EHX - Microsynth (early version)

MiceInMyMellotron · Post by **MiceInMyMellotron** » 06 Sep 2008, 14:31

Hi Folks, I've just signed up this very day.. aaah...

In at the deep end, then. My speciality is repairing vintage keyboards but I do like nice pedals and have repaired the odd one or two.

My current problem piece is my cheap'n'cheerful ebay Microsynth. It's the early version with built-in mains transformer and I really need a suitable schematic for it. The ones available on the net are appropriate for the later/current version and there are so many differences that it's not really helping me. I've even opened up a friend's new one and the two versions are very different.

If anyone could help I would be most grateful. I'd love to get this working but I'm in the dark without a schematic, there's just so much going on in there.

Thanks in anticipation,

MSW

Dirk_Hendrik · Post by **Dirk_Hendrik** » 06 Sep 2008, 15:39

Interesting.
I just fixed a really old one with reference of the current versions schematic. That's the one from the collection of schematics that's not to be posted by the force of wrath from Erlangen, Germany

There may be slight differences but most of the basic principles still stand. Definitely enough to work your way through the thing and get it fixed. Or, in other words, chances are odd that your error is in the schematic differences.

and,
What exactly is the problem you're facing (other than that the double board makes it a bitch to service).

Bernardduur · Post by **Bernardduur** » 06 Sep 2008, 15:52

Dirk_Hendrik wrote:Interesting.
I just fixed a really old one with reference of the current versions schematic. That's the one from the collection of schematics that's not to be posted by the force of wrath from Erlangen, Germany

There may be slight differences but most of the basic principles still stand. Definitely enough to work your way through the thing and get it fixed. Or, in other words, chances are odd that your error is in the schematic differences.

and,
What exactly is the problem you're facing (other than that the double board makes it a bitch to service).

Man, I did the same today......

cool!

Will post some pictures in the thread that still has no pictures

borislavgajic · Post by **borislavgajic** » 06 Sep 2008, 16:02

Mice(inthemellotron) ...the cat would be a logical solution...

.......
...still ...I can not help you ,but please if it is not too dificult for you,post some really good pictures of your EH pcb-s,and who knows....

thanks
boris

Hi Folks, I've just signed up this very day.. aaah...

and yes...wellcome!

MiceInMyMellotron · Post by **MiceInMyMellotron** » 06 Sep 2008, 16:31

Wow, that was quick.. Thanks guys!

The main problems that I have with this unit are as follows..

1) as bought, most- practically ALL- of the wires connecting the boards were missing, especially the wires to the switch and the in/out sockets. Most of the white ones that link the boards were broken off.
2) Somebody has definitely been in there before me. There are lots of pen marks on the board but they don't make any sense. That's why it was so cheap

3) I don't have equivalent numbers for the EH-numbered ICs. I'm not sufficiently experienced to guess what's what and I'm not about to try.

I'm a reasonable tech, I work on old gear for my living, generally have no problem successfully repairing most analogue synths etc, but I'm no electronics genius. I definitely need help on this one! Certainly, correct information would be a blessing.

Probably the best help I could have would be a schematic. A picture of the correctly-wired internals of an identical unit would really help too.

So, anything that you could do would be appreciated. If I could at least get the wiring right that would be a start and then the later schematics (which I do have) might then make some more sense.

Best wishes,

MSW

(http://www.myspace.com/markstuartwallis)

Bernardduur · Post by **Bernardduur** » 06 Sep 2008, 17:13

Check my pics on the other MS post!

Blue = input
Brown = output
purple is GND and is connected to point with the label "Li"
Black is GND for the LED; red is + for the battery
Grey connects the PCB's with each other as do the yellow wires.
Unit is powered by a 24V DC adapter which give out 30V unloaded; 27V loaded. As for the bridge + and - are reversible on the DC input.

I made this one TB as I really disliked the preamp.

MiceInMyMellotron · Post by **MiceInMyMellotron** » 06 Sep 2008, 17:59

Where is 'the other MS post' that you referred to, Bernarddur?

Ah.. Mine has neither LED nor battery. Like I said, it's an early mains-powered version. The board does power up, the transformer/rectifier/regulator bit seems OK.

I can't post pictures today, the pedal's in the workshop and I'm at home.

Anyone got pics of the insides of this version? If I could wire it up properly I'd probably be in with a chance..

Regards,

mark

Bernardduur · Post by **Bernardduur** » 06 Sep 2008, 18:27

MiceInMyMellotron wrote:Where is 'the other MS post' that you referred to, Bernarddur?

Ah.. Mine has neither LED nor battery. Like I said, it's an early mains-powered version. The board does power up, the transformer/rectifier/regulator bit seems OK.

I can't post pictures today, the pedal's in the workshop and I'm at home.

Anyone got pics of the insides of this version? If I could wire it up properly I'd probably be in with a chance..

Regards,

mark

Other MS post

analogguru · Post by **analogguru** » 06 Sep 2008, 18:29

The first Microsynth´s (from 09/1978) are in fact complete different to the well-known versions.

Since I have never SEEN an old on I suggest following deal:
If you promise me, to post some pictures of this unit (including the innards

), I will look what I can do for the schematic.....

So far for now - the chips which were used are:
RC 4558NB
EH 1048 = CA 3094
NE 5554N (for the power supply) and
CD 4013BE

stay tuned,
analogguru

MiceInMyMellotron · Post by **MiceInMyMellotron** » 06 Sep 2008, 18:36

Mr Guru!

You are a true star. I'll take some piccies when I'm next at the workshop and we'll see how it goes. Thanks for offering to help me.
You're correct, the two versions are very different- at least in layout. For example, there are no ICs on the slider board and the interconnections aren't numbered. Mine also appears to have had some traces cut on the sliderboard in the past.

You'll like what you see, it really is a bit of a mess. I don't think it's beyond help though.

Well, here's hoping anyway!

Best wishes,

MSW

drfrankq · Post by **drfrankq** » 13 Sep 2008, 07:22

Ialso have one of the originals, it sounds great except a noticeable AC hum when used going into a fuzz. When the square wave slider is turned up it gradually gets a little quieter...any idea how to get rid of the hum alltogether? Thanks!- Frank

Bernardduur · Post by **Bernardduur** » 23 Feb 2009, 20:14

Hmmmmm, I got one again..... early one, AC powered, great unit. Sounds absolute wonderful on trigger set to 0, when the trigger is raised no sound can be heard anymore.........

I guess the envelope filter is going beserk or something...... I thought maybe the slider was done but she is still OK!

Any thoughts on where to look first?

uncleboko · Post by **uncleboko** » 23 Feb 2009, 21:47

Check connections to trigger slider - outside tags to 0v and -12v, also check if IC26 is faulty

Bernardduur · Post by **Bernardduur** » 23 Feb 2009, 21:53

Thanks!

The slider is OK!

What is IC26????

I got signal (but no trigger) if I disconnect 6 (jumper to the attack modulator); then the trigger has no function over the whole sliding range.

So..... guess the attack modulator is broken (I already replaced the diode and the cap without results) or the 2 IC's before jumper 6 are done (will replace em tomorrow).

Bernardduur · Post by **Bernardduur** » 24 Feb 2009, 14:57

OK, found the fault

A4, A5, A15 and A16 were broken. Replaced em and now the unit works as she should be!

theehman · Post by **theehman** » 25 Feb 2009, 18:32

Was the transformer on that one mounted on the side or hanging from the top? Found what looks like a pretty early one in my garage the other day that has the transformer mounted on the underside of the top. Probably bought it as-is on ebay and forgot about it.

Bernardduur · Post by **Bernardduur** » 25 Feb 2009, 20:51

theehman wrote:Was the transformer on that one mounted on the side or hanging from the top? Found what looks like a pretty early one in my garage the other day that has the transformer mounted on the underside of the top. Probably bought it as-is on ebay and forgot about it.

Side mounted; I posted some pics on the other MS thread!