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Musical Technology

This is a list of articles with teasers.  The headlines below are links to the full articles.

Korg KM-50 quick look

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Posted under Musical Technology at .

The KM‑50 is a simple metronome made by Korg in around 1983–84. I have never seen one in real life, or photographs apart from catalogues from the period. And now this.

front side of the KM-50 metronome

It’s a very basic device, simpler than the earlier Korg RT‑10, consisting of a timing pulse with a higher-frequency sound as a bar pulse. It also has a needle timing indicator like a miniature physical metronome. The timing range is set with a rotary switch, from 40 to 208bpm. The bar pulse can be from 2/4 to 6/4 or off. And it has a tuning pitch, which varies from an unusual 439 to 444Hz, the frequency set with the same linear switch as the bar pulse. The frequency is pretty accurate but wavers very slightly. It takes 9V DC external power or can be powered by a battery. (Unusually for products of this age it seems to work quite well with a rechargeable battery.)

Like many other metronomes, the sound it makes from its built-in speaker is quite irritating and I doubt I would be able to focus on music with that going on. Unfortunately, while the sound can operate without the needle, the sound can’t be switched off separately, unless a cable is plugged into its signal output. It has a 3·5mm TS jack for this, and this rather more interesting. Checked on the oscilloscope, the normal signal is a +8V rectangular pulse which fades out slightly at the end. The bar pulse seems to go through a resonant filter.

I won’t be using this as a metronome in the ordinary sense, but a stepped +8V pulse generator means a timing source that could be run into one or more synth clock inputs, with appropriate attenuation and/or inversion, and even though this seems extremely rare in the secondhand market, it was cheaper than the majority of simple Eurorack timing clocks I’ve seen. I don’t have any specific plans at the moment but it’ll probably be useful at some point.

Hohner Stringvox repair notes (part 1)

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Posted under Musical Technology at . Last updated 2024-01-15 19:15.

The Stringvox (without a space) is Hohner’s rebadging of the ELEX K2 electronic piano/string machine. There were three versions sold by Hohner, the original silvertop (K2r1 by my nomenclature) which seems to have begun production in 1975, the slightly changed rev.2, and the blacktop (K2r3) from 1979. All are in effect built into their own flightcases with lift-off lids, but the r3 case shape is different. (For more information on the series see A Spotter’s Guide to HIPs and Strings.)

I have only been able to find out a little about these previously; in particular I have little information about the interior. [1] Recently an r3 came up for repair, so let’s have a look.


This Stringvox (K2r3)

K2r3 with lid on, top front view, on arrival

This was sold as in need of repair, with the main known problem being that the plug side of the appliance fuseholder is missing. [IN1] Other than that, it came with a stand but without its case-attachment bolts. [IN2] (This is one of those instruments which can rotate on its stand to get a better playing angle, though the stand doesn’t adjust vertically so that’s of limited use.) It could of course do with a clean up and maybe some repair of the vinyl. [IN3] There’s a bit of rust on the hinges. [IN4] As is quite common with these keyboards, the plastic edging on the ports hole on the rear is incomplete. (This is what you get when you put the ports on the outside of the flightcase.) [IN5] It hasn’t been used for some time, perhaps several years, so given its age we can expect some capacitor issues. [IN6] However, under the cover it’s in better condition than most I’ve seen. It would originally have come with sustain and volume pedals and a music stand but these have all gone. It has a socket for a bass pedal board but they were optional extras, and since I’ve never seen Hohner or ELEX bass pedals, they probably weren’t popular.


On arrival . . . it’s heavy! This is certainly the heaviest single-manual keyboard I’ve ever handled. Its lid is slightly misplaced as if something has bent but we’ll see. Some of the keys have slight dents and scratches but they all operate well and the bushes don’t seem to have hardened. This one originally had two bass-range marker tabs but one has gone. [IN7: Consider missing tab replacement.] There’s a vaguely mushroomy smell, but I understand this has been sitting doing nothing for about ten years so not surprising. The switches all operate without too much stickiness and only two of the faders are a little sticky. [IN8: Clean faders.] There’s a power cable under the lid (nice plug, 5A cable, 13A fuse . . . replaced with 5A for now), and what looks like part of the handle in the packaging. I reattached that before going any further. I can’t do much else without replacing the fuseholder, so time to get it open.

K2r3 open, front view, on arrival
Lid off (and handle repaired).

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Dark Time firmware update

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Posted under Musical Technology at .

The Dark Time had a failure. Not really sure why at this point, but when I started it up yesterday it showed an odd pattern of lights and was entirely unresponsive. Repeated power cycling. And I’ll admit that’s a kind of pretentious-sounding phrase especially as applied to something you do just switch on and off rather than go through a more macroscopic shutdown/startup process, but it’s a bit more concise than turning off and on again. Did nothing apart from eventually change the pattern of lights.

So I thought, let’s check the power supply. The Dark Time according to the labelling uses 12V AC and this is the original adaptor I got with it (though not Doepfer own-brand, which may be normal), and is also rated 12V output. It’s actually giving 16V no-load, which seems a little high. Some PSUs limit the voltage until they detect a load but starting up high is less common. But a little online probing tells me that some Dark Times were supplied with 15V supplies, so maybe this is in-range. Powering the thing with 12V AC from a lab supply makes no difference. [1]

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Estradin Altair 231 controls translation

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A quick and dirty drawing and translation for non-Cyrillic-readers of the main control panel of the Estradin Altair (Эстрадин Альтаир) 231. (A rough copy of the Minimoog built in Ukraine in the mid-1980s.) The drawing focuses on clarifying the points I’ve found confusing while trying to learn about the instrument, either due to my limited Cyrillic or the unexpected nomenclature.

There were different versions of the box lines and dials, and the original cap colours seem to vary. This concentrates on the words,[1] and doesn’t show one specific version of the rest. Or screws. For clarity, I’ve simplified the waveform shapes. It may not be exactly to scale or be too accurate about knob shape, as I’m working from some indistinct photos. (Hoping to get exact measurements one day.)

Estradin Altair 231 Panel, multilingual animated

(Full size here).

Like the earlier Estradin 230, the Altair replicates most of the controls (if not the sound) of the Minimoog, but leaves off the output switch and has only a single key CV → cutoff switch. [2]

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Yamaha DX7S repair notes

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Shall I say that DX7s need no introduction? Well there are more web pages than I’d try to count about them, so if you need an introduction, go get it.

The original DX7 to me was that sleek thing with membrane keys which, in 1984, someone I knew got hold of, praised the capabilities of effusively, and left me with unattended for a few minutes. Being sort of used to simple analogue synths, I thought, well, let’s see? And pressed a few membranes, managing to change what it was doing out of all recognition in a few short seconds, with no idea how to change it back. Up DX7 creek without a manual, nor any clue how I got there. I thought it would be best to just leave it rather than make things worse. Leave it at speed, before its owner returned. I never saw it again.

Clearly this memory has stayed with me. In fact it’s probably the exact reason why I prefer proper synthesisers, i.e. ones with a control for each parameter, that you can see, adjust, and then put back where you found it. Nevertheless, I’ve always thought that one day I would like to find out how to use the thing. One of my own, without the risk of wrecking someone else’s setup again.

So, here we have one of the DX7 updates, a DX7S, from 1987 I think, and in need of some attention. I’m not sure what’s wrong with it, but battery replacement is apparently wanted. Hopefully not much more than that.

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A Spotter’s Guide to HIPs and Strings

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Posted under Musical Technology at . Last updated 2024-01-16 00:49.

A summary of investigations into Hohner analogue electronic pianos and string machines. (To date. There have been some updates and more may happen.)

Not long after the technology became reasonably affordable, Matth. Hohner AG sold several electronic piano and string machine models under their Hohner International brand, apparently aiming at cheapness and portability compared with their acoustic and electromechanical keyboards. Most of these were either rebadged ELEX (Excelsior) products or perhaps Hohner-inspired modifications of them. Some were rebadged ELKA and Logan instruments, and they produced a couple of their own designs. [1] (Hohner did also produce organs, bass keyboards and pedalboards, acoustic and electromechanical piano-like instruments amongst others, but that’s not today’s topic.)

The plethora of these instruments from different manufacturers, some sold as Hohner, some not, some easily distinguished, some not, some modified either inside or out over their production span, has led to a certain amount of confusion in the years since, and poses a puzzle for people interested in obtaining and/or repairing them. But perhaps it’s possible to construct a comprehensive overview which will allow them to be identified and distinguished?


HIPs and Strings

Ephemeral photos of most of these instruments turn up occasionally in online sales; longer-term linkable references are given below. Different branding seems to have been used by Hohner in different national markets, however, so even if this list is comprehensive technically (which I can’t guarantee), there may be alternative names in some parts.

Drawings below are sketches intended to highlight the distinguishing characteristics of the instruments, rather than be exhaustively accurate. Or get the perspective right or anything. Slider caps in particular may vary even in the same model; so I’m not trying too hard to represent the exact types found — where they haven’t all vanished or been replaced.

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