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Gulbransen Rialto Model K

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  • Gulbransen Rialto Model K

    I'm in the process of trying to restore a Gulbransen Rialto Model K to working order. I currently have two remaining issues. The first is that for any of the C channel notes (C,D,E,F#, C#, and A#), when any of the string voices are used, the notes cut out (volume is reduced to nearly zero) when the expression pedal gets past halfway or a bit more. This doesn't happen for any of the C# channel notes on the strings or for any of the tibia voices. The same issue occurs for all of the bass pedal notes (they cut out when the expression pedal gets past halfway).

    The other issue I'm having is that whenever any of the string voices are used, there is a persistent C# that plays in the background (gets louder as the expression pedal is increased). This doesn't occur for any of the tibias, except for the Tibia 8 minor.

    If anyone has any ideas for what might be causing either of these problems, I'd appreciate the feedback.

    Thanks!

    Mark

  • #2
    The first problem is likely due to a broken wiper on the potentiometer mounted to the expression pedal. Cleaning won't help, you'll probably have to replace it. Note: some models used the pot to control a lamp and a light-dependent resistor (LDR); if I recall correctly, they required a linear pot, while models with the pot in the preamp signal chain used a logarithmic taper. Be sure to examine the type info on the device carefully, or connect an ohmmeter and try to determine whether it is log or linear taper (since you only have 1/2 the range to work with, that may be tough to discern).

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    • #3
      Oops. Forgot the second part. If turning on a stop within a family (e.g., strings) results in cipher, then you likely have a bad diode switch for that note on the offending stop. Try to determine which actual stop is the source of the cipher, and check the diodes for that note. In this case, it sounds like it may be a shorted diode, and if so, you won't even have to remove the diode from the PCB in order to check it - it will show as 0 or near 0 ohms, regardless of voltmeter probe polarity. If the diode is open, you may be able to find it by a similar method, comparing readings across similar diodes from other notes. If it's a leaky diode, this won't work and you'll either have to examine the circuit to see if you can temporarily short it out to get the same effect, or you'll have to remove the diode(s) to test them. Be careful of the old phenolic PCBs - the copper traces are easy to damage.

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      • #4
        I know this is an old post - if you are still having problems with the Rialto K and have not given up or gotten rid of it, please let us know as I have some service manuals I can scan and post that may help.
        Jimmy Williams
        Hobbyist (organist/technician)
        Gulbransen Model D with Leslie 204

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