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Electrohome-Kinsman "Concerto" Schematics

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  • Want To Buy: Electrohome-Kinsman "Concerto" Schematics

    As the latest in a line of long-shot requests, I am now looking for technical info on the Electrohome-Kinsman "Concerto" organ, a tube-based model introduced in 1962. It has the distinction of being a Canadian-made instrument, perhaps among the very first commercial electronic organs built in our country, though as the name suggests, it is quite similar technically to the American-made Kinsman organs. However, it is not a direct copy of any Kinsman models that I know of. Based on advertisements, it seems the Electrohome-Kinsman "Rhapsody" and "Nocturne" models may be identical to the "Concerto" except in different cabinet styles. Documentation for other later Electrohome organs of the tube era may also be of interest.

    I must admit that I've already brought the instrument to full functionality without documentation. However, I made some assumptions that I would like to check against the schematics if possible. Firstly, some previous technician made a mess of rebuilding the high-voltage supply, replacing a dropping resistor with one much too low in value (and replacing the filter cap sloppily with a salvaged one of about the same age as the original), making the B+ to the preamp section excessively high and ripply. This resulted in significant 120 cps hum in the output. So, I first rebuilt this mess with fresh capacitors and terminal strips while keeping the old dropping resistor, and as expected, the hum was still unacceptable. Then, not knowing the "real" value of this dropping resistor, but knowing roughly what to aim for voltage-wise, I replaced the replacement with one more than 10 times as large, which eliminated the hum and brought the B+ of the preamp/bus-amp/pedal-divider circuits to a reasonable level (yet still higher than the voltages specified in Kinsman's designs). However, I then had some very odd behavior of the pedal divider, where it would not divide on a few non-adjacent mid-range notes. I verified that this was not because of low signal level (to the divider preamp) of the affected notes, nor slow edges of the waveforms, nor tube problems, nor obvious component problems. The flip-flop simply wouldn't divide properly on all notes without bringing the B+ unusually high again. Rather than doing that, or trying to troubleshoot further, my solution was to increase the gain of the divider preamp tube, using a 12AT7 instead of 12AU7. That did the trick.

    Also, initially multiple neon dividers were incompetent. I suspected incorrect supply voltage, so I traced out the relevant regulated power supply circuit, and found that its output voltage is adjustable via the trimpot nearest to the 0A2. (By the way, the Kinsman designs that I know of simply use two 0A2s in series to give 300V, with no adjustment possible.) The voltage measured about 295V to begin with, and I increased it to 300V, assuming the same desired voltage as Kinsman organs. Sure enough, this small change fixed the divider issues.

    A few other small and ordinary faults were found and corrected, not worth mentioning here. What I will mention is that the instrument's sounds, while somewhat lacking in variety, are quite satisfying. For once in a tube spinet, there are enough pitches to cover the entire range of both manuals at all pitch registers without resorting to any "tricks" – this is thanks to the economy and small size of the neon dividers, such that the designers did not find it necessary to skimp on them. And their somewhat arched and rounded sawtooth waves provide a good basis for formant filtering, with none of the "hollow" character typical of squarewave-divider organs, and also without the typical "Hartley sound" bestowed by the lumpy pulsewaves from such oscillators (as found in Conn organs and some of Hammond's chord models).

    But what really makes the instrument special is its upper-manual "sustain percussion", the behavior of which is unlike any other keyboard instruments I know of. The sustain length actually increases significantly the longer a key is held, up to a point. For example, even with the sustain length control at maximum, a short staccato note will die off quickly upon release, while a note held for a second takes more than a second to fade away. I believe this is because the resistors through which the sustain capacitors charge are so large (relative to other designs with similar sustain capacitor values) that it takes a rather long time for the caps to charge to full voltage and hence full sustain length, yet practically full loudness is reached before full sustain length. This can be explained by the behavior of the sustain tubes – yes, amplitude modulation for each key is achieved by a triode, so there are 22 additional tubes for the sake of upper-manual sustain! The sustain tubes are 12AX7s, normally biased at cutoff so that no signal comes through. As the bias voltage moves positive, gain starts increasing, at first slowly, but then quite fast. Past a certain threshold, the tube amplifies linearly, bias no longer having much effect on gain. Another interesting effect is that, because of "outphasing" between certain register lines, combining certain stops (especially percussion + non-percussion) results in unexpected timbres. For example, non-percussion 8' Clarinet + percussion Strings results in a hollower timbre than either stop alone while the key is held. Because of the percussion/sustain behavior, at the attack, the Clarinet tone shifts into this hollow combination gradually, while at the release, the hollow combination instantly turns into the Strings tone. The result is really unique and "synth-like", though I can't say I know of any synths that sound quite like this, nor any with the same unusual sustain behavior. Anyway, eventually I will do a demo of this organ, and use it in some musical recordings!
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