[sdiy] Integral scope on VCO

[Thomas Dunker]

On Fri, 16 Apr 2004, KA4HJH wrote:

> Thomas, you're in good hands on this list. Take it from the "old"
> timers--having a small scope in your modular synth is cool looking--and
> even useful--which is why you probably shouldn't build it into another
> module such as a VCO. Avoid the issues Harry and others have raised and
> make it a module all by itself. That's why we call them MODULAR synths! If
> you want to you can normalize the output of a VCO or a VCF to it behind the
> panels.

 Sorry about the late reply, been putting in lots of hours at work over
the weekend, preparing for a field test/demo the company's having with
lots of military brass tomorrow (tuesday).
Lots of antenna gear and power supply stuff for automobile use had to be
built, prepared and packed.

 Anyway, yeah, I can build a separate scope module, but it would still be
within inches of something else, so we'll see how it works out. I would
have liked to use toroid transformers to keep the stray fields as minimal
as possible. I'll have to dive into my junk stash to see if I can find
something I can use.

> This is IMHO more useful and will let you concentrate on getting the scope
> working and eliminating any problems with RFI, EMI, etc. It will also move
> the high voltages even further away the rest of your modules, reducing the
> possibility of a catastrophic short.

 Well, I am used to working with tubes, it's what my DIY electronics
projects have mostly been about up until the more recent synth related
projects. I haven't had any actual disasters yet. I have learned a thing
or two about the wisdom of adding fuses in strategic parts of the circuits,
however... And about the poor reliability of high voltage electrolytics -
some components are definitely not meant to be recycled...

> Finally, if you get it all working and post the design then anyone can
> incorporate it into their modular in any way they chose. I'm sure lots of
> people would love to have one.
> All this is purely my opinion but I think you'd be better off in the end.

 It would certainly be adaptable to any modular however I choose to do it.
I have been discussing some issues with a friend of mine and think I have
come up with a good scheme for the X sweep. Since I will be generating suboctave
signals, I would want to be able to display multiple fundamental VCO
cycles on the scope, so as to get the full picture of the waveform
including suboctaves. I think it will be most convenient to derive
suboctaves and X sweep signals from the same frequency divider fed by the
sync output of the VCO. The square signals from the divider can
simultaneously be used as suboctave audio outputs and be fed to saw
shapers to provide selectable saw sweeps at 1x, 1/2x, 1/4x and 1/8x the
VCO frequency.  These signals can also double as suboctave saw audio
outputs from the VCO. All this circuitry will be located on the VCO
module with just the sweep saw signals internally wired to the
scope module and suboctave audio brought out on jacks on the VCO's panel.

 In addition, the scope will have an internal sweep generator. A saw
output LFO with the appropriate frequency range should be sufficient.
Whenever the sweep freq. is locked to the VCO frequency, there will be no
need for triggering circuitry to provide a stable image. In my case, there
is the matter of how much front panel space I can allow for the scope,
since whenever I add something I have to drop something somewhere else.
(I chose to build a portable synth with a finite number of modules, so
there's no "extra space" for the scope, really. I'll just have to see if
adding the scope still allows me to keep everything else that I think the
synth should have.

 At the moment I am most concerned about having enough room for mixer(s).
Ideally I'd like to have separate "input mixer" and "output mixer", both
with pannable 2 channel outputs. Since the synth has two different VCFs,
two VCAs and a number of LFOs, I hope to be able to simultaneously process
two different signal mixes (say for instance splitting the suboctaves from
the fundamental VCO signal and filtering/modulating/processing the sub
signals and fundamental VCO signals independently, and mix or pan them into
separate output channels at the output, that sort of thing).

 On the other hand, I have a decent
Soundcraft Spirit Folio mixer that will probably go into the rig anyway,
so maybe I can keep it down to a single mixer module on the synth itself,
and just a small master 2-in-2-out volume/panning and headphone amp output
mixer at the output, taking up only a half module.

 Anyway, I just got the missing parts I needed for the VCO (tempcos
and the CA3046 transistor array) I ordered from Tony
Allgood (thanks!), so that's one of the first things to complete now.

> BTW, I don't know if anyone else mentioned it but Eric Barbour used to have
> a collection of old schematics that had a lot of useful info on building a
> small scope just like this. He's not on the list right now but you can
> email him at synth at metasonix.com. I have a copy myself but I'm not sure
> where it is right now...

 I have been in touch with Eric in the past on tube audio stuff. I tried
to mail him about the tube synth stuff out there (the thyratron VCO, and
pentode based VCA and VCF) a while back but just got a reply from some
secretary at the metasonix address above.
 I can always do a little Googleing and see if I get any more ideas...

 In the mean time I'm waiting for a reply from that Norwegian ham who
built the X-Y scope (for RTTY tuning purposes) with the CRTs I have.

 Anyway, thanks for the suggestions. I think you may be right that I
should just as well make a separate scope module (otherwise a combined VCO
and scope module would take forever to complete, I imagine).

 Probably the single most pleasant thing about modular construction is how
there isn't a point of "completion" way ahead in the distance, but the
whole job is broken into a number of manageably small projects that are
completed one at a time. It ensures a good sense of steady progress and
the fact that modules can be put to use as they are completed, multiplying
the number of possible sounds each time, is also very motivating.

 I've also found that the nicest mounting/construction work I've ever done
has been on this project. My tube amps always look like a total mess
inside once they've been through the usual number of tweaks and
experiments.

 Oh, on the topic of mumetal shielding the CRT - I have no experience with
mumetal, but I do have at least one scope that I can cannibalize for parts
(unless somebody can help me find the manual for a Dumont 190) including a
big mumetal CRT shield that could conceivably be recycled somehow...?

Thomas