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 Forum index » DIY Hardware and Software
Oberheim OBX pots not working...
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confinedscommute



Joined: Jun 03, 2013
Posts: 5
Location: los angeles

PostPosted: Sat Aug 03, 2013 4:15 pm    Post subject: Oberheim OBX pots not working...
Subject description: need help repairing!
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Hello!

I've restored a completely broken OB-X but I cannot get any of the pots to work. All of the buttons work. I'm sort of a newbie at working on synths.

The pots are getting proper voltage and changing the output voltage when turned. And I replaced the CD4051s they are connected to, but I'm not sure where to go from here. On the service manual the output of the 4051s feed a LM311N voltage comparator. Maybe I should replace that next, but I don't understand enough to know if that's the problem.

From my limited understanding I believe the output voltage on pin 3 of the 4051s should change when the pots are turned, right? It doesn't ever change, so it doesn't seem like the address lines are getting fed anything.
(actually on one patch when I change the VCA release, I noticed a 0.2V change from min to max, and the release of the sound does change slightly, but not nearly enough.

Maybe someone could tell me if there are any other crucial ICs that could take down all the pots at once. Or maybe a crucial transistor?

One thing I've noticed is that the OSC2 Detune LED will go on and off when I change presets accordingly, but turning the knob will not affect the LED in edit mode, so my guess is the CPU is probably not registering the knob changes at all. So probably not a problem with the DAC, right?

Anyways, any help would be really appreciated...I'm stuck after spending a ton of hours on this thing, rebuilding the power supply and replacing capacitors....replacing many ICs and sockets and installing the encore midi.

Thanks - service manual here:
https://www.dropbox.com/s/fzg5534lel...e%20Manual.pdf

I also am starting this thread to eventually post a conversation I have been having with a member on these forums, in case it might be of help to anyone else working on a vintage synth.
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sneakthief



Joined: Jul 24, 2006
Posts: 525
Location: Berlin

PostPosted: Mon Aug 05, 2013 3:00 am    Post subject: Reply with quote  Mark this post and the followings unread

1. Can it recall any of its patches and actually change sounds accordingly? If so, then the LM311 and the DAC is working.

2. On early vintage synths with patch storage, the DAC is also used as an ADC: to put it really simply, the DAC can sweep through it's range of outputs until one of the voltage level "matches" the current pot it's trying to scan.

Then the LM311 sends a digital signal saying it has a match between the pot value and a particular digital value that the digital bus sent out. The CPU then holds that value in memory as the current value of a specific pot.

3."From my limited understanding I believe the output voltage on pin 3 of the 4051s should change when the pots are turned"

The pots are being scanned in a KHz range, so unless you have proper metering equipment, it's unlikely you'll be able to recognize any signal on the 4051 outputs. In other words, the 4051's are being switched so fast that you won't be able to tell what's going on.

The data busses that switch the 4051's are labelled AR0, AR1, AR2, AR3, AR4 and AR5. For what it's worth, the chip labelled A26 is a 74C174 is a HEX D-Flipflop that's responsible for sending the A1-A5 data buss to those 4051's. However, it's also helping read all the buttons and keys, so if those are working then the AR0-5 data buss is OK.

I suggest tracing the outputs of the 4051's to make sure they're all linking together properly and going to the analog input of the LM311 without being short-circuited or something.

Or try replacing the 4051's again and making sure they're getting proper power and that the circuit traces for the AR0-5 busses that go to the 4051's are intact.

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confinedscommute



Joined: Jun 03, 2013
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Location: los angeles

PostPosted: Mon Aug 05, 2013 11:24 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks for the in depth reply!

Yes it can change patches fine so yeah the 311 and DAC must be working, I see what you mean now that I understand how the scanning works.
I was thinking the CPU somehow selected an "active" pot when you turned one of them, but now I understand it is checking all of them nearly instantaneously all the time for changes.

Yeah the keyboard and other switches work fine. Also, I logic probed the AR0-5 busses and they are definitely sending information as expected. What voltages should AR0-5 be tested with? I was using +5 and 0. And is it TTL or CMOS? the 74c174 is TTL and the 4051 is CMOS...I'm guessing TTL because that is producing it.

One last thing to note that I thought was strange was that one of the 4051s has a voltage of around 1.5V on the enable pin-6, but the other two have a voltage around 4.5V. I would imagine these should all be the same....

Hopefully the issue is not the midi board, because I have no idea how to trouble shoot that, and I don't have working roms to replace it.

EDIT: actually the pin that has a different voltage is receiving AR3* (not AR3) which looks like it should be a different voltage after going through the 47K resistor.

Am I wrong in thinking that if the switches are working, then A4 on the pot boards (74C42) must be working, which means AR0, AR1, AR2 and AR3* must also be working correctly?
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confinedscommute



Joined: Jun 03, 2013
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Location: los angeles

PostPosted: Mon Aug 05, 2013 1:47 pm    Post subject: Reply with quote  Mark this post and the followings unread

wow so I replaced the 4051s again and it works. There was a bad one out of the new batch. So annoying! Thanks for the suggestion.

It seems the pots connected to A5 still wont' work no matter what 4051 I put in there, but that might be a different problem. I'm going to see if I can control the filter frequency with midi, and if I can't then I would imagine it's not related to the pots.

EDIT: Wow..I saw all of A5s pots eventually get to A44 and there was ANOTHER bad 4051 there even though I had already replaced it! I did read on someone's site that the 4051s on the control board are not properly protected, so they go out all the time.
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sneakthief



Joined: Jul 24, 2006
Posts: 525
Location: Berlin

PostPosted: Tue Aug 06, 2013 1:49 am    Post subject: Reply with quote  Mark this post and the followings unread

Glad to hear that helped.

4051's do blow rather easy at times. I think I've killed a dozen.

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confinedscommute



Joined: Jun 03, 2013
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PostPosted: Tue Aug 06, 2013 2:23 am    Post subject: Reply with quote  Mark this post and the followings unread

I just checked all the filter VCA control inputs on the CEM3110s and found the sustain input wasn't changing and traced that back to a TL084 and replaced it and now my synth plays for the first time like normal!!!!!!!!!!

Wow so cool sounding...this thing is amazing. 2 of the voices are still giving me trouble out of 8, but I'm happy with 6 for now.

Thanks again for your help
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sneakthief



Joined: Jul 24, 2006
Posts: 525
Location: Berlin

PostPosted: Tue Aug 06, 2013 2:26 am    Post subject: Reply with quote  Mark this post and the followings unread

Nice work!

If the voices aren't repairable, there's always this DIY-option:

http://www.muffwiggler.com/forum/topic-83363.html&highlight=

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confinedscommute



Joined: Jun 03, 2013
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Location: los angeles

PostPosted: Thu Aug 08, 2013 3:21 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks, yeah I've actually been following that thread. Pretty cool stuff!

Though I do love the shape of the Curtis envelopes on the originals...I wonder how lively these alternatives will be.

I wonder if you could help me with the remaining problem:

There is a slight bit of vibrato on all the time, but it's not just vibrato. It changes shape when I change the LFO waveform, so it doesn't seem related to the mod lever, which is only connected to the triangle wave. It is present on all patches even when OSC1 and 2 are not selected in the mod section. And those buttons work fine for allowing me to increase the modulation depth when they are turned on.

I would imagine maybe it's a resistor somewhere or maybe one of the opamps in the path from the LFO to the 2 VCOs. A11 or A5 look suspect to me from the circuit diagram.... maybe it has to be A5 because A11 looks like it comes before the filter modulation, and that does not seem to be affected. Only A5 affects just the VCOs..... but I tried replacing A5 and no change.
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radek tymecki



Joined: Mar 22, 2009
Posts: 70
Location: PL

PostPosted: Sun Aug 11, 2013 12:00 am    Post subject: Reply with quote  Mark this post and the followings unread

4051 is common problem in OB's series. i had few ob8's where 4051 was damaged and panels controls were not working
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oldcrow



Joined: Sep 24, 2008
Posts: 22
Location: Chicago, IL USA

PostPosted: Wed Aug 28, 2013 7:16 pm    Post subject: Reply with quote  Mark this post and the followings unread

confinedscommute wrote:
Though I do love the shape of the Curtis envelopes on the originals...I wonder how lively these alternatives will be.


They're the same envelope profile, I used JH's inverse-parallel exponential converter trick, which he based on the way Curtis did things in the 3310:

http://www.jhaible.com/tonline_stuff/hj_vc_hadsr.html

--Crow
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