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Hohner Stringvox Repair Notes (part 3)

Hohner Stringvox repair appendices

Posted under Hohner Stringvox repair notes (part 1) at . Last updated 2024-01-11 07:41.
Tags:Hohner, piano, reference, repair, string machine, Stringvox, String Orchestra (ELEX)
This is an update to Hohner Stringvox Repair Notes (part 3).

Appendix: Confusions

Occasionally there have been comments posted online to the effect that the Stringvox is just a rebadged ELKARhapsody 610. The confusion probably arises from the fact that the original Hohner String Vox (with a space) was in fact a rebadged ELKARhapsody 490, and both the K2 Stringvox and the 610 are Piano+Strings. However, the 610 does not have a separate Bass voice (except by connecting the optional bass pedals), and in many ways is quite different in controls and circuitry. Since the 610 came first, the rumour may have started with someone commenting that ELEX/Hohner were doing a copy. Copy of the basic idea, yes, but expanding on it and altering just a bit; and they weren’t the only ones producing Piano+Strings keyboards at the time.

Another point of confusion seems to be with the Vox String Thing, of which there may also be two models. It doesn’t help that Vox initially marketed their rebadged Jen J‑600 electronic piano as the Pianovox.

And then there’s the String Orchestras. This r3 is not so easy to confuse, but it is a revision of an instrument that was sold as a String Orchestra. But not so far as I’ve able to tell, by Hohner. (Or if otherwise, evidence needed.) The other actual String Orchestra was by Logan, also rebadged by Wersi.


Appendix: Capacitor Tests

PSU
code ratings (F|V) measured (F | DF)
pre-power test mains test
C1220µ25V 301µ0·052 301µ0·076
C2220µ35V 256µ0·119 255µ0·132
C32m035V 2·23m0·104 2·23m0·22
C41m025V 1·19m0·104 1·19m0·057
C5100µ35V 166µ0·070 164µ0·064
C61m025V 1·16m0·105 1·15m0·073
C71m025V 1·22m0·086 1·21m0·057
C82m250V 2·43m0·085 2·42m0·158

Appendix: Voicing Board Variations

Those components which vary between the different envelope groups are as follows. Non-varying components are not listed. Capacitors C4, C5, and C6 are not always present.

Group Capacitors Resistors
C3C4C5C6C9 R12R18
A 4n7 8n2 56n 560K 560K
B 4n7 6n8 47n 560K 560K
C 4n7 5n2 39n 560K 560K
D 4n7 4n7 33n 560K 560K
E 4n7 3n9 27n 560K 560K
F 4n7 3n3 22n 560K 560K
G 4n7 2n7 18n 560K 560K
H 4n7 2n2 15n 560K 560K
H 4n7 1n8 12n 560K 560K
L 4n7 1n2 10n 560K 560K
M 4n7 820p 8n2 470K 470K
N 4n7 470p 6n8 470K 470K
O 6n8 220p 5n6 330K 390K
P 6n8 100p 4n7 270K 390K
R 8n2 6n8 8n2 4n7 390K 330K
S 6n8 6n8 6n8 3n3 330K 330K
T 1n0 4n7 1n0 2n2 330K 270K
U 1n0 2n2 1n0 2n2 330K 270K
W 1n0 2n2 1n0 1n8 390K 220K
Z 1n0 22n 1n0 1n5 270K 220K

Appendix: K2 rev.2 and 3 Boards

Some boards appear to have been changed between the K2r2 (based on detailed photography provided by Sebastian of the Silvertop String Orchestra) and the r3. It is not currently known what changes other than the position of the tuning trimmer and bass pedal connector occurred between the K1r1 and r2. Some of the boards found here also appear to have been used in the K4r1 (which is likely to have been similar to the K2r1 in those ways), but this is based only on the rather faint schematics. No information so far about later versions of the K4.

Board FunctionK2r1K2r3K4r1
power regulatorK2-0
voicingK2-1 × 61 (20 types)K4-1 × 49 (18 types)
dividers, envelope & oscillator backpaneK2-2K4-2
key contactsK2-3K4-3
strings pre-modulation preampK2-6
strings output preampK2-7
piano/harpsichord voices filter & amplifierK2-9
bass voice filter & amplifierK2-10
radiobutton switchesK2-12
volume & decay slide controlsK2-13
controls, routing & pianos backpaneK2-14
pitch controlK2-18?
chorus delayK2-4 × 2K2-20-2 × 3K2-4 × 2
switches and power & signal routingK2-110
chorus effect LFOK2-5M174K2-5
strings backpaneK2-8M175K2-8
PSU-top fuse boardM282
oscillatorOS-5

The only difference between the K2-4 and K2-20-2 seems to be that the K2-20-2 had 8 pin holes for a DIP-8 TCA350Z rather than 6 for a TDA350Y. Conversely, the M174 is a very different design than the K2-5, producing three rather than two modulation signals, based on op amps, and using a different voltage supply, which also required the M175 backpane redesign.


Appendix: Controls

The K2’s controls are potentially confusing. The eleven faders on top are:

  • five volumes for the bass-side voices in split mode;
  • decay time for the Cello and Strings voices (affecting both sides), plus
  • four volumes for the treble voices in split mode and all voices in single mode.

The left four buttons are a four-way exclusive group, i.e. pressing any one cancels the others. They set the four operating modes for the K2:

  • Split Keyboard Mode has no direct switching function but mechanically cancels the other three. Pressing it places the Stringvox in Split mode with normal voices, i.e. both bass and treble range have their four pianos and strings voices available, controlled independently with the bass and treble faders and cancels.
  • Full Keyboard Mode applies the settings of the treble-range faders and cancels to all keys.
  • Ensemble Mode might be described as a strings unison mode — each key in the bass range plays its basic pitch plus the second and third octave up. The bass-range pianos are also still playable. However, in the treble range, the strings voices become unavailable and you’re left only with pianos.
  • Bass Mode replaces all other voices on keys 1–24 with the Bass voice.

(It is possible to keep more than one of the Full Keyb., Ensemble, and Bass switches on at once, but nothing very interesting seems to result from that, although it is useful to have all three in when removing the top panel.)

The other eight buttons are cancels for the four voices in the bass and treble ranges.


Appendix: Switch Functions

All switches on the K2-110 board. Terminals as numbered there.

Switch OFF common ON note
Split Keyb. Mechanically cancels the Full Keyb., Ensemble, and Bass switches. No direct electronic function.
Full Keyb. a NC left GND right GND
b t1 Cello (bass) fader t3 Cello (bass) audio t4 Cello (treble) fader ON unites Cello bass and treble
c Bass switch (d) common t10 PHB (bass) audio in t11 PHB (bass) fader
d t6 Strings (bass) fader AND
Bass switch (c) common
t7 Strings (bass) audio in t9 Strings (treble) audio in OFF allows Bass switch to ground the signal;
ON unites Strings bass and treble
e Bass switch (e) common t20 Strings (bass) decay on/off left GND OFF allows Strings (bass) decay to be switched on with Bass switch off and Strings (bass) switch on;
ON switches it on immediately
f Bass switch (f) common t18 Cello (bass) decay on/off left GND OFF allows Cello (bass) decay to be switched on with Bass switch off and Cello (bass) switch on;
ON switches it on immediately
Ensemble a NC
b NC t13 Cello (treble) decay on/off
AND Cello (treble) switch (a) common
left GND OFF allows the Cello (treble) decay signal to be switched on with Cello (treble) switch on;
ON switches it on immediately
c Cello (treble) switch (b) common Cello (treble) fader NC OFF allows the Cello (treble) signal to be grounded with the Cello (treble) switch off; in Ensemble mode (ON), this cannot happen
d +20V to strings treble busbar +20V input +20V to ensemble busbar
e Strings (treble) switch (b) common Strings (treble) fader NC OFF allows the Strings (treble) signal to be grounded with the Strings (treble) switch off; in Ensemble mode (ON), this cannot happen
f NC t16 Strings (treble) decay on/off
AND Strings (treble) switch (a) common
left GND OFF allows the Strings (treble) decay signal to be switched on by the Strings treble switch;
ON switches it on immediately
Bass a left GND t1 Cello (bass) fader AND
Cello (bass) switch (b) common
NC OFF grounds the Cello (bass) audio signal;
ON allows it to be grounded with Cello (bass) switch off
b left GND right GND NC
c NC t6 Strings (bass) fader
AND Strings (bass) switch (b) common
left GND OFF allows the Strings (bass) audio signal to be grounded with Strings (bass) switch off;
ON grounds it immediately
d t21 PHB (bass) audio to K2-9 PHB bass audio IF Full Keyb. switch is OFF t19 PHB (bass) audio to K2-10 Bass mode sends the PHB (bass) signal to K2-9 for filtering, amplifying, and sending to the Bass output.
e Strings (bass) switch (a) common Strings (bass) decay on/off IF Full Keyb. switch is OFF NC OFF allows Strings (bass) decay to be grounded with Strings (bass) switch ON and Full Keyb. OFF
f Cello (bass) switch (a) common Cello (bass) decay on/off IF Full Keyb. switch is OFF NC OFF allows Cello (bass) decay to be grounded with Cello (bass) switch ON and Full Keyb. OFF
Piano (bass) a left GND t22 Piano (bass) fader
AND K2-110 PHB preamplifier
NC OFF grounds the Piano (bass) signal
b NC
Harpsichord (bass) a NC
b left GND t23 Harpsichord (bass) fader
AND K2-110 PHB preamplifier
NC OFF grounds the Harpsichord (bass) signal
Cello (bass) a NC Bass switch (f) off left GND ON switches on the Cello (bass) decay signal IF Bass is off and Full Keyb. is off (Full Keyb. on switches it on anyway)
b left GND Bass switch (a) common AND t1 Cello (bass) fader NC OFF grounds the Cello (bass) audio signal
Strings (bass) a NC Bass switch (e) off left GND ON switches on the Strings (bass) decay signal IF Bass and Full Keyb. are off (which can also be done by switching Full Keyb. on)
b left GND Bass switch (c) common AND t6 Strings (bass) fader NC OFF grounds the Strings (bass) audio signal
Piano (treble) a left GND t28 Piano (treble) fader
AND K2-110 PHB preamplifier
NC OFF grounds the Piano (treble) signal
b NC
Harpsichord (treble) a NC
b left GND t26 Harpsichord (treble) fader
AND K2-110 PHB preamplifier
NC OFF grounds the Harpsichord (treble) signal
Cello (treble) a NC Ensemble switch (b) common
AND t13 Cello (treble) decay on/off
left GND ON switches on the Cello (treble) decay signal IF Bass is off and Full Keyb. is off (Full Keyb. on switches it on anyway)
b left GND Ensemble switch (c) off NC OFF grounds the Cello (treble) audio signal IF Ensemble is off
Strings (treble) a NC Ensemble switch (f) common
AND t16 Strings (treble) decay on/off
left GND ON switches on the Strings (treble) decay signal (which can also be done by switching Ensemble on)
b left GND Ensemble switch (e) off NC OFF grounds the Strings (treble) audio signal IF Ensemble is on

Decay on/off switching occurs by switching the transistors on K2-110 off and on for each key group: Q1 — Cello (treble), Q2 — Cello (bass), Q3 — Strings (bass), Q4 — Strings (treble).


Appendix: TOG Pitches

TOG chips are not perfectly accurate to the 12-ET chromatic scale. On the K2, the 50240 will produce IPN A6 with the clock tuned to 667,040Hz, and the other pitches as below:

top note 50240 pin divisor output (Hz) 12-ET (Hz) difference (Hz)
(12-ET − output)
F6 44511,479·02441,479·9777−0·9533
G6 54261,565·82161,567·9817−2·1601
A6 64021,659·30351,661·2188−1·9153
A6 73791,760·00001,760·00000·0000
B6 83581,863·24021,864·6550−1·4148
B6 93381,973·49111,975·5332−2·0421
C7 103192,091·03452,093·0045−1·9700
C7 113012,216·07972,217·4610−1·3813
D7 122842,348·73242,349·3181−0·5857
E7 132682,488·95522,489·0159−0·0606
E7 142532,636·52172,637·0205−0·4987
F7 152392,790·96232,793·8259−2·8635

(50240s also produce a /478 division which here would be F6, but the K2 produces that as a division of F7.)


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