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Vermona Synth Boards

Posted under Musical Technology at . Last updated 2019-09-10 00:00.
Tags:hardware, instrument, Synthesizer, Vermona, KHW, synthesiser, artwork
This is an update to Notes from an Exploration of a Vermona Synthesizer.

In spite of the apparent quietness of the last few months, I’ve been working on synths in between other things; in particular on the Vermona. After the initial post, in the absence of any written technical information, I was planning to draw my own circuit diagrams. Actually I did some board drawings, because that’s easier when all you have are last year’s photos and enough space to sit. (happy emoticon)[1] A couple of weeks ago, this was complicated by my discovery of a new (I think) and rather great old manuals page at vermona.com, which includes circuit schematics [2] for both versions of the Synth. The full update I was slowly working on will be rewritten in the light of the new information . . . but it will be better. For now, here are my drawings — with the component numbering brought into line with the schematics, as far as possible. (My earlier numbering was entirely arbitrary as there is no screening on the boards.)


Points to Note

Views are from the component side as seen with the synth open from the front, and transparent to solder side. (Single-sided PCBs.) Clarity is prioritised over precision in trace position, and the scaling may not be exact.

These boards are the later A109 (rather than B81) versions; using A109D rather than B081 op amps, together with differing supporting components. Clarifying an uncertain point in the initial Notes article, only the Octave boards (FW1/2) remain the same between the two versions of the synth. (Update, 2016-04-10: To clarify the clarification, the VCF uses an MAA741 rather than the B081 on the older schematic, but I haven’t seen the older board.)

The schematic component codes are:

ISintegrated circuits
Ttransistors (triode?)
Ddiode (including diode arrays)
Rresistors and variable resistors
Ccapacitors
Sswitches (including radiobutton sets)
Sifuses

Jumper wires are not identified, so they’re Jn in the drawings. Sadly, none of the board terminals are numbered either, so for current purposes they’re TLn.

Radiobutton switches are numbered as complete sets in the schematic; the sides are a or b, and the individual pins are 1–6. (The switches are 4P2T.) In the drawings, individual switches are identified with plain numbers; there’s only one set per board, but the set number is also noted. [3]

There are a series of circled numbers on the schematics which I would usually expect to be annotation references, but there were no annotations. If anyone can tell me what they are, please get in touch.

The scans for the A109 version appear to be four quarter-scans of a larger original, and unfortunately leave out a stripe down the middle; there are some missing components and junctions in it, but they seem to be identical to the B81 version, and can be copied from it if you’re reassembling the whole page. At this resolution there are some ambiguities in the type to watch out for, especially 6s and 8s.

I think the various little Es and As by some terminals on the schematic probably stand for eingang and ausgang — mostly for audio rather than control signals. They don’t seem to be quite consistently used, though; and there are also some Ps which don’t suggest anything to me . . . 

I’ve listed component details below where I’ve wanted them, but I’ve no immediate interest in the values of fixed resistors; many of which are inserted with the printing down on the boards, rather than, y’know, visible or anything. Schematics to the rescue if we need to know. The schematic gives tolerance values for fixed but not variable resistors — I’ve included tolerance based on case codes. Capacitor rating information has been assembled or reasoned out from various sources. All electrolytic capacitors are aluminium, so I haven’t specified that below. Where packages have multiple codes only the most relevant are given, e.g. datecodes may be ignored. (The DRn codes on transistors don’t seem to be dates; not sure what.)

Corrections or further information in any of this — welcome!


The Boards

There are a few points to bear in mind regarding individual boards and schematics, so I’ll take them one at a time:

Regulator (NT6)

power regulator board diagram

Updating my initial notes, the NT6 board takes a supply of around 18V from the transformer (depending how you wire it and what you plug it into) and according to the schematic should output −12V, +12V, and two +5V supplies to the other boards, along with a −20V to ground supply for the power LED, and a 7·15V reference.

The exact position of Si1/2, C1/2, and D1/2/3/4 is indeterminable from the schematic; they are numbered here arbitrarily.

An anomaly is that while in the schematic, the A109D is shown with a capacitor/resistor pair connected across pins 3 and 12, in the order pin 3–resistor–capacitor–pin 12 (3–R–C–12), on the board, the order is reversed to pin 3–capacitor–resistor–pin 12 (3–C–R–12).

(Update 2019-09-10: I’ve corrected the numbering for the Tesla MAA723 on my diagram. The schematic uses the numbering of the board holes (and MAA723CN version of the chip) bracketed within the chip outline, while the exterior numbers are the pin numbers as in the MAA723/MAA723H datasheet. It’s not clear from the schematic which of the exterior numbers are 6&8, but this can be checked on the datasheet. So the diagram shows DIP holes 1, 8, 9, 14 empty, but has pins 10,1–5 in holes 2–7, and 6–9 in holes 10–13. The diagram is also updated, as in the earlier version I missed an almost invisible trace between pins 7&8, which does however (sort of) appear on the schematic.)

Components

numbercodes / ratingtype
caseschematic
SemiconductorsD1, D2, D3, D4SY 360/1diode
T1, T6, T7BC211NPN BJT
T2, T3DR7 307SC307dPNP BJT
T4DR7 C36SC236dNPN BJT
T5DR8 C36SC236dNPN BJT
ICsS1MAA723MAA723voltage regulator
S2A109DA109op amp
Trim PotsR5, R104·7K 20%4·7K
Capacitors C1, C247nF   10%   ?Vpolystyrene
C3, C41000µF   ?%   25Velectrolytic
C5, C90·1nF   10%   ?Vpolystyrene
C6, C710µF   ?%   40Velectrolytic
C84·7nF   10%   ?Vpolystyrene
C102200µF   ?%   6·3Velectrolytic

VCO

VCO board diagram

The VCO board handles various tuning and keyboard CV/gate functions, and has two oscillators which each generate a square-wave signal at a pitch determined by key-press (an octave above the highest setting on the octave switches), which is fed out to the two octave boards.

The board includes some hand-soldered floating components (four for each VCO); these are not directly indicated on the A109 schematics, but they are on the B81 schematic, though it’s not apparent whether they were part of the B81 PCB. I’ve listed these as FT for transistors and FR for resistors, and used the numbers from the B81 schematic.

extra handwired components on the VCO board

This VCO board has two anomalies which may be unique:

  • A trace has been cut with a soldering iron between R6 and R14/TL5; this seems deliberate, and is presumably a later mod (it leaves R6 as a dead end; which could have been achieved by leaving R6 out during manufacture — or desoldering one end instead of cutting a trace, for that matter . . . :-/); the effect is to prevent direct contact between the tuning circuit and the impedance transformer circuit. Why, or what that implies for function, I couldn’t say.
    VCO board, cut trace
  • A solder connection in the VCO2 circuit between TL21 and C26, which also connects to IS6 pin 10; this seems accidental and probably a manufacturing flaw resulting from the spindly and overlong tails of the polystyrene caps. [5] This connection is not made in VCO1.
    VCO board, missoldered capacitor

Once everything else is sorted I’ll try reversing these and see what happens. So these are marked on this version of the VCO diagram, but there should be an amended version in future.

One further anomaly, which I assume is normal for these, is that the pin 3–12 capacitor/resistor pair order for IS1 is inversed from the schematic here too (3–R–C–12, not 3–C–R–12).

Components

numbercodes / ratingtype
caseschematic
Semiconductors D130 YSSAY 30diode
D241 BBSAL 41series dual diode
D341 BSSAL 41series dual diode
T1, T3, T4DR6 307SC307dPNP BJT
T2DR6 C37SC237dNPN BJT
T5–T7KP303EN-Channel FET
FT9, FT11DR6 C36(B81: SC236d)NPN BJT
FT8, FT10DR7 307(B81: SC307d)PNP BJT
ICs IS1–IS5A109DA109op amp
IS6, IS7D121D74 121flip-flop
Trim Pots R271K 20%2K
R2822K 20%22K
R33, R54, R60470 20%470
R51, R594·7K 20%4,7K
Capacitors C1–C3, C26, C274·7nF   10%   ?Vpolystyrene
C4–C6100pF   10%   ?Vpolystyrene
C70·47µF   10%   ?Vpolystyrene
C8, C21–C231nF   10%   ?Vpolystyrene
C9–C1710µF   ?% 40Velectrolytic
C18, C192·2µF   ?% 25Velectrolytic
C24, C2547pF   5%   ?Vceramic disc?
C28, C29100nF   ?%   ?Vceramic plate

VCA

VCA board diagram

The VCA board provides the main output and headphones amplifier, and also contains the LFO, the Vibrato circuit, and the Noise Source. (I think the actual noise source is T6; which is only connected on two legs, intentionally.)

There are also some mismatches between the VCA schematic and the actual VCA board:

  • In the schematic, IS1 pin 4 connects to R8 and R10, while pin 5 connects to R9 and R11. On the board, pin 4 connects to R9 and R10, while pin 5 connects to R8 and R11. Or put simply, R8 and R9 are swapped at one end; and which end you consider as swapped determines the number you assign to the resistors. (I’m assuming they’re swapped at the IS1 ends, so the numbering here matches the schematic at the T2/T3 side.) In effect this inverts the input signals to the op amp compared to the schematic.
  • The pin 3/12 capacitor/resistor pair order on the A109Ds is also anomalous here: In the schematic, IS3 and IS6 are shown with 3–C–R–12; but on the board, the order on all A109Ds which use pairs (rather than a single capacitor) is 3–R–C–12.
  • In the schematic, R40 is shown as connected between TL22 and the junction of R39, C22 and R34; on the board it is connected between TL22 and the junction of R39, IS3 pin 10 and C19 — i.e. at the other end of R39.
  • There is no −12V decoupling capacitor shown on the schematic, but there is one on the board, between C30 and IS6. I’ve called this CX1.

On this board (but perhaps not all):

  • R4 is 2·5K on the schema but 2·2K on the board.
  • R21 is 250K on the schema, but its case marking is just 220. It measures (in-circuit) 4·36KΩ, so I expect it’s a 220K.
  • C28 is a 100µF 10V cap on the board. It’s a Frolyt of a similar age to the others, so is probably original, though below spec.

Components

numbercodes / ratingtype
caseschematic
SemiconductorsD1SAL 41series dual diode
D230 YSSAY 30diode
D3SAL 41series dual diode
T1, T5DR7 307SC307dPNP BJT
T2–T3DR8 C39SC239dNPN BJT
T4DR9 C39SC239dNPN BJT
T6R9 S00SS200NPN BJT
T7KP303EN-Channel FET
ICsIS1–IS6A109DA109op amp
Trim Pots R2, R1410K10K
R31M1M
R42,2K2,5K
R21220 [K]250K
R34100K100K
R481K1K
Capacitors C26, C29, C3247nF   10%   ?Vpolystyrene
C3–C547µF   ?%   16Velectrolytic
C24·7µF   ?%   40Velectrolytic
C12–C14, C164·7nF   10%   ?Vpolystyrene
C6–C9, C11100µF   ?%   16Velectrolytic
C28100µF   ?%   10Velectrolytic
C17–C19, C21100pF   10%   ?Vpolystyrene
C222·2µF   10%   100VPET
C15, C2347pF   5%   ?Vceramic disc?
C20, C2410pF   ?%   ?Vceramic disc?
C25, C30, CX1100nF   ?%   ?Vceramic plate
C110µF   ?%   40Velectrolytic

Octave (FW1, FW2)

octave board diagram

The Octave boards divide-down the signal from the VCO board, allowing transposition over five octaves, and use the divisions for waveshaping, producing a sawtooth waveform and a 25% pulse-width. Although the boards are shown and numbered separately on the schematic as FW1 & FW2, they are identical circuits, so this diagram serves for both. n.b. the D100D chips on these boards are Quad NAND gates, and are represented as logic gates on the schematic.

The missing stripe should include R57; its position is as in the B81 version.

Components

numbercodes / ratingtype
caseschematic
Semiconductors D1, D5(FW1) 42 MP
(FW2) 42 MS
SAM42common-cathode dual diode
D2–D4(FW1) 43 MR
(FW2) 43 ML
SAM43common-cathode triple diode
D6(FW1) 30 YS
(FW2) 30 YP
SAY30diode
ICsIS1–IS3D100DD100Quad NAND
Capacitors C1–C1282pF   5%   ?Vceramic disc?
C13100nF   ?%   ?Vceramic plate

EG

EG board diagram

This board receives a gate signal and outputs a CV envelope signal in response. It has five presets on switches 1–5, and a manual setting on switch 6, controlled by external pots.

Some issues are:

  • There are two R1s on the schematic, first and second in line from the signal input at TL1; I’ve made the second R2.
  • There are also two pairs of components, D1 and R7, and C2 and R10, which appear in reversed order on the schematic compared to the board.
  • The switch-pin numbering on the schematic does not match a simple ordering of the actual pins on the switches, and the group of contacts for sustain level (lower right on the schematic) are not numbered — they’re actually the 5a/6a group of pin-pairs. So using the schematic numbering, the on-board pin order for each switch ends up as:
    EG switch diagram
    I assume the schematic predates the attempted implementation.
  • The missing stripe should include R17; its position is also as in the B81 schematic.

Components

numbercodes / ratingtype
caseschematic
SemiconductorsD1, D230 YSSAY30diode
ICsIS1A109DA109op amp
Trim PotsR2310K10K
Capacitors C1100nF   ?%   ?Vceramic plate
C21nF   ?%   40Vpolystyrene
C310µF   ?%   ?Velectrolytic
C4, C5100µF   ?%   16Velectrolytic
C6, C710pF   ?%   ?Vceramic disc?

VCF

VCF board diagram

This board contains a ladder filter, with presets/manual/envelope/LFO cutoff control, and presets/manual resonance control.

  • The order of C9 and R36 is reversed on the board compared to the schematic.
  • There are two R27s on the schematic, at either end of the filter ladder; I think the upper one, connecting between R22/T7/T12 and the +12V rail, should be R21.
  • One of the polystyrene caps in the filter ladder has no number in the schematic; I make it C2.
  • In the schematic, the connection through the ladder from the audio signal input runs up the left side (T3–T6), and connects to the non-inverting input (pin 3) of op amp IS1 through C8 and R32; while the feedback input connects to the right side (T8–T11) and ultimately to the inverting input (pin 2) through C7 and R31. (Obviously this is a simplification; the sides connect repeatedly to each other.) However, on the board, the audio-in chain connects to the IS1 inverting input, and the feedback chain connects to the non-inverting input. As the two chains are identical between the filter inputs and the final resistors before the op amp, these component numbers are indeterminable from the schematics; the swapover could occur at any of several points in the signal path. I’ve gone with the last possible — in the drawing, the connections between the final resistors and the op amp inputs are R31→pin 3 and R32→pin 2. This is also about the simplest way of drawing a correction on the schematic.

Components

numbercodes / ratingtype
caseschematic
Semiconductors T1DR7 307SC307dPNP BJT
T2–T12DR3 C39SC239dNPN BJT
ICsIS1MAA741 Y10MAA741op amp
Trim PotsR18, R3610k10k
Capacitors 1, 34·7µF   ?%   40Velectrolytic
2220µF   ?%   6·3Velectrolytic
4, 5, 7, 1147nF   10%   ?Vpolystyrene
6, 91µF   ?%   25Velectrolytic
8, 10100µF   ?%   16Velectrolytic
12100nF   ?%   ?Vceramic plate

Full Set

All diagrams as multilayer PDF here. (483KB)

Other Components

There are a few components in the synth which are not on boards — keyboard, control pots, the actual waveform and noise selection switches, modwheels, and some power supply components. I’ll go into these in more detail in the next installation, which will also go over the rest of the refurbishment. This had to be left half-done a few months ago pending accommodation/workspace availability, but should be possible to complete by Summer.


Appendix: Transistors

Technical details of these are available at the mildly wonderful radiomuseum.org.

BC211NPN BJT rm.o page
KP303E [a]N-Channel FET rm.o page
SC236dNPN BJT rm.o page
SC237dNPN BJT rm.o page
SC239dNPN BJT rm.o page
SC307dPNP BJT rm.o page
SS200NPN BJT rm.o page

a: USSR-made, labelled in Cyrillic: КП303Е


Notes

  1. Renovation is proceeding.  
  2. This actually says Gesamtstromlaufplan, which seems to be an overall circuit diagram (er, now in 4 parts), but I’ll go with schematic . . . possibly it’s not intended to be as precise as a schaltplan; not sure of the different translations.  
  3. So my references to these pins are of the form: S(switchgroup)-(switch)-P(pin)(side). (e.g. S4-5-P6b is the output terminal for the sawtooth signal on the Octave 1 (FW1) board.) As it’s worked out I haven’t needed to include these references on this page, but it’ll happen.  
  4. Several others of these caps were a bit wiggly [5], and may have been making intermittent contact with other components, which might explain some of the odd noises. To be glued.  
  5. Is it just me, or is the English deficient in simple and sensible-sounding terms to represent the possible qualities of insufficiently secure connection of a load to a fixing point (or component in a circuit), or is it just that time of night? The only other term I can think of is floppy, and that’s not quite right. Either way — sleep.  

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