VCO High Frequency Tracking modification

Hi everyone.

Well, I thought I was done, but when I finally got to playing with the few modules I actually have racked, I found out first hand that the original Steiner VCO was not immune to the dreaded high frequency tracking problem in exponential converters. I approached Nyle to see if he had any ideas and he had actually explored this problem, and had added a correction circuit to the Microcon. He drew a couple of ways to add the correction and I undertook to investigate. I ended up using the one which was the most direct to implement and did not require cutting PCB tracks. I also have built a very small PCB which can be added as a modification to the VCO if the HF tracking error bothers you.

I'm not going to re-issue the VCO PCB with this correction modification on the PCB. The original did not have it, so mine won't either. This is an easy modification and only requires you to solder three wires to the main VCO PCB. The best thing is it works. I get excellent tracking from 50 to 3200 Hz.

I should also note, using a Polystyrene capacitor for the VCO timing capacitor, and good metal film resistors with a low temperature coefficient makes this a very stable VCO. I left mine on in an open rack for a couple of days and had a drift of under 1 %.

I also updated the calibration section of the VCO page, in particular the Sine shaping section. I'm planning a video for the YOuTube channel going through the calibration procedure, so stay tuned.

Anyway, see the web site on the VCO and in particular the HF tracking page.

http://www.xmission.com/~dingebre/VCO_A_1.4.html
http://www.xmission.com/~dingebre/HF_CORR.html

If you bought a VCO PCB from me and I haven't written you, please PM or email me. I'll send you a PCB when I get them. Probably two weeks.

David

DONE! FINI! FIN! IT'S ALIVE!

I pronounce the Synthasystem fully resurrected. The last module was the Ring Modulator. Nyle spent a couple of hours with me today and it works. I can now make the order for a small production run of the last three PCBs. The panels are done and now I'm going to build my modules.

I'll update the web pages over the next few days and get the module information up to date.

Technically, I suppose I have to do a keyboard, but for all practical purposes, the Synthasystem modules are all done and tested.

Keep watching for more videos, build photos, etc.

David

First Videos are up

The first YouTube videos are up. Have a look and watch for more!

http://www.youtube.com/watch?v=U0uZzLIi0jk

http://www.youtube.com/watch?v=QimHwslIuoE

http://www.youtube.com/watch?v=G3FSFm6JH-Q

MC1495 blues

Nyle got the Ring Modulator prototype and right now, it looks like the 1495 is bad. I tried a second one which did the same thing. Tonight, I'll take some of the other ones and will test them to see if I just have a whole batch of bad chips. Good news is the ON Semi pieces are on their way from ArchadeGameChips.com and I'm hopeful they will work.

One bug, AAARRRRRRRRRGGGGGGGG

The Input Amp works, the Voltage Follower works, the VCO Bb (saw/square) works... the ring modulator doesn't. I'm confident the capture and layout are faithful to Nyle's design, but I was sure on the ENVDT, too and I had missed a diode completely and had one net wrong. Nyle took the PCB from me today and is going to play with it.

Honestly, I just think I have some bad MC1495s. They are the older Motorola parts, but are from a reputable dealer. That said, ArcadeGameChips has a big stash of the ON Semi ones. I ordered a bunch and if Nyle can't find something I did, I'll try the ON Semi chips.

On the bright side, the only module I haven't built is is the VCO Ba (sine/triangle). Because it was derived from the VCO A, and had no new circuitry, I'm very confident it will work. I'll build one this weekend just to make sure.

I'm going to hold off just a bit on the Duo-Phonic keyboard and I want to get all my modules built and racked. I need to build a couple more oscillators, filters, VCAs and then I'm calling the modular part good.

I'm still conflicted on what format exactly to use for the keyboard electronics. I could do a typical keyboard, or build it in its own standalone box with midi inputs, or even as a racked module. I want to put a Pitch Bend wheel in in place of the panel pot Nyle used, but that might be awkward on a vertical panel. Anyway, that's why I want to wait. The way it will work, I think I will build it either as a separate box or racked module. But, that's for later.