The trunk is the main body of the pump system, in a pedal-operated player piano.
It is basically a long box of wood with holes (ports) cut in it, over which various pneumatics and bellows are placed. The trunk will contain:
- Exhausters: The exhaust bellows are the lungs of the player system; they are what constantly evacuate the atmosphere air from the system, to create the suction which in turn powers the system. Despite the fact that one pushes the treadles with one’s feet, the air is not pushed in to the system (a misconception), it is in fact pushed “out”. This is done by two sets of large flap valves. On the push motion of the pedal, the atmosphere from the system is drawn into the bellows, through the first one-way flap valve. The second step is that the bellows (which is under constant spring tension) closes, forcing the air which just entered to now evacuate via the second flap valve. More here.
- Reservoirs: as the name suggests these large pneumatics hold a reserve of suction to help regulate the airflow in the system. Otherwise the direct action of the exhausters would be too jarring and irregular. Unlike the exhausters (which are spring loaded to close), the reservoirs are spring loaded to constantly try to open. This makes them want to tend to constantly draw in suction, which is after all their job! They only have one duty, but it is an important one.
- Expression boxes and other peripherals: these have already been discussed in previous posts. On my pump unit, the soft and theme expression boxes are mounted directly on the pump, while the governor/accelerator is mounted separately on the underside of the keybed, connected by hosing.
- Treadle linkage: I will discuss this separately in an upcoming post.
Restoring: the first and most important task before reassembling everything is to clean and seal the trunk body. If the trunk envelope is not airtight, the piano will not play as it should. A leaky pump will be noisy and ineffective. Using your sealant of choice (traditional is shellac but I chose to use Phenoseal), go around the entire trunk box and check every nook and cranny where a joint is discernable. The surface of the wood itself is likely already sealed with black lacquer or shellac, but over time the joints may open up just enough to leak, so those have to be sealed up. If there are any splits or holes in the trunk they should be repaired (wood glue is suitable here).
Then it’s time to recover the exhausters and reservoirs (which you will have previously removed). If the boards were not damaged during removal from trunk or removal of old cloth, then they can be reused.
Let’s start with the reservoirs. You have removed the old cloth with heat, or water, or a power sander (let’s take it easy with that last method!). In the process of removing the cloth it’s best to leave the open end until last and take out the powerful V-shaped springs once the sides are first cut open. Do this carefully! Once cloth is gone clean off the old glue, which hopefully was hide glue. If it was a modern synthetic glue, you have my sincere condolences. There is probably leather gasket material left on the stationary board which needs to be cleaned off. Scrape off what you can, mechanically, without damaging the wood.
If there is residue left it could either be hide glue or thick shellac; glue comes off easily with water, but shellac will need denatured alcohol or methanol.
Check the condition of the hinge; replace as necessary.
Verify the condition of the springs, they are probably fine but if not replace them as well.
Before recovery I chose to apply a bolstering strap (about 2″ wide) across the open end, to help resist the strong tension from the springs, using the same ticking material as the hinge.
Reinstall the springs.
Prepare the pneumatic cloth, cut to rough shape then glue it back on.
Because of the large amount of surface area to cover with hot hide glue it is handy to have an iron to help set the glue, and smooth out any inconsistencies in the glue seam. Trim the excess cloth with a new razor blade, after the glue sets up but before it hardens completely.
Copy and make a new mounting gasket for the stationary board, and if the reservoir has a panel cover (to allow access to mounting screws), make a new edge gasket for it, and set all aside for now.
Here are a few photos from the process: