Gulbransen Theatrum expression pedal not working

[AMDguy]

Hi all,
I recently found a Gulbransen Theatrum on craigslist for free. All of the functions on the organ work perfectly, except for the expression pedal. The expression pedal just does not do anything. Is there a specific combination of stops or buttons necessary for it to work (i do not know much about the Theatrum)? I would assume that something is wrong with it. The pedal has what seems like a lot of play when moved back and forth.

[andyg]

It's faulty. The pedal should affect the volume of everything, it doesn't matter what sounds are selected. The pedal should move freely throughout its travel, but stay where it is when you take your foot off it.

The pedal will either have a volume control potentiometer (perhaps a dual gang one as the Theatrum is two channel) or will control the volume with a light bulb and a light sensor. Bulb failure is the most common issue. Many organs are designed to default to full volume if the bulb fails.

Maybe one of our Gulbransen owners will be able to tell you what you'll need to fix.

[geoelectro]

The pedal itself has no lamp or pot. Instead, it has a series of switches with graduating resistors to create a variable resistance. On the preamp board, there is a lamp under a metal cover with the LDR's. It could be a bulb failure or the lamp driver circuit. You can locate the preamp board and pull off that cover and see the lamp.

Geo

[andyg]

Thanks, Geo. Trust Gully to use a different way of doing things!

[AMDguy]

interesting... I will check that out. The pedal does move freely throughout the entire travel, and it does stay put, but there is a ton of back and forth play in the pedal... I will post a video showing issue. A bulb failure sounds like that's what it is, as the organ has indeed defaulted to full volume, and it is rather annoying, as it is so loud that all of my family members complain that they cannot have normal conversation...

thanks,
Christian

[AMDguy]

ok, so after being away from home (and the organ) for a while, I finally got the back off. I have a service manual coming in the mail, but I am impatient, and I cannot seem to fine the preamp board, and I really don't want to break anything.

[geoelectro]

You have to lower the electronic panel once the back is off. Black frame with two screws on the left and right holding it in position. It will hinge down. Behind that panel should be the preamp board on the shelf. It will have a couple of rectangular metal "boxes" that house the lamps etc. They just lift off with a mild snap. This is all located about mid height and to the left or treble end of the organ.

Geo

[AMDguy]

perfect, thank you