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Thread: Rodgers 990 Stops Issues

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    ppp Pianississmo Hurdy Gurdy Man's Avatar
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    Rodgers 990 Stops Issues

    I've been gradually restoring a Rodgers 990 organ that had various power supply problems, but am now trying to troubleshoot some of the stops. Four different stops are operating improperly, exhibiting various combinations and permutations of stuck on, stuck off, and indicators that disagree with what is being heard. I've got the Technical Manual, but it appears to be quite deficient, so I'm requesting 15 minutes of discussion with somebody who still remembers how these beasts work. My background is retired electrical engineer with Wurlitzer training, so I should be able to find my way around a 990 with minimal assistance. Looking forward to working with folks on this site!

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    ppp Pianississmo Hurdy Gurdy Man's Avatar
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    Update: Working on Stop stuck ON, found weird voltages around Q25, D25, & D26. Removed transistor -- it's fine. Diodes seem to be OK. Can't figure out what's downstream that could be causing trouble. Help!!!

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    ppp Pianississmo Hurdy Gurdy Man's Avatar
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    OK, I was looking at weird voltages because I was probing a spare circuit! (Guess I don't quite know everything about a 990 yet.) So all is well with Q25, D25, & D26 on the actual stuck Stop. The Stop switch works fine, and the indicator turns on and off. But the sound is still stuck on, and it's very annoying, because it just happens to be the 1 1/3' Nineteenth Stop. So here's the real question: On schematic drawing number 1396F, the cathode of D26 is shown to connect "TO STOP." Where exactly is that destination? (Which schematic, and which component?) That's where the problem is, but I can't figure out where "there" is! Thanks in advance to anybody who knows their way around this beast.

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    fff Fortississimo toodles's Avatar
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    1 members found this post helpful.
    Check Q301 for the 1-1/3 ft. stop on Rack 5, the Great Ganging Board. That's the transistor that ultimately stops keying voltage from reaching the Great Diapason Keyer when the stop is off. If it is open, the stop would always be on. See schematic 1295, upper LH corner. If you ground the bus that the diodes collect (emitter of Q301), it should turn off the stop. That would tell you that Q301 isn't turning on to shunt the keyer voltage to ground. Q301 could be the problem, or the capture system voltage to turn on Q301 could be interrupted--possible broken wire or defect in the capture system.

    For this problem, the manual suggests QCR 25 might be defective, and R's 26, 24, and 42 might be open if the stop will not turn off--these are all on the combination action system.
    Last edited by toodles; 09-30-2018 at 03:58 PM.

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    ppp Pianississmo Hurdy Gurdy Man's Avatar
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    Wow -- Thank you so much!!!

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    ppp Pianississmo Hurdy Gurdy Man's Avatar
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    OK, I'm down to my last problem. Everything on the 990 now works except that the Lighted Capture System will not capture a Stop that is turned OFF. In other words, I can set a Stop to ON and save it to memory, but if I set the same Stop to OFF and then save it to memory, it's still on when I read back the setting. This is true for all Stops and all keyboards. Needles to say, my memory is filling up with ON Stops! Something has failed that writes "zeros" to the core memory. Any ideas?

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    Moderator jbird604's Avatar
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    Can you set a stop "off" with the "Hold-and-Set" method? (Press a piston that is already set, then, while holding in the same piston but NOT the set piston, turn the stop off, wait a half a second, then release the piston. Then check to see if it set as "off.")

    The magnetic core memory erases all settings each time a piston is depressed as part of the "read" cycle (so-called "destructive read.") Then it immediately re-writes the registration memory by checking for "on" stops, continuing to do so as long as the piston is held in. Thus, holding a piston while changing combinations will alter the recorded registration for that piston without the use of the set piston.

    If this method works, and I suspect it will, you probably don't have anything wrong with the memory system itself. Other than that, I can't tell you why it isn't setting properly with the set piston. Maybe some kind of trouble in that circuit. But you can theoretically do without the set piston by doing all your registrations with hold and set.
    John
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