Repairing a Viscount Organ

I recently helped Andrej from Emšo Blues Band to repair his Viscount organ. Well, actually he did all the work, I just gave him some tips to figure out how the problematic chorus / vibrato section of the organ works. Eventually he found the issue and managed to get the organ fully working again.

vibrato komande

The main issue was that, since some repair work on the other boards of the organ, the vibrato/chorus effect was having issues. Without using the effect the organs sound was correct, however with any of the two effects being used the low tones were heavily attenuated and also the sound was plain wrong.

Viscount organs are not that common so their repair manual / schematics are a bit harder to find (hint: You can try contacting the original manufacturer.). And even with the schematics in hand one still needs to track the signal down its path and make assumptions on what may be wrong.

brum

The organ uses a TDA1022 BBD delay line to generate the two effects. Depending on the effect selection, different deviation frequencies and different mixing modes of delay line output and input signal are used.

VIBRATO

Manual effect switch controls both the deviation frequency and the mixing of the original signal and TDA1022 output signal. One of the lower opamps is used as a LFO. CD4046 is used as a VCO to turn the slow LFO deviation frequency into higher frequency control signal used for driving TDA1022. CD4016 is used as analog switch to set mixing of the signals and to modify the frequency of CD4046 depending of the active effect.

After quite a bit of troubleshooting he found out that the control signal from the effect (vibrato/chorus) selector switch was not getting to the CD4016. Actually the signal from the wire was not getting to the PCB although the wire seemed soldered properly to the board.

He even made this nice image of the fix he made:

cold_joint

I guess there are two options why there was no connection – either a cold joint or corrosion of the joint and copper over time.

Cold joints usually happen with improper soldering techniques (wrong temperatures, poor wetting, unclean soldering surfaces), but the problem may not develop immidiately. Sometimes the defective solder joint may even mechanically hold the component in place while still not being conductive. This was also the case with Crumar Organizer organ I repaired some time ago – the wire started conducting only after the board warmed up…

Corrosion of the copper may happen because of too hot soldering temperatures or even slowly over time due to air and humidity in contact with two different metals. The PCB trace may sometimes develop hairline crack / corrosion directly adjacent to the soldering pad. The crack may be thin enough to be almost impossible to see without magnification.

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2 Comments on “Repairing a Viscount Organ

  1. Hi Rei Labs Net !

    Like Andrej, I own this Viscount Intercontinental OP-6 organ. I have a question regarding the chorus-vibrato section, given your experience with it.

    Any organ that seeks to emulate the Hammond tonewheel organs (like the famous B3), needs to get three sounds rights – the tonewheel sound, the leslie sound and the scanner chorus/vibrato. As it turns out, the Viscount, although being a transistor made organ, is really pretty pretty good at the pure raw organ sound. The leslie simulation of the Viscount is also not bad for the time – but a very complex sound effect to emulate electronically. Recently, digital modeling has achieved much much better leslie simulators, the best one probably being the Ventilator. (if you want to hear a Viscount OP-6 through a Ventilator as compared to its own leslie simulator, go to youtube and search for “synthmania” and “organ”)

    Now, having read about the original (mechanical) scanner chorus/vibrato on Hammond organs, it is famous for being much more subtle than regular chorus pedals (circuitry) of the time. So, inspired by the big gain in sound quality by using a Ventilator leslie sim with the Viscount OP-6, I am interested to use a better, more recent external chorus/vibrato that likely comes much closer to the Hammond scanner chorus-vibrato.

    There are three options here – a hardware DYI scanner vibrato designed by synth module and guitar pedal guru Jurgen Haible, or a VST run on a laptop (PSP B-Scanner or Martinic Scanner Vibrato). Now here comes my question:

    Could the signal be taken from where it enters the Viscount chorus-vibrato section, put through a soundcard for A/D convertion, processed by the VST, get converted back D/A and get reinserted into the circuitry post-Viscount chorus-vibrato section ?

    Is there any problem with impedance, or how hot the signal should or should not be to pass effectively though convertion on the way in, and suit the organ circuitry when it returns back to the organ ?

    Thank you for considering this idea of matching gorgeous old skool analog transistor sound with the bets the digital world has to offer !

    Pim van Harten

    • In circuit the sound information may be present in two different forms, as a current (this was more common in transistor era, but is often used in analog devices) or as a voltage. Generally for interfacing external equipment and measurements it is much simpler to use the voltage variant. Adding a high impedance voltage follower to get an intermediate output is reasonably easy. It is basically an amplifier connected to a specific circuit, which provides isolation (no feedback) from the new output.

      The current variant heavily depends on the “receiver” having specific feedback on the “transmitter”, so adding an additional receiver to the circuit is not that simple. Here the receiver has to be replaced with imitator circuit that will drive both the original receiver and the newly created output. If you want voltage outputs / inputs converter circuits should be used (eg. transimpedance amplifier).

      I don’t have Viscount documentation anymore, but I think the logic here is voltage based. Old organs are mainly pretty nicely sectioned, so it should not be too hard to make it modular. There are some considerations about that, you should be careful not to introduce too much noise into the circuit, also external connectors may need protection from statics to give the device a long life. For outputs and inputs standard impedances and levels could be used if you plan to use this to interface different effects.

      What you are talking about is a standard DSP scheme, using PC as the DSP processor. I don’t see any problem with that, but in addition to the noise consideration, you may also notice some delay in processing. This will be regardless of the PC / specific DSP hardware, as most DSP algorithms themselves need some delay. Dedicated DSP hardware may be slightly faster as it directly interfaces the DSP processor with the ADC / DAC and thus works in real time. You can easily test this by connecting the effect on the output of instrument.

      Regards,
      Dejan

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