Help with vintage soviet Synth

[angelothewolf]

Hi there!
i got no oscillator on this baby (was plugged on 220v by mistake), can someone give me a hint of where the oscillator is generated? every thing works except osc:


[ATTACH=CONFIG]38944[/ATTACH

[angelothewolf]

nobody???

[Enzo]

Well, to be honest, Russian schematic and synths not being what most of us work on, it is hard. I used to be great working on DX7s, and got pretty good on old Oberheims and Sequentials. This thing is ancient.

I don't understand your question. To me the oscillator is the heart of tone generation - no oscillator means no tone generation. And no tone generation means no sound coming out, and that leads me to wonder what we mean by "everything works except..."

[angelothewolf]

well, the noise generator works, so the filter section, with the modulation, etc. but the tone generator is not working

[dmeek]

Looks like the oscillator is built around DA4, DA6 and DA7. What are the
numbers on those chips? They appear to be regular op-amps, but I'd
like to see the datasheets.

Doug

[angelothewolf]

alright! this chips are K140UD7 the soviet version of the 741

http://www.radiant.su/files/images/Voshod/K140ud7.pdf

[dmeek]

DA4 is a 741. DA6 and DA7 have a different pinout.
Interesting page here:
https://en.wikipedia.org/wiki/Soviet...it_designation

looks like the oscillator generates a sawtooth wave, DA9 converts it to triangle and
DA10 converts it to square/pulse.

[angelothewolf]

thanks man!

[J M Fahey]

Not exactly a keyboard guy here , but:
1) this is the beast in question

as a side note Russians seem to have been *hot* on synthesizers, I found a page with tons of them, and appears to be well made; this one was an "educational" model, could be played live but mainly intended for classroom demonstrations and sound effects (Soviet UFO movies anybody?)

2) it IS a 220V powered synth, so don't understand the possible cause mentioned

3) that said, first of all check that supplies are working as intended, then that suspect area Op Amps receive proper +/-15V as intended, then that audio is generated straight at Op Amp pins, they might work yet some switch or connector might cut the chain somewhere.

4) scope Op Amp outputs with scope set to DC and on a full screen scale of, say, 20V , we want to see some waveforms and we do NOT want to see an output stuck to +15 or -15V and staying there.

Not a keyboard guy by any means but that is basic/generic troubleshooting.
Post results and, just out of curiosity, some gut shot if possible.

PS: if not lucky, there is a Synth-only Forum, google it, they might know it or recognize the basic modules , as in: "hey, it uses a Spritzen Nitzen oscillator, wow!!!" followed by "check C14, if out of spec it will kill the oscillator" type answers ... to each his own and these guys eat, sleep and breath Synthesizers.
Lots of Germans hang up there and they might be acquainted with funky Soviet stuff.

[olddawg]

Usually in any old stuff no oscillator means dry caps. Just sayin'...

[Mick Bailey]

A nice thing is the waveforms are marked on the schematic, so at least something to go on to see if any of these are present at all.

I've had a few older synths that have suffered from poor electromechanical contacts - pots and switches. Especially switches.

No note output can also mean a keyboard fault even if the oscillator is working - you need a voltage to give the note value (usually 1v/Oct) and a gate or trigger pulse. Does the keyboard have a connector to the main circuit board?

[km6xz]

It is called the Model "Rythym 2" and is a teaching synth. The power supply is critical, verify the output is what it is supposed to be or nothing is going to work properly. The parts are pretty common and low cost. Don't adjust any internal trimmers unless you know what it is supposed to be since proper circuit function depends on those timmers being set specific voltages and in some cases, currents. I find that old Soviet era electronics is pretty solid and caps don't seem to dry up and stop working. They were popular for pop music based in Moscow, but the term "Moscow pop" is used in about the same condescending manner when rockers in the US referred to "bubble gum". It is the crossover stuff, performed lipsync by singers every night on Moscow based TV variety shows. I have seen dozens of brands and each having a number of models in the 70s. For those circuits using current sources, the subbing of a 741 might be so helpful since the bias input current is a lot different on these than a western 741. The original ics are cheap and still produced. One of the semiconductor producers is the known by their vacuum tubes they don't produce any more "Svetlana" which in the west went by SED or Wing-C.

You can still hear these types of synths in restaurants or old style resorts where musicians play all those classics from the 60s and 70s of Russian pop music.

[J M Fahey]

Interesting.
Care to share a playlist of those "moscow pop" songs?

Al google shows is Moscow based "pop" radios ... playing brit/US modern or classic Pop in English.

Thanks.

[Mick Bailey]

With regard to dried out caps - synths don't use electrolytics in critical situations, particularly in frequency-determining circuits where precision and stability are needed to maintain correct pitch. They are used for signal coupling though and are a clue to where the main functional blocks connect.

Can you get the VCA/EG to trigger off the keyboard? I would do this by setting it up to 'play' the noise generator. A key press should give a shaped noise output. If this works you've tested that the keyboard is triggering.

[angelothewolf]

It works!!!!

thanks all of you for the help! when it came i found several ICs with 13vdc on the ouputs i replace those but it came back to life when i replace the Fet VT16, (with a 2n3819) i dont understand exactly what this fet is doing there, but it works!

a few years ago, the same client brought this synth for 220v to 110v conversion, it was working perfectly so i replace the mains transformer and everything work just right. Now he plugged on 220V, the fuse blow up, but it took two psu filter caps with it.

this model has some different components than schematic but is all about the same. very difficult layout to troubleshoot btw...

thanks all for the help!