Soundlab minisynth plus VCO2 problem

nop.debouwer · Joined: Dec 06, 2012 Posts: 5 Location: the Netherlands

Hi all,

I'm trying to build the soundlab minisynth plus. Actually it works almost perfectly except for VCO2. It doesn't track 1V/Octave. If I calibrate over a 2 volt range (0-2V) It does get calibrated. However, every voltage between 0- and 2V is way off! except for 0- and 2V? I went over the wiring and parts values and can't find anything wrong. Does anyone have an idea what could cause this strange behavior?. I saw a post with similar problems but apparently he/she misplaced two parts wich is not the case in my soundlab.

any help would be greatly appreciated.

elmegil · Posted: Thu Dec 06, 2012 6:24 am Post subject:

Funny you should ask, I had something similar with an original SLMS recently....

O1 would track over 2 octaves and then fall apart, and O2 wouldn't even do that. Turned out that R56 & R64, and R59 & R65 had wrong value resistors in them. They're supposed to be 1M and 10k respectively in each pair, but there were two 10ks for R56 & R64, and two 1Ms for R59 & R65. Lucky for this conversation, the resistor numbers and values are the same for both schematics (SLMS original and SLMS+).

So triple check the resistor values there, and probably throughout the rest of the expo converter section (everything around IC5A & B and IC6A & B), and I bet you will find your problem.

nop.debouwer · Joined: Dec 06, 2012 Posts: 5 Location: the Netherlands

Thanx for your reply,

I think I was reffering to your post Wink

. Do you mean that R56&64 should be 1M and the other two R59&65 10k?

elmegil · Posted: Thu Dec 06, 2012 7:29 am Post subject:

Nope

http://www.musicfromouterspace.com/forums.html?MAINTAB=SYNTHDIY&PROJARG=SOUNDLABMINISYNTH_PLUS/slmsplus_pg2.html&VPW=1300&VPH=686

56 & 59 are 1M

64 & 65 are 10K

Sorry for my reading comprehension problems I just jumped right in....

I looked at those resistors a few times before it sunk in what was wrong with them, so I think triple checking is still a good idea. Are the resistors that go on the panel with the pots also checked? the only other thing I would suggest is possibly a problem with the MPF102.

Pete

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nop.debouwer · Joined: Dec 06, 2012 Posts: 5 Location: the Netherlands

You are confusing me a bit. As I said in the first message: I did not switch/mix-up these resistors. Or do you mean that there is an error in the layouts/schematics from Ray? Embarassed

I will take a closer look at the expo-con's and see if I made a mistake there.

The thing that gets me puzzled is the fact that at 0V (I know, can't be wrong at 0V) and 2V gives the correct frequencies ([f(V=0)]x4) but anything >0V & <2V is off.

nevertheless, thanks again.

elmegil · Posted: Thu Dec 06, 2012 9:37 am Post subject:

If you've checked and rechecked then that's fine. Ray's schematic is right.

Many times I have been baffled by troubleshooting something, been totally convinced it was wired correctly, and then turned out I did have a problem somewhere that I only saw the third or fourth time I looked at it.

Perhaps there's not a component value problem, but a solder bridge or something in that area?

What I saw with my own mistake here was that I could calibrate O1 so that it was right on at 0 & 2V, but 1V was a bit sharp (but not so much that it worried me a lot), and everything after 2V was flat. O2 originally was so bad I couldn't calibrate it without changing the 2K feedback resistor to 2.2K, and then it behaved more or less the same; calibrate at 0 & 2V, but then 1V was off (more than O1 at 1V) and after 2V was WAY off.

nop.debouwer · Joined: Dec 06, 2012 Posts: 5 Location: the Netherlands

I will take a look at it tonight. I've learned about solderbridges and cold solderjoints the hardway (too) Mad

Now I tripple check every trace and joint with a BIG magnifying glass and multimeter Very Happy

.

Hope I will find the error/faulty part toninght.

I will keep you posted Wink

nop.debouwer · Joined: Dec 06, 2012 Posts: 5 Location: the Netherlands

Wow, it did turned out to be a wrong resistor value Embarassed

. Problem was R61. I had a 1k instead of 2k. Red and brown color codings are hard to distinguish in poor artificial light. Embarassed

elmegil · Posted: Sun Dec 09, 2012 2:42 am Post subject:

Glad you have it sorted Very Happy

Uncle Krunkus · Posted: Sun Dec 09, 2012 3:36 am Post subject:

I was about to say that I agreed with you elmegil, in that I thought it unlikely that the MPF102 had gone, but that it was more likely the resistors you were pointing out.
Resistor values are taken for granted more often than they should.
Sure,.. if you're bashin some idea around on a breadboard, play with values as much as you like. Even find the odd strange value you might not have tried,.....but,....
When you're building a tested PCB design for a specific end,....... check everything!!! Shocked

Why not?

Are you planning on re-building this again real soon? ( to get a working one?)
It's hard to put into words, but when you've troubleshooted things for a while, you'll see that most faults are value/connector related, and these are both things which you have the power to eliminate from you're troubleshooting before you even start building.
Okay,.. I'll stop my ranting now and return you to your regularly scheduled viewing! Laughing

Just hope it helps someone! Very Happy