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Problem with ADSR
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LetterBeacon



Joined: Mar 18, 2008
Posts: 454
Location: London, UK

PostPosted: Fri Jun 19, 2009 1:45 pm    Post subject: Problem with ADSR
Subject description: Infinite sustain....
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I've built two ADSRs and have mounted them behind a single panel making a dual ADSR module. They both share a power connector.

ADSR 1 seems to work fine - according to the LED on the front and my oscilloscope.

ADSR 2 however does not! If I have the Sustain pot set to anything greater than 9 o' clock, the output keeps on sustaining after the gate signal ends and only stops when I turn the Sustain pot back to 0.

I've checked all the resistor values and they are all correct - including the correct values so that two PCBs can be run in parallel. I'm stumped!

I am using a different brand of 7555 in ADSR 2 than I am in ADSR 1 - could that be it?
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yusynth



Joined: Nov 24, 2005
Posts: 1229
Location: France

PostPosted: Fri Jun 19, 2009 11:19 pm    Post subject: Reply with quote  Mark this post and the followings unread

It's very probably a bad component, bad solder joint or a solder bridge.
According to the symptoms you mentioned I will try to find some ideas of thing to check but until then since you have two circuits, one working and the other not working, what you could do is to measure voltages at various points on the working one and compare these measurements with those on the non-working one. This can give us a clue.

One thing measure (with an ohmmeter) the exact value of the sustain potentiometer, and also check the value of the 4.7K resistor that is in series. Check that diodes are oriented in the right directions.

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Last edited by yusynth on Sat Jun 20, 2009 4:20 am; edited 1 time in total
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LetterBeacon



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PostPosted: Sat Jun 20, 2009 3:15 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks Yves, I'll give this a go later on today.
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LetterBeacon



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PostPosted: Sat Jun 20, 2009 4:16 am    Post subject: Reply with quote  Mark this post and the followings unread

Sustain Pot = 10.51K
4.7K Resistor = 4.67K

yusynth wrote:
According to the symptoms you mentioned I will try to find some ideas of thing to check but until then since you have two circuits, one working and the other not working, what you could do is to measure voltages at various points on the working one and compared these measurements with those on the non-working one. This can give us a clue.

At which points shall I take the measurements?

Last edited by LetterBeacon on Sat Jun 20, 2009 4:31 am; edited 1 time in total
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yusynth



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PostPosted: Sat Jun 20, 2009 4:25 am    Post subject: Reply with quote  Mark this post and the followings unread

For example on the collector of the three BC547, then on all the pins of the 7555
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LetterBeacon



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PostPosted: Sat Jun 20, 2009 5:12 am    Post subject: Reply with quote  Mark this post and the followings unread

Ok, here we go:

ADSR 1 (the one that works):
Q1 = .72v
Q2 = 0v
Q3 = 14.93v

7555
Pin 1 = 0v
Pin 2 = 14.93v
Pin 3 = 0v
Pin 4 = 0v
Pin 5 = 9.83v
Pin 6 = .02v
Pin 7 = 0v
Pin 8 = 14.96v

ADSR 2 (the one that doesn't work):
Q1 = .70v
Q2 = 0v
Q3 = 14.93

7555
Pin 1 = 0v
Pin 2 = 14.93v
Pin 3 = 0v
Pin 4 = 0v
Pin 5 = 9.83v
Pin 6 = .01v
Pin 7 = 0v
Pin 8 = 14.96v

Sustain pot = 10.51K
4.7K resisitor = 4.67K

I measured the values for ADSR 1 while it was the only PCB connected to the power, but I had to measure the values for ADSR 2 when they were both connected. Do you think this will make much difference?
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yusynth



Joined: Nov 24, 2005
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PostPosted: Sat Jun 20, 2009 7:41 am    Post subject: Reply with quote  Mark this post and the followings unread

I suppose that you measured these for a GATE OFF status, it would be interesting now to repeat these in the GATE ON status with the ATTACK,DECAY, REALEASE set to min pos (0) and SUSTAIN pot at mid-range, include the voltage value on the wiper of the SUSTAIN pot too.
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LetterBeacon



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PostPosted: Sat Jun 20, 2009 8:50 am    Post subject: Reply with quote  Mark this post and the followings unread

Ok I'll give that a go next time I get a chance - might not be until tomorrow evening now though.

Thanks for your help so far.
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LetterBeacon



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PostPosted: Sun Jun 21, 2009 3:06 pm    Post subject: Reply with quote  Mark this post and the followings unread

The following measurements were taken with all the pots at 0 except for the Sustain pot that was at its mid-range position. The first number is the gate off value while the second is the gate on value.

ADSR 1 (the one that works):

Q1 = .72/ 0
Q2 = 0/ 14.92
Q3 = 14.94/ 14.92

7555
Pin 1 = 0/ 0
Pin 2 = 14.94/ 14.92
Pin 3 = 0/ 0
Pin 4 = 0/ 14.92
Pin 5 = 9.84/ 9.81
Pin 6 = .11 (and falling)/ 6.20 (and falling)
Pin 7 = 0/ 0
Pin 8 = 14.97/ 14.95

Sustain pot wiper = 5.52/ 5.93

ADSR 2 (the one that doesn't work):

Q1 = .71/ 0
Q2 = 0/ 14.92
Q3 = 14.93/ 14.92

7555
Pin 1 = 0/ 0
Pin 2 = 14.93/ 14.92
Pin 3 = 0/ 0 sometimes flashed up to .77 but that could be my DMM
Pin 4 = 0/ 14.92
Pin 5 = 9.83/ 9.82
Pin 6 = 4.67 (and falling)/ 5.56 (and falling)
Pin 7 = 0/ 0
Pin 8 = 14.96/ 14.95

Sustain pot wiper = 5.21/ 5.20

So it looks like the only figures that are different are the ones on the pin 5 of the 7555.

All the diodes are orientated the correct way and I can't see any solder bridges or bad connections. I'm a little stumped!
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yusynth



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PostPosted: Sun Jun 21, 2009 3:28 pm    Post subject: Reply with quote  Mark this post and the followings unread

You probably mean pin 6 not pin 5 ?
Wht do you mean by "and falling" ? How much is it falling ? It should not fall the sustain voltage should maintain a steady value. If there is a significant drop either C4 could be dead (if it's a tantalum be sure to use a 10µF/25V or even better a 10µF/35V) or the TL072 is faulty.

Anyway, the voltage at pin 6 of the second ADSR is not normal it should 0V or 0.1V when the GATE is OFF. It looks as though there is a permanent voltage source charging C4. Check that the diodes are not in short-cut state (test them with a diode tester : the voltage drop should be something between 0.550V and 0.720V in direct polarization and 24V in reverse polarization), in particular check D2 and D4.
You can also use a continuity tester to check that there is no shortcircuit between neighbouring tracks (sometimes when DIYing PCB there may be tiny copper bridges that are hardly visible with naked eye.

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LetterBeacon



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PostPosted: Sun Jun 21, 2009 3:38 pm    Post subject: Reply with quote  Mark this post and the followings unread

Yes, sorry I meant pin 6 Embarassed .

When I say 'and falling' I mean the voltage value is falling very slowly while the gate is on and once the gate is off. This seems to do this on both PCBs - looks like there might also be a problem with ADSR 1 too!

Quote:
Check that the diodes are not in short-cut state (test them with a diode tester : the voltage drop should be something between 0.550V and 0.720V in direct polarization and 24V in reverse polarization), in particular check D2 and D4

Is this something I should do with them connected in the circuit, or shall I take them out?

I'll give all this a go tomorrow morning, thanks!
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yusynth



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PostPosted: Sun Jun 21, 2009 11:32 pm    Post subject: Reply with quote  Mark this post and the followings unread

LetterBeacon wrote:


Quote:
Check that the diodes are not in short-cut state (test them with a diode tester : the voltage drop should be something between 0.550V and 0.720V in direct polarization and 24V in reverse polarization), in particular check D2 and D4

Is this something I should do with them connected in the circuit, or shall I take them out?

I'll give all this a go tomorrow morning, thanks!

Normally, this can be tested on board but it is better to unsolder a leg in order to isolate the diode from the circuit.

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LetterBeacon



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PostPosted: Mon Jun 22, 2009 4:32 am    Post subject: Reply with quote  Mark this post and the followings unread

yusynth wrote:
Wht do you mean by "and falling" ? How much is it falling ? It should not fall the sustain voltage should maintain a steady value. If there is a significant drop either C4 could be dead (if it's a tantalum be sure to use a 10µF/25V or even better a 10µF/35V) or the TL072 is faulty.

This happens on both PCBs. On ADSR 1 (the one that is supposed to be working!), if I have just powered on and the gate hasn't been pushed, I get a reading of 0.1v on pin 6. When the gate = ON I get a reading of around 6v which then decreases by 0.2v at a rate of about 1v every 20 seconds or so. When the gate = OFF the voltage drops down to about 0.20v and then very slowly drops down by 0.1v until reaching 0.1v. I tried replacing the TL072 on ADSR 1 but there was no difference. Looks like it could be the tantalum cap? Perhaps we should leave this error until we figure out what is the problem with ADSR 2?

Quote:
Anyway, the voltage at pin 6 of the second ADSR is not normal it should 0V or 0.1V when the GATE is OFF. It looks as though there is a permanent voltage source charging C4. Check that the diodes are not in short-cut state (test them with a diode tester : the voltage drop should be something between 0.550V and 0.720V in direct polarization and 24V in reverse polarization), in particular check D2 and D4.

Here are the readings from my DMM after I unsoldered a leg of each diode:
D1 = .598v
D2 = .593v
D3 = .591v
D4 = .590v

My DMM didn't have a decimal point before those figures but I assume it means to put one in...? I couldn't measure the diodes in reverse polarisation as my DMM would bring up an error.

Quote:
You can also use a continuity tester to check that there is no shortcircuit between neighbouring tracks (sometimes when DIYing PCB there may be tiny copper bridges that are hardly visible with naked eye.

The continuity tester seems to think everything's ok!
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yusynth



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PostPosted: Mon Jun 22, 2009 4:58 am    Post subject: Reply with quote  Mark this post and the followings unread

Yes "fall" this problem can be taken care of later Wink

It looks like your diodes are also good, the direct voltage drop is in the normal range and it is not in short cut when you reverse the polarization.

Now it's getting tricky, the fact that on pin 6 you have 5V at GATE OFF is definitly not normal and this voltage must come from somewhere. Because the diode are OK there is no explaination either than the 7555 but I suppose you already swapped them between the two ADSRs ?
Can you send me a good (detailed) photo of your PCB (component and track side) ?

Anyway try to replace C4 and see if there is any improvement...

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yusynth



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PostPosted: Mon Jun 22, 2009 5:01 am    Post subject: Reply with quote  Mark this post and the followings unread

By the way did you check the polarity of C4 ?
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LetterBeacon



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PostPosted: Mon Jun 22, 2009 11:34 am    Post subject: Reply with quote  Mark this post and the followings unread

yusynth wrote:
By the way did you check the polarity of C4 ?


Shocked Oh my God... I just can't... I mean I was SO sure I had checked it. It turns out that C4 was orientated the wrong way! I can't believe it! I'm so sorry, I'm sure I double checked everything - I wouldn't even be asking for help on here before checking everything was round the right way. So sorry to waste your time.

I've orientated it the correct way now, but now when I trigger the gate, the LED lights up and the fades very quickly. Do you think this is because C4 is now dead from being the wrong way round all this time?

I'll go to Maplin tomorrow and get another cap.
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yusynth



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PostPosted: Mon Jun 22, 2009 11:49 am    Post subject: Reply with quote  Mark this post and the followings unread

LetterBeacon wrote:
Shocked Oh my God... I just can't... I mean I was SO sure I had checked it. It turns out that C4 was orientated the wrong way! I can't believe it! I'm so sorry, I'm sure I double checked everything - I wouldn't even be asking for help on here before checking everything was round the right way. So sorry to waste your time.

This is classical because when we check something we built ourselves we are blind to our own mistakes : our brains tell us that it is not possible that we could do such stupid thing and therefore we don't see it even though it's there in front of our very eyes... And we need someone else to tell us that the King walks naked Wink

Eventually I thought of this polarization inversion because that was the only possible explaination left to me Wink

LetterBeacon wrote:
I've orientated it the correct way now, but now when I trigger the gate, the LED lights up and the fades very quickly. Do you think this is because C4 is now dead from being the wrong way round all this time?

I'll go to Maplin tomorrow and get another cap.


Yes the cap is dead. Tantalum cap do not tolerate inversion of polarization.

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LetterBeacon



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PostPosted: Tue Jun 23, 2009 4:01 am    Post subject: Reply with quote  Mark this post and the followings unread

I swapped out the dead cap with a freshly bought one from Maplin, but unfortunately I still have the same problem Rolling Eyes

I've attached pictures of both sides of the PCB of ADSR 2 - hope they're good enough as my camera isn't particularly great.


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yusynth



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PostPosted: Wed Jun 24, 2009 4:58 am    Post subject: Reply with quote  Mark this post and the followings unread

Probably some other component have suffered.

When you say it is still the same do you mean that the voltage measurements at pin 6 are still the same 5V gate off/ gate on 6V ?


I'll check the photos of the PCB too.

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LetterBeacon



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PostPosted: Wed Jun 24, 2009 5:12 am    Post subject: Reply with quote  Mark this post and the followings unread

yusynth wrote:
When you say it is still the same do you mean that the voltage measurements at pin 6 are still the same 5V gate off/ gate on 6V ?

I get the same problem as before yes.

I think I might have made a mistake when taking the readings at pin 6. When I took the readings on pin 6 I had already fired the gate to take the readings from the previous pins and transistors, this is why it showed 5V on gate off. If I turn the module on and take the readings before I fire the gate then I get a reading of 0.0V or 0.01V
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yusynth



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PostPosted: Thu Jun 25, 2009 9:20 am    Post subject: Reply with quote  Mark this post and the followings unread

I check your photos and all seem OK. Therefore I don't know what's wrong, tomorrow I will do some measurement on one of my ADSR and compare them with your measurements. May be it'll show some anomalies...
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LetterBeacon



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PostPosted: Thu Jun 25, 2009 2:49 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks Yves, I appreciate the time you're spending on this. I'm not that experienced with troubleshooting so I'm completely at a loss!
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yusynth



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PostPosted: Fri Jun 26, 2009 12:25 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi again

I checked the voltage and they are about the same as for your ADSR1 (even for the falling on pin 6 which therefore is normal).

Can you repeat the measurement at pin6 with the SUSTAIN at maximum (CW) ? you should get something like 10V and falling.

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LetterBeacon



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PostPosted: Fri Jun 26, 2009 3:21 am    Post subject: Reply with quote  Mark this post and the followings unread

yusynth wrote:
Can you repeat the measurement at pin6 with the SUSTAIN at maximum (CW) ? you should get something like 10V and falling.


ADSR 1
Before gate is triggered = 10.38V
During Gate On = 10.36V

ADSR 2
Before gate is triggered = 10.35V
During gate on = 10.35V

The gate on voltage started falling very slowly on both circuit boards.
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yusynth



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PostPosted: Fri Jun 26, 2009 3:50 am    Post subject: Reply with quote  Mark this post and the followings unread

LetterBeacon wrote:
yusynth wrote:
Can you repeat the measurement at pin6 with the SUSTAIN at maximum (CW) ? you should get something like 10V and falling.


ADSR 1
Before gate is triggered = 10.38V
During Gate On = 10.36V

ADSR 2
Before gate is triggered = 10.35V
During gate on = 10.35V

The gate on voltage started falling very slowly on both circuit boards.


Are you measuring at pin 6 ????? it must 0V to 0.1V at GATE OFF and 10V to 10.4V at GATE ON

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