Rodgers Exeter 770- Krummhorn locked on

[Billsacto]

Hello,

I hope someone can direct my repair efforts, a little bit. My dad plays a Rodgers Exeter 770 in the local church here in Roseville California. He asked if I could help him fix an old nagging problem. The Krummhorn is stuck on, but the stop works fine. The stop indicator light goes on and off normally when the stop is actuated. One more symptom: it makes a slight popping sound in the speaker when the stop is actuated.

I have the schematics, but I'm not familiar with organs in general or the logic flow of the schematics. I see in the Logic Block Diagram the Krummhorn Stop Control is listed under Chain 4 coming from the CPU. However, I'm not able to find any kind of on/off control in the Krummhorn keyer schematic.
Again, any direction from the community would be immensely helpful and appreciated.

Bill

[jbird604]

"Straight" stops on the Rodgers LTG/serial keyed organs are generally switched by a 4094 chip, which is a sort of latching switch controlled by a specific bit in the data chain to which it belongs. The 4094 chip in charge of the Krumhorn audio may be the culprit. It could be either on a krumhorn keyer board or on the audio pre-amp board, I don't recall.

You should begin by following the krumhorn audio path from the output of its keyer and voicing cicuitry to the audio pre-amp board. In a pinch, since it's surely better to have have "no" krumhorn than one that's on all the time, you can clip a lead somewhere to disable it until you're able to fully troubleshoot the problem.

[toodles]

The Krummhorn is switched on and off by an FET audio switch on the Krummhorn keyer board (Q123 on my 840 schematic, probably the same on the 770). The switch is controlled by the data stream, so either the data stream is corrupt (i.e., defective U11 4094 as per JBird's response), there's a physical short on the board between the 4094 and the FET, or the FET transistor is shorted.

Should you want to disable the Krummhorn until the issue is resolved, you can just turn down the pot controlling the Krummhorn volume on the preamp board.

[Billsacto]

John,
Thanks! I turned down the Krummhorn for now. Great! But it also killed Carillon. Bad for the Christmas season!

I saw the 4094 shift register chip on the Krummhorn keyer board. Seems like there are sets of 4094s for each of the stops that may be selected and which data chain it belongs to.
So is it the 4094 the chip that "turns off" the Krummhorn? On the Logic Block Diagram, I do see "Krummhorn stop control"

[toodles]

The data chain that is delivered to the krummhorn keyer via the 4094 IC's includes the note on and off information for each note plus the stop on and off info for the krummhorn stop. The stop on and off info is the "krummhorn stop control". The microprocessor bundles the note on and off info along with the stop on and off info into shift register the datastream.

There should be a krummhorn level pot on the krummhorn keyer board--turn that all the way down and you can silence the krummhorn without affecting the carillon and harp.

[Billsacto]

Looks like you were correct. I am getting switching (high/low) at the FET, but not turning off.

Do you have any idea what the part number of the FET is? It's not listed on my print for some reason.

Thanks again.

Bill

[toodles]

Well it took quite a bit of looking in various Rodgers tech manuals, but that FET is a J175. Rodgers Part No. 1414-151, for reference.