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TM-3030 Filter Sweep Problem
MUFF WIGGLER Forum Index -> Oakley Sound Systems  
Author TM-3030 Filter Sweep Problem
Grumskiz
I have finally finished my dual TM-3030 desktop synth.
Two version 3 PCBs are stacked in one case. Since I built them almost one year apart from each other, there are some component differences. Both are calibrated and most of the time seem to work alright, but one has a slight problem which is noticeable at high resonance settings:
The filter seems to not sweep any lower than ~500Hz. The FREQ preset trimmer does not really help.

I have attached a short recording demonstrating this, but please listen to it with caution as it contains loud high frequency content. First you hear the well working one, then the flawed one with very similar settings.
Does anyone have any idea whether this can be fixed and what components I should check?
Synthbuilder
Possibly a problem with Q16 or Q11. Make sure they are the right devices in those positions. Also, perhaps resistors R46 and R57 could be the wrong values.

Tony
Grumskiz
Thanks Tony.
I checked Q11 and Q16. You recommend BC560C, but I used BC557B, because it was what I had on hand. Is the low noise, high gain variant critical here?
I have taken out R46 and R57 and they were exactly 10K, so now they are back in there. I touched up the solder joints on the transistors as well, but no change.
Should I replace Q11 and Q16 with BC560Cs?
Synthbuilder
Grumskiz wrote:
I used BC557B...


Those should work just fine. So unless they are broken the problem lies elsewhere.

If you turn the ENV MOD pot down can you manually control the filter cut-off frequency down lower so the sound almost dies out. If so I think the problem is in the envelope generator. In this case, it could be a bad U8 or Q13, 17 & 18. Check also that U8 is a 4066 and not a 74HC4066.

Does the mod wheel control work OK?

Tony
Grumskiz
The manual cutoff control works equally well on both units as far as I can tell.
Regarding transistors: Q13 and Q17 are BC547Bs and Q18 is a 2SK30AY which I got of ebay. I am pretty sure I used the same FET on both units.
U8 is a CD4066BCN by National Semiconductors manufactured in 1980. I have a huge stash of NOS CMOS ICs and I'll have a look through it for another 4066. U8 is socketed so I'll just try a different one and see what happens.

Edit:
I've got a CD4066BE by RCA from 1991 in there now, no change.
Mod Wheel seems to work on both units.
Synthbuilder
It's difficult to say what's wrong. The good thing is that you have two units and you can compare between the two. I'd start out comparing two filter envelope circuits.

What's the voltage across D11. It should be 0.6V or so. If it's not then check that D11 is the right way around or swap it out.

Tony
Grumskiz
D11 is soldered the right way according to the silkscreen and I got 0.625V across it.
I'll have a closer look at the schematic of the envelope circuit and look for something that seems odd. Thanks for your tips so far!
Grumskiz
I have had this put aside for a while but decided to take another look today.
The problem is still present.

I've gone through all the parts in the envelope section with the schematic at hand and they at least seem to be the correct values.(I have not taken out or replaced anything besides the 4066 and the resistors R46 and R57)
D11 now has 0.53V across it...is that unusual?

Could a sketchy electrolytic cap for C20 cause this issue? (just a wild guess)

The 4066 seems to work just fine.

I've tried to measure the actual envelope at the "ENV_OUT" point, but always got about 3.2V there, except when the accent hits which causes it to be 1.6V. I'm unsure whether my multimeter is just to slow to show the envelope or whether my assumption that it is a voltage envelope (opposed to current) is wrong.

I don't know what to look for anymore, do you have anymore tips for me?
Synthbuilder
Just to verify what I was asking earlier - when you turn down the ENV MOD pot do both units work identically? Also do both units behave identically when using the mod wheel. The key word is identically. If they do behave the identically then the fault is most likely in the envelope section of the circuitry.

Grumskiz wrote:
D11 now has 0.53V across it...is that unusual?

That will be fine.

Grumskiz wrote:
Could a sketchy electrolytic cap for C20 cause this issue? (just a wild guess)

I don't think so. I think Q18 is probably still the most likely suspect.

Grumskiz wrote:
I've tried to measure the actual envelope at the "ENV_OUT" point, but always got about 3.2V there, except when the accent hits which causes it to be 1.6V. I'm unsure whether my multimeter is just to slow to show the envelope or whether my assumption that it is a voltage envelope (opposed to current) is wrong.


It is a voltage but, as you correctly say, trying to measure it on a voltmeter is not going to give you too many clues because your meter can't move fast enough. A scope is the tool for this job.

That said you should be able to see whether the voltage from the working TM3030 is the same as the one that isn't working. Don't trigger any notes - just check the voltage on the ENV_OUT on each machine. If Q18 is working correctly they should both be at the same voltage.

Also the voltage difference between the + pin of C20 and the ENV_OUT should be around -1.2V. If not try changing Q13 and Q17.

Tony
Grumskiz
I decided to gamble on Q18 being the problem, because I didn't feel like taking the whole thing apart again. (The TM3030s are stacked on top of one another in a single case, so it's impossible to measure the lower one without taking the top one out)

So I ordered an new 2SK30AY and replaced the old one. It worked! Both units sound perfect now. I don't really know why the old transistor wasn't working right. Maybe it was just a bad part, but I suspect I damaged it during soldering or maybe case assembly.

Anyway, I am happy now smile
Thanks so much for your help Tony!
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