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Issues with the diy ms-20 type filter by rene schmitz and LM
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Issues with the diy ms-20 type filter by rene schmitz and LM
Bardan
Hi everyone!
I've recently attempted to make the ms-20 style filter from René schmitz and LMNC: right here

But i've come across Some issues:

- first the TL074CN gets really hot really fast !
I heard that it might be because its oscillating, should il try and inspect the output capacitors?

- also, and that might be linked, but one of the Leds lights up constantly unless something is plugged into the panel..

Did i use the wrong TL074 ??

Pictures here (it was a pain to make it for some reason so excuse the ugliest stripboard layout you've seen aha)


mskala
Bardan wrote:
- first the TL074CN gets really hot really fast !
I heard that it might be because its oscillating, should il try and inspect the output capacitors?


Although that's possible, I think a short circuit is more likely - something causing one or more of the op amps to be forced to produce way too much current. First thing to look for is solder bridges between adjacent connections. I didn't see any in your photos but I could have missed one, it's worth looking at the panel wiring too, and it really calls for a multimeter instead of only unaided eyes.

Do you know if there is a a schematic diagram for this circuit? Trying to debug with only a stripboard layout is not easy.
Bardan
I did forget two GND wires between the resonance, cv input and cv level, but still gets hot though..

I didn't see any shorts around the IC so i'll have to check with the multimeter.

Now pardon my noobness, but what should i look for with the multimeter? I'm not sure what voltage to expect on the different pins of the IC


hmmm.....

But this is all necessary learning so yeah smile

Thanks! grin
mskala
I was thinking to check - with the power off - whether there is zero or nearly-zero resistance between any two points that shouldn't be connected to each other. A multimeter on "beep continuity" or low-resistance ohmmeter range can sometimes find short circuits that aren't obvious to a visual examination.

If you know the voltages to expect on different pins then checking those with the voltmeter range is valuable too, but as you say they may not be known. Finding or figuring out the schematic would be an important first step to that.

One reasonable place to check voltages would be at the inputs of each op amp. In most op amp circuits the normal expectation is for the two inputs of the amplifier to be at the same voltage. If they are significantly different, that's a warning sign - a few circuits are meant to operate with a significant difference but it's not common and I wouldn't expect it in a VCF. If you find a case like that it can be a clue to which part of the circuit needs to be checked more closely.
AlanP
It's veroboard, so also double check all your track breaks with a multimeter if need be.
Bardan
So i've checked the voltages on all of the pins from the 74, and here's what i got:


1 out : 0v
1 IN- : 0v
1 In+ : 0v

Vcc+: 12v

2 out : 0v
2 In- : -12v
2 In+ : -12v


3 Out : 0v
3 IN- : 0v
3 IN+ : 0v

Vcc- : -12v

4 out : 0V
4 In- : ~8v
4 IN+ : 0V

Something doesn't seem right, looks like it's narrowing down the problem. What do you guys think?

Also checked the tracks breaks but nothing suspect!
wackelpeter
just had a quick look and the cap between Pins 4and 5 isn't correct… should go to gnd and not +12V... would advice to print out the schematic and trace all your Connections and components on the board and mark each one that is correct and by the way also check that there are not any other Connections because the trace wasn'T cut where it should have been.

IF you don't have a Printer you can load the schematic into paint and do it there.

Another advice, Always use some 10-15 ohm resistors on the power lines, this will save you in case you made a short somewhere. and well some electrolytes for stabilising the power will also Benefit any build.

had just last week a brave 15 ohm resistor saving my board while a srewdriver was laying underneath my pcb when i powered it on...
Bardan
I'm not sure to follow: There's only a side of a capacitor going to pin 5 and the other side is going straight into 1 OUT.

The other capacitor is going from pin 14 to pin 13 (From 4OUT to 4In (-))

Although i'll definitely mark each component for a future build!

I had trouble finding actual 4.7nf and 470nf, i could only find the big blue one, and an even bigger one that i removed from the finished build. Maybe they're actually wrong but the values are the right ones so idk...

I guess i'll just try to rebuild it from scratch..
unauthorised-service
Having built this myself , Something just came back to me . You may have to reduce the input level . Ive put a small trimmer V/divider on the input jack . I may be right that overdriving the input may cause it not too work at all so to speak. Definitely try that solution that worked for me.
Synch59
unauthorised-service
I ran into this same problem today with my LMNC LPF. I had my 3340 VCO running into a mixer and all three waves mixed to the LPF. It was sounding cool with the maxed out res, then the leds got bright then phoompf it cut off. It still works but it is finicky. I’ll probably look at doing the voltage divider.
Gonzbull
I know this may be of no help but LMNC has released a DIY Panel and PCB kit for this filter. I'm building one and am just waiting on pots from Thonk.
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