CYGN-A

[laserist]

20210219_171638~2.jpg

The freq pots are not grounded. Oh and my drawing has a error. I'm pretty sure the 1meg pots were dual reverse audio taper.

The spiral select switch was the image source for the spiral card. Originally on the Mark IV the spiral source was a fixed quadrature osc on the spiral card with the Mark VI and maybe Laserock the existing switch was retasked as a spiral source select. Originally three positions and eventually five. CYGN-A, CYGN-B, KQO, and two others I can't quite recall were the new sources.

[laserist]

1. CYGN-A
2. CYGN-B
3. KQO
4. DANUBE 8's
5. DANUBE 0's

[Greg]

And thus, a slice of history is revealed in a log book entry. The photo shows this circuit built using a KIA324P quad op amp. The output is a beautifully tunable quadrature. But the output amplitude is only slightly over +-1V. Is this correct? I notice the 10K in the summing amps suggests some gain there.

As the SPGN schem receives my closer attention, I realize that I don't have a clear idea of what this circuit actually does. I see the inputs for an external cycloid. It is like a VCA in which a shaped ramp modulates gain? Or is the ramp for the "sweep"? Is the sweep a varying offset in one axis? What is PDM out? Spiral in rate control? How was the reset used? Your comments, as always, and as your time permits, are very much appreciated.

[laserist]

The spiral card had 3, 0 to 10 volt inputs from the 351 data, and two digital inputs. Out rate, in rate and sweep were the analog signals, spiral reset and diamond spirals were the digital inputs. So you could control how fast the ramp generator ramped up and down, reset the ramp, and the sweep was summed to the image input. The ramp generator produced a ramp and an inverted ramp. Two pairs of 2QMMs modulated the input/sweep as spirals and inverted spirals. Analog switches could apply the other ramp to one input of the multiplier pairs for diamond spirals. I've never looked at the ramp gain. I assumed it's 0-10 V, but the key is the resistance before the 2QMMs. The PDM output was a square wave at the spiral frequency that was used as an offset on one axis of the fixed cycloids. Originally used in "Summer Madness" in Laserium I.

[Greg]

An error causing the low amplitude of the CYGN-A oscillator was found. Photo shows the three oscillators.

Other photo shows the collection of the less common semiconductors which should be able to become the SPGN, except for the Burr Brown, with which this build will have to do without. If I may ask, what is the part of the circuit that contains the dual transistors doing?

[laserist]

The dual transistor in the spiral ramp circuit is similar to the 2QMM circuit. I'm guessing, but I believe it was to get a log frequency response from the 0-10 volt up down ramp control voltages. You see it in audio synthesizer circuits where they're after one octave per volt VCOs.

[laserist]

I'll send the BurrBrown chip with my next shipment of Laserium artifacts. It just occured to me that it might be a cool effect if you used the ramp shaper on just one axis. It might be similar to the KQO symmetry effect.

[Greg]

I'll send the BurrBrown chip with my next shipment of Laserium artifacts. It just occured to me that it might be a cool effect if you used the ramp shaper on just one axis. It might be similar to the KQO symmetry effect.

Definitely I'll try that if / when we get that far. And thanks for offering the last known remaining Burr Brown chip!

The info in many instances is not possible to read with certainty from the SPGN schematic. Thank you very much Ron, for the great scans of your board, without which this would be much more difficult. After much deductive reasoning using the scans, parts placement charts, component lists, and the schematic, the following points of interest turn up:

The documentation contains an earlier and later version of the SPGN. Ron's board is the later version. Some parts in this version have different values, and there is a resistor (R52 470 ohm) that does not appear on the schematic. There is also a diode and some jumpers on the back of the board, but I haven't enumerated the diodes and caps yet.

R30 = 330K on the schematic I assume must be a mistake. It's listed as 82K in the parts list.

A comment is sought regarding the Y shaped schematic symbol appearing on the analog switches. I'm not sure how to interpret that. (see attached)

[laserist]

Definitely I'll try that if / when we get that far. And thanks for offering the last known remaining Burr Brown chip!

The info in many instances is not possible to read with certainty from the SPGN schematic. Thank you very much Ron, for the great scans of your board, without which this would be much more difficult. After much deductive reasoning using the scans, parts placement charts, component lists, and the schematic, the following points of interest turn up:

The documentation contains an earlier and later version of the SPGN. Ron's board is the later version. Some parts in this version have different values, and there is a resistor (R52 470 ohm) that does not appear on the schematic. There is also a diode and some jumpers on the back of the board, but I haven't enumerated the diodes and caps yet.

R30 = 330K on the schematic I assume must be a mistake. It's listed as 82K in the parts list.

A comment is sought regarding the Y shaped schematic symbol appearing on the analog switches. I'm not sure how to interpret that. (see attached)

Look on the right side of the drawing under the conspicuously unused revision block, and those are the inputs to the "Y"'s. The gates generate an inverted version of the spiral reset, sweep direction, and diamond spiral signals to turn the 5012 switches into SPDT switches. R-30 is on the "delta frequency" input. Now there were laserists and there were Laserists. Laserists tended to expand the capabilities of their machine. Sometimes by fixing things that were just wrong, and sometimes changing things to make it better. An 82K resistor instead of a 330K resistor would give a much greater range in the "fine frequency" adjustment of the spiral rate. There was a diode or two added in 1979 to prevent a really unlikely lockup of the spiral ramp circuit. I remember this because it was put in backward on the machine that was my first installation. Spirals didn't work - and "Laserock is almost all spirals", said the tech on the installation - and BTW the tech who put the diode(s) in backward - but in the end he was a hero - we discovered that if you shorted the input to a comparator with +15V there was a ramp. No control, but "you can't always get what you want..." He sat at my ankles for the shows that night with two test probes stuck into his hand. As CR4 & CR5 aren't on the parts list bat are on the layout, I'm guessing they're to keep the up and down ramp voltages from going negative. Perhaps never a problem before changing to an 82K resistor?... Not really sure about the 470 ohm resistor.

[Greg]

With regret, I never saw a performance of Summer Madness. The rhythm of that bass is also in something very well known that appeared a couple of years later. (Something about diamonds I think.)

Might that be a test point on the output of A4?