Need help debugging an EH Microsynth

[slacker]

I've got an old MK1 Microsynth that I bought "spares or repair" a while ago, it was a complete mess when I got it but after a bit of work I got it mostly up and running.
The only things that don't appear to work are the distortion and the attack delay, as I don't really know what the attack delay is supposed to do I thought I'd have a go at the distortion first.

Here's the schematics, courtesy of Mr Giles.
http://img.photobucket.com/albums/v517/uncle_boko/EHmsynth1d.jpg
http://img.photobucket.com/albums/v517/uncle_boko/EHmsynth1c.jpg
http://img.photobucket.com/albums/v517/uncle_boko/EHmsynth1b.jpg
http://img.photobucket.com/albums/v517/uncle_boko/EHmsynth1a.jpg

I get no sound at all from the distortion. I've checked the pot and traced through the circuit for bad solder joints etc and everything looks OK. The voltages on the relevant ICs look good. Using a audio probe I get sound up to pin 3 of IC17 but I get nothing on pin 1.
I don't really know anything about OTAs so is this likely to mean that the chip is damaged or could a problem with another part of the circuit cause this?
Any suggestions on anything else I should be checking out?

[oskar]

This is somewhat of a wet dream...       

OTA's have a diode forward voltage drop on the bias input, pin5 for these fellas, just like the 3080.
IC 17,18,27 pin 5 ~0.6V

oskar

[oskar]

When you turn down the volume on the octave up/ guitar is it no action on the last bit of the pot? The pots are voltage
dividers and there should be no action from -11,3V and down which could be like 20% of the pot if they are logs.

[slacker]

OTA's have a diode forward voltage drop on the bias input, pin5 for these fellas, just like the 3080.
IC 17,18,27 pin 5 ~0.6V

Do you mean pin 5 should be a diode drop higher than pin 4, which is -12volts?
I'll have a look tomorrow, presumably if pin 5 isn't the correct voltage that's bad?

[oskar]

OTA's have a diode forward voltage drop on the bias input, pin5 for these fellas, just like the 3080.
IC 17,18,27 pin 5 ~0.6V

Do you mean pin 5 should be a diode drop higher than pin 4, which is -12volts?
I'll have a look tomorrow, presumably if pin 5 isn't the correct voltage that's bad?

yes, sorry... 0.6 volts over -12volts...
Well, if pin 5 isn't -11.4V then there is a problem with either the resistor feeding pin5 or, well the chip is bad or not connected to -12v properly.
bedtime...

[oskar]

I'll have a look tomorrow, presumably if pin 5 isn't the correct voltage that's bad?

Actually, that OTA is hooked up as a noninverting gainstage. The 100k resistor after the OTA makes the current out into a voltage.
bla. bla... If it is dead you could just substitute it for a basic OP voltage follower (or noninverting gainstage if you need gain)...
If you can stand the thought of it not being the original stuff inside that is...
It'll work, I promise!   

[slacker]

I get about a diode drop between pin 5 and the negative supply, suppose the chip could still be damaged though. I'll have a poke about some more and maybe reflow some of the solder joints.
I like the idea of replacing it with an opamp if it is damaged, I reckon with a bit of creative surgery I could fit a TL072 in there.
Thanks for the help Oskar

[oskar]

Depending on what pins are connected... 3080 ( probably just one patchwire needed ) would be better or a single OP because many pins would be just
right where they are. I saw one today in a store but since I can't afford one for a very long time, I didn't try it... it
would just have broken my heart.     

I'm thinking of building one though.

About the controls...
Those faders/pots (?) that control the other OTA/level. Do you get full control of the soundlevel of the octave up and guitar channels or does
the sound go out completely noticeably before you've reached the bottom of the fader?

[slacker]

My thinking was that a dual opamp would be easier than a single because, pins 1,2,3 and 4 of the CA3094  that's in it now match half of a dual opamp, and it would be easy enough to add the 2 resistors needed to make it into a gain stage. I'd just have to reroute the power that goes to pin 7 to pin 8 of the opamp, pin 5 is already tied to +12 through a resistor so I could just solder pins 6 and 7 together to stabilise the spare half of the opamp.  Smallbear does sell 3094s but unless I can think of some more things I need to make up a decent sized order its too expensive compared to the cost of an opamp.
You're right about the level controls they only work over about the top 70% of the travel, I haven't measured the voltages but I guess below that there's not enough current to turn the OTAs on.
It would be interesting to build one, I don't know how different they sound but the second version would probably be easier. The original version is completely over engineered, like using the OTAs for level control when they could just have used the sliders as voltage dividers like on the later versions and using OTAs simply as gain stages.
To be honest the filter part is the thing that is worth building, I think the rest of the stuff could be simplfied a lot whilst keeping most of the sound. 

[StephenGiles]

You can't simplify the sweep generator that much. Why quibble over the cost of a CA3094 - that never ceases to amaze me!

[slacker]

You're right it's only about 4 quid including shipping for one CA3094, just seems a bit silly getting one shipped half way round the world if an opamp will do the job. If it was in mint condition I wouldn't hesitate at buying one but as it's very far from mint I don't mind the odd bodge if it works.

[StephenGiles]

I can't help thinking that there must be an old fashioned radio supplies shop somewhere in England that still has a stock of ICs including CA3094 lurking in it's stock room - I wonder where??

[oskar]

Why quibble over the cost of a CA3094 - that never ceases to amaze me!

Thank's! I ACTUALLY found these little bastards (3094) here in Sweden from one supplier for 20SEK... ~3.50$...  I had no idea they were
that cheap!     I know of ONE radio store in sweden that carry obsolete components...   

You're right about the level controls they only work over about the top 70% of the travel, I haven't measured the voltages but I guess below that there's not enough current to turn the OTAs on.

Yeah, the mod for it is to stick a diode in, at the end, in series with the level control.

[slacker]

Well, I gave in and ordered some CA3094s from smallbear, replaced IC17 and that fixed the distortion. On to the attack delay now, hopefully that will be an equally easy fix.

[StephenGiles]

 

[slacker]

I thought you'd approve