Hex EH Micro Synth

[Vsat]

Fixing up an EH Micro Synth (chip datecodes are 1979-1980)... this one has 25 chips, 14 of which are CA3094.. the rest are RC4558, plus a CD4013 and NE5554.

This is different from the schematics I've seen on the net, which have seven of the CA3094. I traced the schematic from the PCB and the main difference is that the additional OTA's provide remote level control for the octave, suboctave, distortion and guitar signals, rather than sending the signals directly through  slide pots. The regen level is also controlled by an OTA. Wonder why they did it this way? Perhaps EH was developing a polyphonic synth with a hex pickup - with a single group of sliders setting the parameters for all six channels at once... anyone EH historians on the list?
Regards, Mike

[puretube]

are they "integrated" in the PCB layout?

(the datacode on the ICs themselves just tell you, that EH bought them quite a while ago :wink: )

StephenG., and Howard Davis have some genealogic knowhow...

[Vsat]

Fairly nice looking light-green epoxy-fiberglass PCB with soldermask.... all of the parts with datecodes have about the same date... including the RC4558. I assume/hope this isn't the reissue. The 14-pin regulator chip sure runs toasty though... +/-22V in...
Mike

[jimbob]

Sounds like my re-issue. Takes a 24v which actually only used around 15. I have the same green epoxy board. I didnt think the originals had that king of board.

[Vsat]

Hi Jimbob,
The unit here has a power cord attached and a built-in transformer,
does your reissue use a wallwart? This one says "made in USA" near the bottom, doesn't look that old, except for the chips.
Regards, Mike

[jimbob]

I dont have the built in trans w power cord but from what i understand they can be built that way differently though.  I have some Eh stuff ( Del Memory man) that has a wall wart  and is new but there are also others that have one built in that are new.

example-http://www.musiciansfriend.com/srs7/g=guitar/s=effects/search/detail/base_pid/153306/

I think ..Im not going to try to figure out EH.. I see a lot of variety with that company.

[puretube]

afaik: powerchord=vintage; wallwart=re-issue... (cc. microsynth)

[jimbob]

thanks Puretube- wasnt sure bout that one. Is that the rule? New ones have the wall wart  and old ones the built  in cable?
'
So according to the musicians friend web site there selling vintage?

//www.musiciansfriend.com/srs7/g=guitar/s=effects/search/detail/base_pid/153306/

[puretube]

afaik: powerchord=vintage; wallwart=re-issue... (cc. microsynth)

:roll:

maybe it`s better to ask the "E-H Man"

[jimbob]

maybe it`s better to ask the "E-H Man"

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You lost me there? Answer =?

[puretube]

check The EH Bulletin Board here for possibly definite answers:
http://electroharmonix.ronsound.com/


I didn`t say "rule", but "concerning MS"

But as a rule, I can say: there is no rule... :?

[StephenGiles]

Mike, I have the circuits for that version at home. I had a couple of non working units from EH UK, and yes, the regulator chips certainly ran hot. I remember changing it for more traditional 78 & 79 types which cured that problem. It would have been nice to have had an envelope follower function on the VCF, as it was all there in the control circuitry.
Stephen

[Vsat]

Hi Steve,
Seems a bit excessive to put +/-22V into the regulator for +/-11V out. Swapping it for a pair of 3-terminal regulators probably wise in a "pre-emptive surgery" sense.

Unit is working nicely now.. a lot of fun really...even though the VCF sweep is the same for each note

Total of three blown RC4558 in there. So, no "EH Poly-Micro" in the archives?
Regards, Mike