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Organ Farfisa 251S

Discussion in 'Troubleshooting and Repair' started by André Fischmann, Jun 12, 2020.

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  1. André Fischmann

    André Fischmann

    Jun 12, 2020
    Hello. A friend gave me an electronic organ Series 251S - 252S . I have the schematic diagram 1st edition. It's not working : The oscillator ICa1 SN 29730 (board PA-633) doesn't work, but probably because the voltage regulator SN72723N gives 0V on pins 2 3 4 10. I don't know the voltage to put in pin3 of oscillator : I have try to put a 3,3 kohm resistor between 12V and the potentiometre of 100 ohm but the voltage stays too low (0.9V). Could you help me because I don't find the data shets of theses CI, or tell me where boy a SN 72723 or a board PA-633 (6729*1124661).
    I have tried to cut out the output of the regulator and put 5V (with USB Power) at the Oscillator, without result. I don't find an equivalent IC from the SN 29730 with same pins. Thanks to help me !
  2. bertus

    bertus Moderator

    Nov 8, 2019

    The SN29730 is not an oscillator, but a dual one shot.
    It needs to be triggered to produce an output pulse.

  3. 73's de Edd

    73's de Edd

    Aug 21, 2015

    Sir André Fischmann . . . . .

    eeeWWWWWWWWWWWWW . . . . .dat dooooooo be one old one ! . . . . . . . but . . . . . who am I to talk ?
    Initially, why not check out these most logical possibilities . . . .

    Since your ICA1 ossifrier chip doth runneth at a 5 V supply level , initially check its pin 5 (GREEN box) for that voltage presence.
    If not there, it gets that voltage from pins 2 or 3 or 4 or 10 of the uA723 linear regulator ICA3, which in turn requires ITS own ~ +12.4 Vdc presence at the ( RED box ) that comes from the main power supply.
    Find that as ( RED box ) +12.4 at the main supply which is received from the emitter of Q2 transistor .
    No voltage there . . .? then perhaps Q2 has developed an opened emitter junction.
    If such is not the case, then back track to see if +27Vdc ( PINK box ) is present at its collector and +13Vdc at its base ( BROWN box ).
    No 13Vdc there, then might be due to a shorted Z1 Zener diode.
    The Q2 transistor receives its whole supply voltage back at the raw +38Vdc supply at the ( ORANGE box).
    What you finding ?



    73's de Edd . . . . .

    Ya' know what ? I just don't quite have a final solution yet, but I certainly do admire your problem.

    Last edited: Jun 12, 2020
    Martaine2005 likes this.

    Jun 24, 2020
    Hi Andre,

    I am in a similar situation - FARFISA CH-25 organ stopped working after playing for half an hour. I switched off and then on agains and to came back - for a little while. As time went on, the playing time grew shorter and shorter until it was impractical to ply it. I then bit the bullet and started to map the circuits prior to perfomring electrical tests. All d.c. voltages correct . . . from the mains psu. Cutting a long story short, I eventually traced the fault to the Tome Generator p.c.b. (on mine it's on a separate small p.c.b.and very accessible. Monitoring the +5 volt line from the CN27273 N (pins 4 and 10 connected to it) and the organ's musical output (hold a key down) they both fell at the same time, the +5 vot line fell to +3 volts. I cleared the SGSM086ABI (Circuit showed it as CIE AY-I-0212) synthesiser
    by checking that the 1 Mz or 2.5 Mz ? input was also absent when everythign went down (believe it or not I used the a.c. range of a cheap multimenter and it measured 4.0 volt rms). I now realise that the voltage regulator can trip out if the load goes above some threshold - a afety feature of regualators. ERGO, I suspec that it's not the regulator that's at fault but the "oscillator" i.c. - which is also an obsolete component. Could this shed light on your problem? Bob

    Jun 24, 2020
    Hi Bertus, I cannot find a circuit for the (obsolete) SN29730 "Oscillator" so cannot comment on what's inside it. However, it must be a some sort of v.c.o because the pins 3 and 11 are taken to an array of precision resistors that transpose the organ's pitch in semi-tone intervals. On the other hand, the stability of the tone must be less than 2% since a 4% frequency variation is detectable as a musical quarter tone. That made me thin that a crystal reference might be embedded (amzingly) in the i.c. It's astonishing to think that a mere R-C oscillator - multivibrator, etc. can achieve that degree of stability. Perhaps I'm reflecting my own poor attempts!

    Anyway, somehwere in this conversation a querie was rasied about pin 3.

    My SN29730 is certainly malfuncting and I am in earnest to find a pin-compatible replacement . . . failing that I cold bird's best a pin-incompatible but electronically functional equivalent if anyone can recommend one. It must be runnign off +5 volts but need not have the transposer facility - I can live outwith that!

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