Technics SX-70GW F# stopped working across all octaves

[croolis]

I recently bought a Technics SX-70GW which was fully working at the time I got it.
I suspect it hadn't been used for a long time but after activating the vibrato/wah switches a few times, the initial problems it had with manuals not working/crackling went away, and I've been happily trying to teach myself to play Bach's Toccata and Fugue in D Minor on it .. There is precious little info on this type of organ on the net, but I have worked out it must have been built around 1973 and was a reasonably high end model at the time. Circuitry is 100% discrete and I don't think I've seen a single IC in any of the boards.

Today I was just noodling around and noticed that F# was not working. A bit of further investigation showed that F# was not working for any combination of stops on both manuals though it does work fine on the pedals or when activated using the pedal that links pedals with the lower manual.

Pressing F# gives either a very soft F# sound at high frequency (not related to the octave being played) or nothing at all.

I think the problem would be one of the tone generators .. none of the boards are labelled, if someone with a bit more experience could help me by pointing out where to start probing (I am reasonably competent with a soldering iron) I would be grateful as I am not feeling too confident at the moment.

I am uploading some pictures of the various circuit boards present.

There are a couple of other boards in the lower cabinet with the speakers - I think those are related to the foot pedals and, in that case, they work OK.

What sort of synthesis does this thing use? I am guessing it starts with a square wave and then forms the notes subtractively.

Thanks in advance for any and all replies and for allowing me to post on this forum (my first post)

[croolis]

Well .. a little update ..

I found this article really useful :
https://reverb.com/news/diy-vintage-organ-repairs

It finally made sense what all the boards in multiples of 12 are about .. each note (of the scale) has an oscillator, the output of which is passed through frequency dividers to generate lower octaves. A fault with an oscillator would make all the keys for that particular note go dead. The pedals have separate oscillators so are not effected.

I opened everything up again - luckily with these old organs it's quite easy to work on them as everything hinges or comes apart - the tone generators are beneath the upper manual, covered with a hum shield - you can see the bunches of wires for them in my second photo, behind the lower manual.

I counted the 7th board (F# is the 7th note on the scale) and gently touched the components on the board .. at one point the notes came back on .. and soon went dead again. Definitely a dryjoint though I wasn't brave enough to actually start pulling boards out considering the amount of wiring. A bit of further fiddling brought the F# notes working and I left it that way. I also went through the contacts of some keys that were crackling a bit with contact cleaner - pressing a key activates the sound in 2 stages, first a basic note and then harmonics come in. Quite interesting!

So, for now my organ is working fine :) If anyone finds this note helpful, do please leave a reply!

[andyg]

We saw those organs in the UK with the name 'National' on them, around 1971/72/73. That then changed to Panasonic and finally to Technics some years later. They were solidly built with good quality components. The overall sound was the usual rather thin late 60s/early 70s Japanese sound, but the spec was big enough to produce some good tonal colours.

Yes, it uses subtractive synthesis, starting with square or sawtooth waves and then filtering them. If some of the notes produced a high frequency F# as you describe, then the fault might be with the dividers rather than the tone generator for that note.

Best contact cleaner for organs is Caig De-Oxit D5, but don't use it on any sliders, your need Fader Lube for that.