abyone familiar with this tone generator?

[davessp]

A drummer/synth player friend asked to build a tone generator off of an old 1982 Electronics Experimenter's Handbook.  I laid out the boards and spent weeks checking everything.  Made a small +/- 9v supply.  Ordered 10 boards and a few parts- cool trapezoid box.  After the boards arrived I found 2 wiring errors- easily fixed.  After fixing those- it still doesn't work.  Not knowing what is actually going on in the circuit- I can't tell if the schematic is any good.

I was wondering if anyone out there is familiar with it (attached).  Around the transistor/diodes- that, I don't get- and I'm pretty sure i can't measure it.  But remember, I only know exactly enough to be dangerous, mostly to myself.

Thanks!, DS

[Mark Hammer]

Not that familiar with it in particular, but it sure looks like one of those James Barbarello e-percussion circuits that showed up with regularity in Popular Electronics.

I like to joke that even though they only have 3 pins, there are probably 837 different pinouts for any given JFET, depending on manufacturer.  If it were my circuit, I'd begin by verifying that the pins I think are at each pad on the board are actually the ones they ought to be.  I'll just add that I doubt the circuit demands an MPF-102, since it is simply being used as a voltage-controlled resistance between drain and source.  If, for instance, you had some 2N5952 JFETs left over from attempts to match a set for a phaser, that could work just fine.  So could many others.  Of course, you'll want to verify their pinout as well.

[PRR]

> doesn't work

Insufficient information.

Use your Listening Probe to poke the opamp output pins. It looks like some should thump and some should whine, controlled by the knobs.

[Rob Strand]

Superficially it looks like it should do something 

You need to start by making sure each of the oscillators work (ie.  IC1 and IC3).   

Some things will be working and some not.

Measure the DC voltages.

[davessp]

Exactly- and those two IC's have voltages on them.  IC 2 has nothing, and I'm not sure what the transistor should be showing me.

[Rob Strand]

Exactly- and those two IC's have voltages on them.  IC 2 has nothing, and I'm not sure what the transistor should be showing me.

Zero volts around IC2 is good.

The way the circuit works is IC1 provides a ticks.  Imagine ticks at a slow rate like twice per second.  At extreme settings it becomes more of a tone oscillator but just stick with the slower rate for now.  If you connect two LEDs in parallel but opposite polarity then connect that in series with a 10k resistors you should be able to adjust the rate control R4 so it oscillates at a regulate rate - you will see the LEDs flickering.  Like this ckt.
https://www.multisim.com/content/6gkt9yCtnDsi5AjecGubYZ/polarity-indicator-back-to-back-leds/

Without this part of the circuit working the whole thing falls in a heap.

Once you see the oscillator working the next step is to get a "tick" out of the circuit R6 to D1.

The tick passes through IC2 which should provide a sound like a bass drum, or, like a "bell" depending on  the resistance of the JFET.   IC2 is actually a high-Q band-pass filter.  It is very common to use this type of circuit to get a drum or "bell" sound.   The Sustain control R10 sets how long the drum or bell rings for.   It the sustain is too short it will just produce a tick or thud sound.

So the problem I see at the moment is the filter around IC2 is set to a very low frequency [EDIT: maybe below what you can hear].  You might be able to make more sense of it if you replace the 100nF caps with 10nF caps.   

For now I would pull out the transistor Q1 and replace it with say a variable resistor somewhere around 4.7k to 47k.  It sets the pitch of the drum/"bell".

When it works the other oscillator IC3 varies the pitch of the drum/"bell".

FYI I think R5 should be called Level.

[Rob Strand]

I had a quick poke around.   

With the 100n caps the frequency is pretty darn low.    Even with the JFET replaced with a small value resistance, like 2k2, I'm only seeing 75Hz or so oscillation frequency.

With low values of the JFET resistance, less than say 2k2, it should be possible to get the band-pass filter to oscillator continuously with the Sustain control maximized.

One thing which seems a bit odd is when the depth control is on minimum, the JFET will be fully on and that will push the oscillation frequency up.    It makes you wonder where the natural pitch of this thing should be.

So maybe a good test for the circuit around IC2 would be to put in a 1k resistor for the JFET then see if you can get it to oscillate by adjusting the sustain put.   Watch out the output of this thing is pretty darn high.

C3 is 100nF not 1uF.

[davessp]

I just noticed the front page of the article which pictures what I assume is a mock-up of the device.  It only shows one rate knob - though there are two on the schematic.  Do you think they just slapped a picture together or is it a clue?

[PRR]

> the front page of the article

Date? Link??

We're not all going to read a whole year of EEH to find this article.

[davessp]

I can't seem to attach docs to the post.  Here is all 3 pages:

[anotherjim]

Typo in the schematic? R5 isn't another Rate control, it's Intensity or something (same as Depth really, but it already has one of those).