This is a 25 year old test tone generator (have several) that I want to turn into a VCO. There is a CA3080 inside and some LM301s. All top shelf components and marvelously built. It sold for a whopping 950.- Deutschmarks back then but some caps started leaking. The rotary has 6 stages (60Hz,1K,3K,7K,10K,14KHz) so the second thing is to replace the fixed resistors to have octave steps. The tricky bit will be to find the right spot in the schematic for the CV input and make it V/Oct.
beautiful beast …
Omg look at that huuuuuuge potentiometer! Nu the way is it a potentiometer?
The red thing is a 6-position, 3-pole rotary switch. The PIKATRON is an output transformer.
Not sure if you can CV this one, though — the thing to the left is a Wien bridge oscillator that produces beautiful sine waves (the LF351, C2, C3, and the precision resistors in the rotary switch), the rest is gain control to make sure it keeps doing that with a minimum of distortion.
(A guy called Hewlett figured out how to do gain control with a simple light bulb in the late thirties, btw, and a similar oscillator was HP’s first product. Yours has a somewhat snazzier circuitry, though )
Yes, Fredrik you are right. It was purely designed for accurate frequency and low distortion. I wouldn’t mind to replace the main circuitry and just keep the output stage and the rotary original. Plus the housing because i have racks and more modules in this format.
I have these big LDRs lying around for decades and thought i build a “dual vactrol” to control the frequency of the wien bridge. Then i rememered these black thingies and it turned out that they’re exactly what i need, 1 LED 2 LDRs
Wouldn’t be my first pick for controlling a vco. I do see vactrol low pass gates in your future. There are also phasers and compressors that use them.
Progress, the vactrol works. As i expected it has a slow slew-rate and i guess also quite a temperature drift, not to mention the crooked volt/octave behaviour but i don’t care
Beeing designed as a broadcast reference generator, the output is hot as hell, enough to drive a speaker directly. The hole between the CV and the output will be a tuning pot or a range switch.
Since i have 2 of these babies, i detached the output transformers (see schematic above) and will use them as a passive old-school ring modulator
Those are the bottom jack sockets on the modules below. The rotary switches will be waveform selectors.
Any suggestions for a square-triangle-saw-pulse-width design ?
The dBm levels in the schematics aren’t that hot (+6 dBm into 150 ohm is ~2 Vpp) so I assume you measured that at the output from the BC328/338 transistor stage? (which doesn’t just provide voltage but also has low impedance; you can get the same signal levels from a TL072, at least if you power it with ±15 V, but that one can definitely not drive a speaker )
yes, the output from the transistor stage is 20 Vpp (without the speaker load).
The sine wave is a bit distorted/clipped, not sure if this is because of the slightly unsymetric vactrol or the missing load from the transformer or both.
And before you mention it, that output transformer unfortunately doesn’t have symetrical windings, so it won’t make a good ringmodulator. I may have other transformers that are more suitable.
Passive ring modulators are a fav fx of mine and very easy to build. I actually dug out a pair of wee transformers today to build a passive ring mod into a friends guitar. Its a brilliantly effective effect.
How did you connect the ringmod to the guitar and what is used as the carrier signal?
I’ve tried out a few "onboard " fx after falling for the vox setups years ago. For the ringmod I put the circuit in series with the bridge pickup wit a true bypass. The carrier is a 9v battery and 555, though I have tried using the neck pickup with another audio transformer.
Interesting, I play guitar in a band. I run seperate signals for bridge and neck pickup over a XLR cable into 2 different paths on the pedalboard and mix them together with the amp channels. I’ll definetly try to implement a ringmod somehow.
Not all pickups give a strong enough signal so worth breadboading. You can add an ldr to the 555 for more fun.
I wish i could change the title of the thread into “The Experimental Rack”
The frequency of the 2 wien bridge tone generators are controlled by vactrols (red LEDs are in parallel). I tried a double Potentiometer in parallel to alter the frequency but this often stopped the oscillation, thus the hole between the jacks. I think i add output volume pots now.
The 2 Modules next to the generators hold the passive ring modulator. There is space left for schmitt triggers and other wave shaping stuff.
The format of the rack is called “Danner” btw. The connectors were very sought after a few years ago, when everybody was racking up vintage broadcast modules, but not anymore. It still feels lavish to use them for that purpose but i have plenty.
In the video you see/hear the output of the ring modulator. I unplugged the control voltage of Gen1 and then Gen2 showing the behaviour of the vactrols. The wien bridges still oscillate on a very low frequency without CV.
I’ve changed it, but also leaving the original title so people know how it started out.
more racks and blank panels. The modules are only about 3,5 units high, that’s why I put the one rackspace panel inbetween. It’s supposed to be 4 buffered multiples or just 2 plus 2 mixers. The 8 jacks in the center will be unbuffered.