Help..... I screwed up my Microsynth

[Bill Bergman]

Well I made the corrections to mine that SWT uncovered and there seem to be a favorable improvement. Then, the low octave got weak....a wire came lose from the blend pot. Fixed it,turned it over, plugged it in again and another pot that was dangling under the pcb shorted something out. Smoked R99 and overheated Q5. I replaced them, but Q5 still overheats. I don't think there is an external short. I'm afraid it another bad component somewhere. I can't trouble shoot it with a probe or I'll smoke Q5.

How do you probe or trouble shoot a pedal that over heats componets when you power it up?  :cry:

[Maneco]

HI Bill,
i'd start by checking the vca controlling components,without powering the unit,by measuring the resistor values around q4,q5 and q6,by lifting one leg of the resistors and the 1n914 diode...if their values are correct,i'd replace c30,because a short in it would force too much current through q5 in the lowest setting of the attack pot...also a short in 1n914 or q6 would produce the same effect...in the worst case,try replacing the three transistors,you can use bc549 in place of them(at least my MICROS. uses that,just remember they are EBC,instead of CBE as in the 2n5088).
This stage seems to be more or less isolated from the rest of the synth by  r102,8k2 and r98,47k...
btw,mine uses lm3080 and lm 13600 as otas,and tl074 as opamps...i have a circuit board drawing in circuit maker,but it's a nightmare because i don't draw components on my boards,just pads spaced for the size of the resistors and caps used,and of course the dil connections...
Best regards,and as always ,congratulations for your work
Maneco

[Rob Strand]

Bill for Q5 to overheat the problem is likely to be isolated around the Q4, Q5, Q6 region.  There's not much extending out from that zone of the circuit which could cause overheating.

Does it overheat Q5 all the time or only when the attach pot is in the "shorted" positition?  It's possible the tantalum cap is fried.

I'd pull Q4, Q5, Q5 and the tantalum and thougougly check they are all OK.

BTW:  I'm *very* sure R12 should be left as 7.5k on swt's fixes, other than that thre fixes look good.  That's would explain why you octave up is weak now too.

[swt]

Ohh nooo!! Bill, i'm sorry!. I didn't want that to happen. Hope you can fix it. I think the guys are right about the places to look at. Maybe changing those few parts will be less of a headache.
I've double checked the value of the 7k5 resitor, and it is a 1k5. But i trust rob as much, that i will change it to the original value to see what happens. Thanks for sharing!

[Bill Bergman]

First off, thanks for the replies! SWT no reason to say your sorry, it's not your fault.  I have good news or at least better news. I got it going although the octave down(my favorite) is still messed up. I was already suspect of the diode Maneco suggested, so I changed it out. I got some signal by probing around before it overheated. Then as Maneco and Rob suggested ....it was the tantalum 10uf cap. When I was removing it, one lead pulled out from the body of the cap. I changed it out with a 47uf electrolytic and it worked except the octave down is very weak.

Maneco, is a 47uf cap too big?

SWT, is R99 suppost to be 47 ohm?

Thank again Rob, Maneco and SWT !

[Rob Strand]

I got it going although the octave down(my favorite) is still messed up.

'm not sure if the lower octave was damaged from the accident, or if it's side-effect from the mods (which I don't expect)?  The filter mod at R8 will change the behaviour of the unit - you may or may not like this, but it's how the original was.

Here's some things to check:

- What you should be seeing is the DC voltage on the caps C19 and C20 going up and down with the signal level.  You might have to turn the pre-gain up pretty high to see this well.

- You could try temporarily connecting the -ve end of C23 to the junction of C21/R55/R56.  When you do this make sure you turn down the SUB-OCTAVE level. You should hear the sub-octave signal coming through.  The lower octave itself will be a little weaker and a little buzzier in this position than when it is working properly.  If you are at least getting the octave through then all that stuff to the left of  R56 is OK - if not that's where thr problem is.   Assuming it is OK then it's likely something has gone wrong around or with A18.

[Bill Bergman]

Hi Rob,
If I'm not mistaken the Octave down was ok before the acciden even with the new part values.  
I tried the jumper but there still little or no Oct down. Seems like there is too much clean signal coming through. Also I notice I've had C23 in backward before. I changed it and no differents assuming the layout was right ....positive on the pot side ..right?

Also my power supply is only positive 7.8 and negative 9.7

[Rob Strand]

I tried the jumper but there still little or no Oct down.

If there's no octave down at all then you may have fried the CMOS 4013 flip-flop.   Before you change it out though, make sure you are getting voltage changes on C19 and C20, if not then the problem is elsewhere.

Also my power supply is only positive 7.8 and negative 9.7

I wouldn't be worried about such a small difference, there are tolerances involved with the zeners.  I would only worry if the supply voltage is known to be different before the fault - besides a fault chip could increase the supply current and pull the voltage down.  It's a bit hard to point the finger unless you knew what it was before.

positive on the pot side ..right?

According to the schematic that looks right to me, I don't there there's much bias on that cap in normal operation.  BTW, that jumper I suggested assumed the cap polarity was as per the schematic.

(hehe, I like this edit feature I can fix my typo's)

[Bill Bergman]

ROB YOU ARE THE GREATEST!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
I can't even count the many time you saved the day!!!!!!

Checked the voltage fluctuation on the 2 caps ....it was correct.

Changed out the 4013 and WE HAVE OCTAVE DOWN.
THANK YOU,THANK YOU,THANK YOU,THANK YOU, Rob!!!

[Rob Strand]

Good stuff Bill, glad to hear you are out of grief!

I'm interested in getting to the bottom of the R12 issue, it's hard to tell, I suppose swt is the man for this since he has a real unit.

[drew]

Pardon my ignorance here, but where's the schematic for this beast?



drew
toothpastefordinner.com

[Maneco]

Hi,Bill,
the attack envelope works as follows:
pin 6 is normally high on silence,so q6 is conduting,so the + terminal of c10 is connectrd to earth via d19,and q4 is not conducting,so  no current through r98 that controls the ota a14....q5 is also no conducting.
When a note starts,pin 6 goes low,q6 cuts,and the base of q5 is connected to v+ through r100,22k ,and it starts conducting...so c30 starts charging through the attack pot,and q4 starts to conduct a rising current proportional to the rising voltage at its base...so with a higher value c30,the envelope will be slower,taking more time to reach the highest volume...i used 10uf electrolytic,i suppose that a higher value electrolytic may have losses,tantalum is preferred...
Best regards
Maneco  

[swt]

There you go Bill!!. I should also say thanks to Rob. He's been really helpful at the forum. I'll ask for the real unit again, to check r12. I remember looking twice, and it was a 1k5. The fact is that measuring it with a dmm, it read something like 6k9 ,so maybe it's a 7k5, or maybe the guy who traced it has some measurements as 7k5 and he put that on the paper. But i'll check it out anyways. Apart from that, the unit works exactly as the original one, because i've tested and ab'd both. Maybe the guitar and suboctave were a bit lower, with respect to the octave, what makes me think Rob is right, as there will be more signal towards that part, and less going to the suboctave section. Although the difference is really minimal . As soon as i got the answer, i'll let you know. Thank you all guys. This was a cool and fun project.
Rob!, i'll ask for the qtron maybe next week, and will take some pictures of it.
Drew!! send me an email, and i'll send the schem to you.
sfeirwt@infovia.com.ar

[Bill Bergman]

swt thanks so much for correcting the component values. It was well worth the trouble. It broght new life to the pedal. The changes added much more depth to the synth effects of the pedal.  Thanks swt, Rob and Maneco.

Oh, I switched R12 back to 7.5K and I don't think there was a noticeable difference.

[Rob Strand]

swt, Bill, thank's for the comments.

The main reason I suspect it's 7.5k is because that's the stock standard value for a rectifier circuit like that.  The fact the original ckt has this value and the 6.9k measurement  *really* makes me think it's 7.5k.

That resistor will only effect the octave-up signal - Bill, interesting such a big change doesn't have an effect - quite possible though.

[Bill Bergman]

Rob
Sorry, I can't be a definate about the difference between the 1.5k and 7.5k, I was anxious to put the pedal  back together and I trusted your judgement. All I can say is my Mircro Synth clone was improved quite a bit with swt's componet changes and your help fixing my shortout accident.
It's a very cool  synth considering it's analog as compared to modern realm of digital synths.

[Rob Strand]

I was anxious to put the pedal back together

I can understand that.

I trusted your judgement

Thanks Bill, but with this stuff it's only my best guess.

It's a very cool synth considering it's analog as compared to modern realm of digital synths.

I was thinking of writing a VST plug-in to emulate that effect on the PC so people can see what it's like before they build it (it takes a bit if dedication to build that one).

It won't happen tomorrow but is anyone interested in such a Plug-in?

[drew]

Doop doop doo... so, can ANYONE POINT ME TO A LINK TO THIS SCHEMATIC?

Hello? Anyone out there?

[Rob Strand]

It's in the Leper's archive,
http://audible.transient.net/archive/

There's guitar version and a bass version, PDF and postscipt formats, 3 pages each.  (It's a  relatively complicated effect).

[hank reynolds 3rd]

a vst version would be cool....
i sold my microsynth (ri) during my stupid drugfuelled uni days....i'm thinking of building something similar but using 13700s for the filter,vca,etc....
i'm  sure i've seen a vst plugin thats similar (had more or less the same controls/sliders) but i cant remeber what it was called (this was about a year 1/2 ago...i'm still learning to use my memory again!!!)



sam