[sdiy] Thomas organ-some stops have stopped working

Tom Wiltshire tom at electricdruid.net
Sun Jul 14 21:49:47 CEST 2013

Right, first off, I don't know anything about this particular organ. So I'm making lots of assumptions about how it works. I'm going to assume it's got some method of generating the top octave, followed by dividers and filters. If that assumption's wrong, then so  is everything else.

If the flutes work, that suggests that the dividers are ok, since you wouldn't have any pitches if those had died. So somewhere in the instrument there are square waves at all the right frequencies. Good start.
The different stops are done using filters, so it's possible that a particular filter board isn't working, if all those stops were on the same board. That might suggest a power failure to that board. If the failures are spread over many different  boards, that's more complicated. 
But if the stops are still audible at high volumes, that means that they still work, but the signals aren't getting through. That might rule out a power failure (since that would probably kill the signals completely). So then you're looking at the signal path. Why aren't the signals getting through? Perhaps the mixers that mix the stops together are at fault. On an old instrument like this, I'd look for symptoms caused by age. Corroded signal wires could have a much higher resistance into a mixer than they did when new. In fact, interconnect wires would be one of the places I'd start looking straight away. Broken wires are quite likely. A reference earth connection breaking off could give the effect you're describing with a load of signals not being properly audible.

Ok, I'm going to stop speculating now. I hope some of those ideas spark some useful idea for you.


On 14 Jul 2013, at 14:54, Dan Snazelle <subjectivity at hotmail.com> wrote:

> I got a Thomas californian 263 a while back...no schematic
> On the upper manual, the RED stops ( 16 trom, 16 bass, 8 trum, 8 diap, 8 clar , and 8 oboe have all stopped being audible EXCEPT at very high volumes.
> The flute stips all still work.
> I am trying to determine general causes of stops failing in organs
> Thanks!!!
> Sent from my iPhone
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