Help with KVINTET vintage soviet synth

[hal18k]

Hoping someone here has experience with these. whenever i play a note it also plays bottom G at the same time. I have no idea what would be causing this. Maybe something with the power converter? any help is greatly appreciated.https://youtube.com/shorts/e_f3cxrah-4?feature=share

[nosaj]

No experience with soviet stuff but. First check the voltage at the power convertor (wallwart) an make sure its correct.

If it was a yamaha or other brand I would try a factory reset first. If no change then
Then I would open it up and clean the rubber contacts then check the diodes on the pcb the rubber contacts hit. I cannot tell you if Soviet stuff is made the same way.

Per Wiki a description of the diode matrix and how it functions The BOLD sentence kind of describes your issue.

Nosaj

Without a matrix circuit, a 61-key keyboard for a synthesizer, electronic organ, or digital piano would require 62 wires to connect (one for each note, and a ground)—an awkwardly thick bundle of wiring. With a matrix circuit, any of 61 notes can be determined with only 16 wires. This is drawn schematically as a matrix of 8 columns and 8 rows of wires, with a switch at every intersection. The keyboard controller scans the columns. If a key has been pressed, the controller scans the rows—and in a manner analogous to the board game Battleship, the controller determines the row-column combination at which a key has been pressed, and generates a note corresponding to that key. This process occurs so quickly that the performer is unaware of any delay.^[1]

The inside of a Yamaha SY77 synthesizer shows the various internal components. The switches for each key are connected to the microprocessor chip using a matrix circuit. Even though the SY77 has 61 keys, only two small ribbon cables of wiring comes from the keyboard matrix.

There are at least two limitations with this system. The first is that it provides only a crude binary on/off signal for each key. Better electronic musical keyboards employ two sets of switches for each key that are slightly offset. By determining the timing between the activation of the first and second switches, the velocity of a key press can be determined—greatly improving the performance dynamic of a keyboard.

The second is that instruments with a matrix circuit can only play in a monophonic fashion without the addition of a diode for each key crossing.^[2] The diode prevents unwanted notes ("phantom keys") from being triggered, or intended notes from being masked ("phantom key blocking").^[1]

Monophonic instruments and most low-cost computer keyboards reduce costs by leaving out most or all of those diodes. To avoid "phantom keys", the keyboard controller in modern low-cost computer keyboards will ignore further key presses once two keys (other than modifier keys) have been pressed, which is known as jamming.

The matrix circuit approach is also used in non-musical keyboards, such as in the keypads for calculators and the "QWERTY" alphabetic and numeric keyboards used to enter information into computers.^[3] The same matrix circuit approach is also used in many pinball machines.^[4]

[Mick Bailey]

My own opinion is not to dismantle the keybed and to disturb as little as possible. Diagnose the problem first before taking any action - parts for Soviet-era synths are really difficult to find if you run into problems. My first though is that you have a shorted diode in the keyboard matrix which is causing the ghost note. I don't know what your skill level is, but you need to take a methodical approach and be able to use a DMM. The first step would be to assess how the instrument comes apart - you need to gain access to the circuit board that houses the diodes (one for each key). Then it's a matter of using the DMM in diode test mode to carefully check each one. Once you gain access the test only takes a few seconds per diode and you have to eliminate this possibility before moving on. The test is carried out without any power connected.

EDIT: Does the unwanted note sound with every key? A diode short wouldn't usually affect every key, but I don't know what the switching arrangement is with this particular model and it could be unusual.

[dmeek]

From the look of the internal photo here https://www.matrixsynth.com/2009/08/...esizer_29.html this looks like a Master Oscillator-Divider type keyboard.
I recently fixed an Elektronika EM-25 which among many other problems had a VCF in the Brass section that was stuck on full resonance-self oscillation, causing an extra "note".
Fortunately schematics were available at Ruskeys.net. Unfortunately no schematics for this keyboard.

[Mick Bailey]

You raise an interesting point with the VCF resonating - that would affect every key. Similarly if it was another shared function that was oscillating. A shorted diode would most likely affect a group of keys - in a single row or column.

[hal18k]

My own opinion is not to dismantle the keybed and to disturb as little as possible. Diagnose the problem first before taking any action - parts for Soviet-era synths are really difficult to find if you run into problems. My first though is that you have a shorted diode in the keyboard matrix which is causing the ghost note. I don't know what your skill level is, but you need to take a methodical approach and be able to use a DMM. The first step would be to assess how the instrument comes apart - you need to gain access to the circuit board that houses the diodes (one for each key). Then it's a matter of using the DMM in diode test mode to carefully check each one. Once you gain access the test only takes a few seconds per diode and you have to eliminate this possibility before moving on. The test is carried out without any power connected.

EDIT: Does the unwanted note sound with every key? A diode short wouldn't usually affect every key, but I don't know what the switching arrangement is with this particular model and it could be unusual.

Yes it effects every key

[dmeek]

The internal photo doesn't show the bottom of the keybed but I suspect there is an organ style keyer circuit with a one or two transistor "VCA" for each key. How the low G gets triggered for any key press is a mystery.
If you are comfortable dismantling the keyboard, post a photo of the underside.

[Jazz P Bass]

Some interesting schematics here: http://www.ruskeys.net/eng/pasp/maestro/pasp.php
If one note plays by itself when aonther note is triggered it could be as simple as the contact is stuck or dirty.

[Jack Harmony]

Did you manage to find a solution to this? I have a Kvintet here with the exact same issue and my tech is somewhat baffled.

I will post updates if I have success

Cheers,
Jack

[Jack Harmony]

Some notes from my tech:

Hi Jack, some duff soldering on these boards, have re-flowed them. Still trying to work out the signal flow, the divider IC’s are working nice and cleanly, fault is something after that. There is also a board right at the back that the other boards plug into, need to get that out and check it.

—

Have found a page of Russian schematics, but this thing is greyed out on the list. An interesting thing is that each key has a magnet under it which closes a reed switch encased in a glass tube. Never seen that on a keyboard before

—

Hi Jack, I’ve finally admitted defeat with your Russian machine and put it back together. I’ve still not fully grasped the signal flow, difficult without a map (circuit diagram). There is a board right at the back which routes signal to the filters and output, I’ve had it all out to check for bad solder joints (and all the other boards). That board is difficult to get to and signal trace when the other boards are in place. Fault is somewhere where the notes sum. There is nice clean signal coming from the divider chips I can get to, but some are inaccessible with the other boards in place and the synth working.

—

I will try to take some detailed shots of the synthesizer this evening, any help is greatly appreciated!

Cheers,
Jack