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voltage comparator troubleshooting, please help
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author voltage comparator troubleshooting, please help
Hello everyone,

I'm having troubles with a dual decay envelope module:

-Channel B : when a trigger signal is sent to the module, this outputs a decay envelope.
-Channel A : when a trigger signal is sent to the module, nothing happens.

Those are a part of the schematics and some of my comments:

This is the Channel A comparator output [5V/div]t:

This is the Channel B comparator output [5V/div]:

Channel B is the correct one, since when a trigger in is sent, some signal makes it through the following diode (D4) and the envelope is then generated.
In channel A, when a trigger is sent, the comparator outputs 0V, and no signal goes through D3 to generate the envelope...but why ??? very frustrating

please help me debug this, It's the first time I need to troubleshoot a DIY module and I don't have any clue on this problem.
what are the powersupply rails for IC9? channel A looks like its going 0-10V, whereas channel B looks like its going 0-15V. if you set your scope to trigger on normal mode, it should hold the image fixed for a better picture.

my first guess is D3 being backwards or busted, as it looks like the comparator is doing its job, its producing a positive spike with a trigger input.
Thanks guest!

IC9 is a TL074, and it's powered with +/-12V.
The two gifs show the output of its two op amps, which in this design are used as comparators.
The channel A output is going from about -10.5V (I suppose this value is the negative saturation level of the op amp) to 0V (when the trigger is "on"), while the channel B is going from about -10.5V to 5V (when the trigger is "on").

Sorry for the bad quality of the gif, I think the image flickers a bit due to the camera on my phone. I take a small video to show a timespan of 1-2 seconds, to show what happens when the trigger signal is "on", and then converted the videos to gif.

D3 is not backwards, since it is still possible to see the orientation mark on the PCB, but could be busted... I'll try to replace it.
If the channel A comparator output is limited to 0V, maybe the node to the right of D3 is shorted to ground. Try measure the resistance to ground with the trig signal removed.

It seems a little odd that the output of channel B is limited to only +5V unless the load has quite a low resistance. It might be worth the effort to measure the resistance to ground on the node on the right side of D4 as well.

If the 500kohm potentiometers set to the minimum resistance, the load will be 1kohm at most. If this is the case, try to set the resistance to a higher value to see if the swing changes.
is that a jumper or a switch across R20? its unclear from the schematic. its possible that component is shorting out.
Thanks you all for the support guys!

Yes, they're jumpers, I've tried to swap it with the one of the other channel, but nothing changed

Isn't 5V control signals a standard in eurorack systems ?
I'll try to measure the resistences as you said in a few days, when I'l be home again, and update this thread.
if the jumper is shorted, remove the short and see if anything changes.
Sciamano wrote:
Isn't 5V control signals a standard in eurorack systems ?

That is possible, but a TL074 wouldn't know it. If the output goes to -10.5V when the input is below the threshold, I would have expected it to go to +10.5V when the input is above, not to +5V, unless it is heavily loaded. Based on fig 6 in the TI datasheet, the output would be limited to 7V below Vdd if the load is 300 ohm. Maybe this get worse at lower supply voltages than 15V or your device isn't typical so this is what happens with a 1k load. But if this is the case, the peak output amplitude could still double when the decay potentiometer is changed to a higher value. This would be less than ideal, I presume. If this is correct, the problem can at least be reduced if the timing resistors are increased to 2kohm and 1Mohm and capacitors reduced to 0.47uF and 4.7uF to maintain the same time constants.
Synthiq wrote:

I would have expected it to go to +10.5V

Did you measure +12V @ pin 4 of TL074 ? if so, pull out pin 1 and 14 of TL074 ( hopefully socketed :-)) and measure unloaded voltages.
If they are >10V, then the outputs are overloaded in the original circuit, which could be the case, because your minimum discharge R is only 1k !
I've notice I've made a mistake, the output of IC9D (channel B comparator) goes up to 10.5V, At the time I measured just 5V because I had set the potentiometer (DEC_B ) to a really low value!
- Yep, pin 4 is at +12V.
- The unloaded voltage at pin 1 and 14 are at +12V when a trigger is on, otherwise is at -12V, as expected.
Thanks to all for your suggestions, the module is now working!

I tried to measure the underload voltage, as suggested by elektrouwe, of both comparators (the outputs of IC9) to check If they were working properly, by pulling out pin 1 and 14 of the TL074.By pulling out pin 14 I've brokeng the leg of the chip, so I had to replace it with another one, and after doing so....It started working!
This is quite strange, since I had already tried to replace the TL074, without results. Anyway, I'll be left with the mistery and with a working module.
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