1v keyboard issue or not

aletropot · Joined: Jun 10, 2012 Posts: 27 Location: Amsterdam

Hello everyone,

I finish the ray single buss keyboard, but i found something strange that i dont know if its a problem or not.
With the multimeter in cv1 out and ground, when i press a key, give me the right voltage, but immediately the voltage starts dropping. Even when i keep the key pressed...and goes to minus. I didnt wait til know how much go to minus, but the (minimum)maximum that i saw was -0.81V...
I dont know if this is normal, but dont looks like.

Any idea?

Blue Hell · Posted: Sun Aug 12, 2012 10:30 am Post subject:

Was the multimeter set to AC or DC?

aletropot · Joined: Jun 10, 2012 Posts: 27 Location: Amsterdam

Blue Hell · Posted: Sun Aug 12, 2012 10:46 am Post subject:

No, DC is good ... but the behavior is a bit strange so I wondered ... the voltage should not drop when you keep the key pressed. And when you release the key it should stay at it's value for a good while too. So I wonder .. maybe an error in the sample and hold part?

aletropot · Joined: Jun 10, 2012 Posts: 27 Location: Amsterdam

JingleJoe · Posted: Sun Aug 12, 2012 11:13 am Post subject:

sounds like you might have put a capacitor in the wrong place or mis-wired something ...maybe even mis-wired your measurement equipment?

aletropot · Joined: Jun 10, 2012 Posts: 27 Location: Amsterdam

Hum, strange. the sample and hold seems to be correctly wired.
Can be a problem with the polystyrene cap?

Blue Hell · Posted: Sun Aug 12, 2012 12:08 pm Post subject:

Even when the cap is fukt it should still hold it's voltage while the key is being pressed ... hmm

aletropot · Joined: Jun 10, 2012 Posts: 27 Location: Amsterdam

The sample and hold its the 2n5457, right?
could be one of those burned?

Blue Hell · Posted: Sun Aug 12, 2012 12:45 pm Post subject:

yeah those FETs .. or connected wrong?

Feifer · Joined: May 16, 2012 Posts: 32 Location: San Diego

Did you use the LF444? It has to be one of those ultra high input impedance opamps other wise the voltage will start to droop. Although it shouldn't be going to negative voltages. I would guess there's something hooked up wrong, perhaps the bottom of your resistor chain?

aletropot · Joined: Jun 10, 2012 Posts: 27 Location: Amsterdam

aletropot · Joined: Jun 10, 2012 Posts: 27 Location: Amsterdam

after check the circuit couldnt find any problem.
I replace one of the 2n5457 because was acting weird.

Can be with the lf444cn? the part list say lf444, unlike the mini controller, dont say "do not sub"

aletropot · Joined: Jun 10, 2012 Posts: 27 Location: Amsterdam

After change both 2n5457 and polyester cap, know I'm receiving negative voltage. -9.30v. When i press a key the value increase and decrease immediately to -9.3v. Looks like now its worst than it was....
Any idea?

Feifer · Joined: May 16, 2012 Posts: 32 Location: San Diego

Yeah, the LF444cn is the same thing.
The problem really shouldn't be with the fets. They're only gates. And the polystyrene cap is referenced to ground, not -12v, so there is no reason why it should be creeping negative. Having a buffer before and after the sample and hold section isn't going to change the voltage either and the cv1 is right after the second buffer. And since you are getting the right voltage initially when you press a key means your current source is working. I would check the value of R7. Make sure it's 10M. From this behavior I'm guessing it might be a lot lower and you're draining voltage out from there.

aletropot · Joined: Jun 10, 2012 Posts: 27 Location: Amsterdam

After replace R7 (it was 10M and it was working, but i replace) the keyboard back to my first problem. Every time i press a key, the voltage goes to the right voltage, but start drooping, and goes to -9.3V. So its not holding. Even when i keep the key pressed...

Some one can give me some reference points to test with the multimeter, to see if i can find the problem?

Blue Hell · Posted: Wed Aug 15, 2012 1:21 pm Post subject:

Start on KBUS that is what it all starts with, and should have 1V / octave.

Then U2-B pin 7 must have same signal and U2-A pin 1 same signal too.

aletropot · Joined: Jun 10, 2012 Posts: 27 Location: Amsterdam

thanks for your help.

On KBUS, i got 1V/oct, the key holds, and when i release goes immediately to -0.48V.

U2 pin 1 and 7 i get 1V/oct too, and when release i get -0.55V.

I think so far so good...

Blue Hell · Posted: Wed Aug 15, 2012 1:58 pm Post subject:

Yeah that seems fine ... unfortunately now it gets a bit more complicated ... need to go into the U4-A, U5-A region ... expecting pulses on their outputs that probably are too short to see on a multi-meter ...

hmm .. maybe better to start on U5-B pin 7 .. there should be one steady level when you press a key and another steady level when release the key.

BTW, are your LEDs doing anything?

aletropot · Joined: Jun 10, 2012 Posts: 27 Location: Amsterdam

Blue Hell · Posted: Wed Aug 15, 2012 2:53 pm Post subject:

aletropot · Joined: Jun 10, 2012 Posts: 27 Location: Amsterdam

I will replace U5 tomorrow because i remember that the first time that i turn on, one of the tantlum caps in PS blow up, because it was with wrong polarity. maybe it burned after this.

Blue Hell · Posted: Wed Aug 15, 2012 3:09 pm Post subject:

To have a look at the pulses ... and to all pins of Q2 and Q3 ...

Could there be a short between G and D of Q3? Maybe the multimeter could tell you that.

aletropot · Joined: Jun 10, 2012 Posts: 27 Location: Amsterdam

After check q2, q3 and c19 i got this diagram:

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Blue Hell · Posted: Thu Aug 16, 2012 1:42 pm Post subject:

Ah ok ... looks not too bad.

Could you temporary pull pin 10 from U3-C? (or U3 as a whole if you had socketed it) and then check again on the capacitor (C19)?

As it may also be that the load on the C is too high ... maybe from a PCB error or a defect opamp.