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Paia Midi/CV prob?
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23isgood



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PostPosted: Sat Jul 07, 2007 11:35 am    Post subject: Paia Midi/CV prob?
Subject description: messed up CV outputs
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Hi folks. My Paia midi to cv 8 is not working correctly. When I patch the cv out to a VCO it modulates the VCO at a high frequency and it sounds like FM. Clearly there is a problem with it. I called Scott at Paia a few times and he thought that maybe it was due to a bad 4051 chip (I think its the 4051 I gotta look at it). So I replaced it with a new one and the problem still continues.

I noticed other people are having this same problem. I would love to get this working again. Paia charges $35.00 just to check and repair it, which is silly when I could fix it for next to nothing. Any ideas?

thanks
pete

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EdisonRex
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PostPosted: Sat Jul 07, 2007 2:18 pm    Post subject: Reply with quote  Mark this post and the followings unread

seems obvious the CV Out is being intruded on by some AC, probably at a reasonably high frequency (or not, it could be just a modulator).

you could hook a scope up to the output and look. It might give some hints.

I'm a fan of diagnostic processes. The first thing you need is tools. I don't suppose you have a scope or something you can use for a scope?

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State Machine
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PostPosted: Sat Jul 07, 2007 4:10 pm    Post subject: Reply with quote  Mark this post and the followings unread

Assuming the power supply is good, always check that first .....

Does this happen on just one of the CV outputs or all of them? The reason is that it might be the sample and hold cap/amplifier combination, correspondent to that CV channel, that may be the problem. I don't have the schematic in front of me (In am sitting in a coffee shop) but the this MIDI-CV device shares a single DAC output by demultiplexing it sequentially to output channels via a S/H amplifier, the 4051, operational amplifier, a holding capacitor, and addressing and enable logic from the microcontroller.

It just might be a single the S/H circuit. The microcontroller continually scans each output by writing to the DAC the digital value that corresponds to that channels output voltage at that instant in time, then it applies the address to the 4051 (A,B,C) and then enables it. The holding cap will charge through the switch then hold that value when the switch is deactivated until the controller comes around back to refresh that channel. If there were a problem with the addressing or the cap or amplifier, the output will not be a DC value. You will need an oscilloscope to see all of this or just substitution if you have no real test gear.

This process saves having to use 8 separate DAC's which was very expensive to do at the time this unit was designed (It's an older design)

I can help further if you answer the first question.

Bill
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23isgood



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PostPosted: Sat Jul 07, 2007 7:54 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hey guys. The first thing I checked was the power. I replaced the wal wart with another one I knew was working (from my Fatman) and I tested the power and everything looked good. The problem is on all the outputs. I do have a scope. Its a very old Heath Kit scope which I don't entirely understand how to use. I have hooked it up to VCO's and was able to look at some wave shapes with it. I don't know the model at the moment, I wont be home till mon.

thanks guys,
pete

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PostPosted: Sat Jul 07, 2007 10:48 pm    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
Hey guys. The first thing I checked was the power. I replaced the wal wart with another one I knew was working (from my Fatman) and I tested the power and everything looked good. The problem is on all the outputs. I do have a scope. Its a very old Heath Kit scope which I don't entirely understand how to use. I have hooked it up to VCO's and was able to look at some wave shapes with it. I don't know the model at the moment, I wont be home till mon.


OK, so the power is good, the 4051 is not at fault and all CV outputs exhibit the same symptom. When you get back, place the scope on a CV output and describe what you see for the next step (waveform [random, periodic, etc], voltage levels, frequency, etc ...). I will get my schematics of this unit out and take a look at them this morning. It could be several things happening here. The DAC itself or the reference to the DAC could be getting hosed. The 4051 addressing could be corrupted. etc ...... Shocked

OK, we will await your report of what junk is on that CV output .... Very Happy

Bill
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23isgood



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PostPosted: Sun Jul 08, 2007 11:35 pm    Post subject: Reply with quote  Mark this post and the followings unread

Ok because I wasn't sure, I changed the wal wart out again today for a brand new one, and 12v is coming out of the new wal wart. I plugged a cable out of the cv output and plugged that into one of my XR VCO's. I noticed that there is no more crazy FM going on but when I hit a key on my keyboard the midi light does not turn on, on the Paia and the cv does not change. Also the self test does not cycle through the outputs.

I think I may have a problem with the 7805. I placed the ground of my volt meter onto the middle pin of the 7805 and the positive from the meter to the out pin of the 7805 and its reading 1v or nothing on the output. Unfortunately I don't have any 7805's so I have to order one. So that means I have to wait to fix this thing, unless i'm not reading the voltages on the 7805 correctly? Its kinda late so I will check again tomorrow.

pete

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PostPosted: Mon Jul 09, 2007 3:54 am    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
I think I may have a problem with the 7805. I placed the ground of my volt meter onto the middle pin of the 7805 and the positive from the meter to the out pin of the 7805 and its reading 1v or nothing on the output. Unfortunately I don't have any 7805's so I have to order one. So that means I have to wait to fix this thing, unless i'm not reading the voltages on the 7805 correctly? Its kinda late so I will check again tomorrow.


OK, good. Just one thing, for proper 5V regulation, the input of a 7805 must be 7.5 volts DC or above. It's not what they call a low dropout regulator (LDO) and it it requires an input to output voltage difference of about 2.5V to operate. Not very efficient for battery applications but fine for this circuit.

Also, you have rectifiers D3 and D4 and C19 and C21 ahead of this regulator so please don't assume they are good. You can check V+ and V- to see if those components are OK. Also, the 7805 may be loaded down (C15 short??) so you may want to lift it's output to isolate IC14 and see if the voltage remains at 1V or less.

Just a few tips ...... Good luck and keep us in the loop ! Very Happy

Bill
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