[HELP] EH Space Drum woes (schem inside)

[Zero the hero]

Hi people,
I'm struggling against the Electro-Harmonix Space Drum!
http://topopiccione.atspace.com/EHSDschem.jpg
(In case of problem, copy & paste the URL).

This schem is based on the files found on the "schematics" section of this site. I chose not to use EH1048 (CA3094) since they are expensive and not very worldwide available. LM13600 / LM13700 has been used, instead.
Another major difference is in the microphone: I adapted this circuit for cheap condenser mics.
In order to obtain Vref (+4.5V) I tied the unused op-amp of the LM324 as a 4.5V voltage follower.
?k is a 20k resistor.
I've built the "non deluxe" version, not connecting the mod in the upper rightmost part of the picture, and using the mod drawn in both bubbles.

I breadboarded everything as in the schem, but I found it was not working. The microphonic signal is correctly amplified, and I noticed that the envelope follower works, and from pin 14 of LM324 correctly (i presume) feeds the VCA.
The "control voltage generator" is working too, and the VCO is feeded by the first OTA stage and the second LM324 op-amp. I don't remember what kind of wave produces, I think it's a ramp.
Woe comes from the VCA: it doesn't oscillate.
I found that:
- swapping inverting and non-inverting pins of the second OTA used in the VCA, and
- swapping buffered and unbuffered connections of the first OTA (of the VCA), i.e. connecting the 0.1uF on the junction between pins 5 and 7, and connecting the "coordinator input" to pin 8 of the same stage,
produces the oscillation!!!

Anyway, the sound is very distorted and gated, I had to strike HARDLY the microphone to obtain a slight and short oscillation.
Any suggestion?
THANKS!

[choklitlove]

sorry, can't help.  but god bless you for tackling a schematic that looks like that: http://www.diystompboxes.com/pedals/spacedrum.gif.

anyway, when you finish it, i'll definitely build it.  i'm building your sonic boomer right now.  great site!

[StephenGiles]

I wonder where that circuit came from, it looks about 10th generation xerox copy. I was at EH UK (near West Norwood) at the time the Space Drum was designed by Joe Truchess. This circuit does not look like his writing though.

[Zero the hero]

  Actually, the writing is MINE 
This schem was hand-drawn by me last week-end and badly scanned at work. I could sell it on e-bay as a "NEVER SEEN ORIGINAL EH SCHEM" since it looks very vintage...
I'll look the input section once again, I think I fried a condenser mic.

[Celadine]

Hi all!  I've also lusted after the space drum, since hearing the demo on

http://www.keyboardmuseum.com/d_machines/vdrums.html

The big thing may be those EH1048's.  If they are pin-for-pin copies of the 3094, then pin 1 corresponds to "external frequency compensation or inhibit in" (as seen on 3094 datasheet).  I'm no pro with electronics, but I have seen "inhibit in" pins on digital chips, usually used to power down the chip.  On the schematic, the 'start freq' 1048 has pin 1 connected to...something.  Either the oscillator, the 'stop freq' 324, or both.  This may gate the 1048 in some way, or change the way it processes signals. 

[christian]

One thing that caught my eye is that you dont connect the collectors of the darlington in LM13700 to +Vcc. CA3094 does this internally and you have to do it externally here.
Pin 1 is just a tap after the G cell, which you replaced correctly with 13700.
The other decay cap is wrong (intentionally?). Original schematic has 2 100nf caps in parallel, other is switchable. Nevermind, there was the mod down there 
Resistor that is between the trigger 324 and OTAs control pin is too small. Original shows 20k to me?

The OTA in the middle seems to be wired wrong. Output to the next opamp with 0.1 cap to ground is taken from pin 1 (G cell output)
The coordinator input is switched with the darlington output (pin 6)

Whats the resistor in series with the "Mod Rate" pot? I guess its 2k2 since it should be quite small, cos the pot is 100k and you wouldnt get a decent range otherwise?

ch.

[Jaicen_solo]

I don't want to sound clever, but why the EH Space drum? In my view it's an overcomplicated design, and lacks features.
I've got an Amdek PCK-100, which is a clone of the Roland/Boss PC-2. That makes all the sounds of the space drum, and many more besides. It has an LFO that goes into the audio range for pseudo-ring mod tones, and decay controls as well as sweep (up&down) and pitch.
Failing that, there's the Coron DS7(?) which seems to be quite a popular percussion synth as well.
Of course, i'm sure you're doing it precisely because nobody else is interested in building one

[Mark Hammer]

Just to be certain, you do realize it is a crystal microphone, right?  Using something like a little electret cartridge/capsule or even a dynamic mic will provide the mic input with signal levels a mere fraction of what a crystal cartridge would provide.

[Jaicen_solo]

That's a very good point Mark. For testing I use the trigger out from my DR-110 drum machine, which gives a solid +5v 10ms pulse for reliable triggering.

[Zero the hero]

THANKS EVERYBODY!!!!!
All of you gave me good points and hints, I'll check them all later this night at home.

I've left this project for a couple of days, since I was stuck, and I repaired my Commodore 64 instead (one memory chip was broken). Now that AT LEAST this one is working I can switch back to the Space Drum prototype.
More comments soon.

[choklitlove]

that is weird... today i almost got a book at the library with a ton of commodore codes/service info in it.  i thought about getting mine out from under the bed and messing around.  but then they told me i owe over $25.  i didn't get the book. 

[Jaicen_solo]

That is indeed weird! I think a memory chip has gone in my old '83 commodore too, only reads 30912 bytes free. Have you got a link to how you can fix that?

[Zero the hero]

http://www.commodore.ca/manuals/default.htm
http://www.commodore.ca/manuals/Tips/c64_tips.htm
for tips and hints on troubleshooting hardware failures
and
http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/c64/manual/index.html
for the complete service manual. I suggst to browse the whole schematic and documents directory at
http://www.zimmers.net/anonftp/pub/cbm/
there are lots of infos about fixing a Commodore computer.

Also:
http://personalpages.tds.net/~rcarlsen/cbm.html
Nice and comprehensive repair info page.

I bought my C=64 on e-bay and it has been sold as "fully functional"...
When the package arrived, I plugged everything and on my TVset appeared a "out of memory error at 0" warning.
I decided to keep the whole package anyway (but I gave no feedback to the seller), since it was very cheap (15Euros for everything) I had a chance to mess around a bit with it.

Spotting the not-working chip was very easy: I tested all the memory chips one by one by simply piggybacking a working 4164 on them and turning the computer on.
Seven chips returned the same "out of memory error" and one of them returned the usual *** COMMODORE 64 BASIC *** logo, but only 4128 bytes were stated as free! Anyway, THAT was the not-working chip!!!!
I desoldered it and soldered a socket instead, where I put a working 4164 mem chip.
THAT'S IT! Now I'm enjoying Paradroid, Montezumas Revenge, Mission Impossible, Lazy Jones and many more again (what about the Wizball main tune?!?!?).

I found a couple of program that allows 1541 or other drives to be connected to modern PCs, in order to transfer .T64 od .D64 files back on real floppies, ready to use on a real C=64!!!!!
Look for XE1541 cables and the "Star Commander" program...

Good luck!

[Jaicen_solo]

Thanks for that, looks like it is indeed a bad memory chip. Probably one at the higher addresses since i'm not getting out of memory errors. I'll see what I can find!

[Dave_B]

Weird timing, as I just packed up my '83 Commodore 64 and it's $400 IndusGT floppy drive. (!)  You'd think it was a vital organ the way I handled it.   

[StephenGiles]

This explains how the peak envelope follower works.

[nelson]

Stephen, you are A GOD!


In all seriousness, your knowledge and experience is invaluable to this forum.


Thank you for your contributions.

[Zero the hero]

Thank you Stephen!!!!!

[StephenGiles]

I just happened to be at the right place (EH UK) at the right time (1980)!

I have been saying for sometime that a combination of the Peak Envelope Follower from the Space Drum and the Envelope Generator in the Microsynth (for instance) should produce a pluck sensitive VCF, with a decay independent of the decay of the guitar note, but with a start frequency controlled by the pluck strength.