windchest bottom was fixed with 'only' 20 screws and was removed. Removing is a little complicated because you have to work above the smallest organ pipes of the Great descant. Here the bottom is removed indeed.

ladebodem van de baslade van het hoofdwerk is verwijderd

Ah! The culprit is a disc pallet that had come loose of the bellow. In this way the duct to the pipe cannot be blocked any more. Do you see the culprit?

een balgje vertoont een half losgelaten ventielschijf een balgje vertoont een half losgelaten ventielschijf

I glued the pallet onto the lower discs that were still glued to the bellow board, without removing the complete bellow from the windchest. We are lucky in the sense that it was the front stop: if it had been one of the rear stops, working on the bellows over the small pipes would have been complicated!

Malfunctioning stop

There was a third problem: the bass of the Octaaf 2' of the Great did not function. Cause: simply a leaking pouch (if that's the correct translation of what we call a membrane in The Netherlands). You do not see it is leaking, but after cutting it loose, it is clear: a leak! This is a frequently occurring defect in (electro)pneumatic organs due to the ageing of the vulnerable thin leather, but it is easily cured. Moreover, it is not not justified, as is sometimes done by protagonists of the slider chest, to condemn stop channel chests like these as unreliable: these pouches function flawlessly for periods of tens of years and over that time these organs function without malfunctions.

registertractuur van de bas van het hoofdwerk simpel: het membraan is lek door veroudering van het leer

This pouche has been renewed.

Silent large prospect pipe

Fourth problem now: the first G of the large prospect Contrabas 16' doesn't speak. The key contacts and the contacts in the couple rail cabinets turn out to function properly (the couple rail cabinets are in the lower right of the back of the console in the photo below). By making other electric contacts it turns out that the key contact shows signal.

This is a large organ, so the console is indeed also complex and a impressive specimen of electropneumatic organ building.

interieur van de speeltafel met koppelkasten

This large pipe is in the prospect on a prospect chest. The pallet turns out to function, so somewhere in between there must be an error. It turned out that there was a bad contact near the magnet. This was repaired.

For comparison: this is the console from the 'normal' side.

De speeltafel van Onze-Lieve-Vrouw ter Sneeuw in Borgerhout

And so we drove home with a satisfied feeling …

> Next page: dismantling of an electro-pneumatic organ

Kind regards,
Rens Swart