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Oberheim OBX-a voice tuning specifics

Oberheim OBX-a voice tuning specifics

2014-01-07 by walter bonacci

Hi,

Someone has the specifics for the voice of an oberheim obxa?
I would like to know which trimmer does what.

In theory is it possible to do the tuning by hear and without an oscilloscope?

Thanks,
Walter

Re: [oberheim] Oberheim OBX-a voice tuning specifics

2014-01-07 by James

Walter,

First, download the 1st and 3rd edition of the service manual. First edition is here:

http://launch.groups.yahoo.com/group/oberheim/files/OB-xa%20files/

3rd edition: http://www.synthfool.com/docs/Oberheim/Oberheim_OBXa/oberheim_obxa_service_manual_3.pdf

You don't need a scope to tune the boards. It is very straight forward if you follow the manual. 

To do this right, you need to perform the full service. The 5 and 15 volt piece always comes first followed by all of the adjustments on the bender assembly. Once those are finished, then you tackle the cards.For your first time, you might want to plan on 2 hours for the whole calibration. 

James




________________________________
Show quoted textHide quoted text
 From: walter bonacci <walterbonacci@...>
To: oberheim@yahoogroups.com 
Sent: Tuesday, January 7, 2014 5:51 AM
Subject: [oberheim] Oberheim OBX-a voice tuning specifics
 


  
Hi,

Someone has the specifics for the voice of an oberheim obxa?
I would like to know which trimmer does what.

In theory is it possible to do the tuning by hear and without an oscilloscope?

Thanks,
Walter

Re: [oberheim] Oberheim OBX-a voice tuning specifics

2014-01-07 by Les Lambert

I have done it without any testgear at all, but if you don't have a good ear, you're in for a rough ride.
The pulse width adjustments and tracking are particularly hard, and there are many interactions of course.
It all hangs off the accurate adjustment of the power supply as mentioned above, and the bender wheel adjustment is crucial. 
Previous answer is the correct one, do what they did at the factory, if you can get the info.
At the time I did it, the info wasn't easily available, and I'd done my OBX a short time before, so felt confident.
Mistakenly some might say.  : o





On Tuesday, 7 January 2014, 12:34, James <fl_799@...> wrote:
 
  
Walter,

First, download the 1st and 3rd edition of the service manual. First edition is here:

http://launch.groups.yahoo.com/group/oberheim/files/OB-xa%20files/

3rd edition: http://www.synthfool.com/docs/Oberheim/Oberheim_OBXa/oberheim_obxa_service_manual_3.pdf

You don't need a scope to tune the boards. It is very straight forward if you follow the manual. 

To do this right, you need to perform the full service. The 5 and 15 volt piece always comes first followed by all of the adjustments on the bender assembly. Once those are finished, then you tackle the cards.For your first time, you might want to plan on 2 hours for the whole calibration. 

James




________________________________
Show quoted textHide quoted text
 From: walter bonacci <walterbonacci@...>
To: oberheim@yahoogroups.com 
Sent: Tuesday, January 7, 2014 5:51 AM
Subject: [oberheim] Oberheim OBX-a voice tuning specifics
 


  
Hi,

Someone has the specifics for the voice of an oberheim obxa?
I would like to know which trimmer does what.

In theory is it possible to do the tuning by hear and without an oscilloscope?

Thanks,
Walter

Re: [oberheim] Oberheim OBX-a voice tuning specifics

2014-01-07 by walter bonacci

Thanks guys!

Ok, I will start with the power supply and then follow the service manual. Yes I tried it before and the pulse weight was the hardest part... (do you have any tip? Searching for the most hollow sound doesn't tell much).

Can I leave the modulation at the end if I disable it?

I know I have a problem with -1 octave transpose button (down), sometime it works more often it doesn't... seems to be random.
The light turns on always but the transposition seems to work only in certain circustances, I couldn't realize when, maybe only when I select certain programs... or sometime after clicking it 20 times or moving down from up :/

Is the mod for enabling self resonance on the filters hard to implement?

Thanks!
Walter


On Tue, Jan 7, 2014 at 2:42 PM, Les Lambert <les_lmbrt@....uk> wrote:

I have done it without any testgear at all, but if you don't have a good ear, you're in for a rough ride.
The pulse width adjustments and tracking are particularly hard, and there are many interactions of course.
It all hangs off the accurate adjustment of the power supply as mentioned above, and the bender wheel adjustment is crucial.
Previous answer is the correct one, do what they did at the factory, if you can get the info.
At the time I did it, the info wasn't easily available, and I'd done my OBX a short time before, so felt confident.
Mistakenly some might say. : o




On Tuesday, 7 January 2014, 12:34, James <fl_799@...> wrote:
Walter,

First, download the 1st and 3rd edition of the service manual. First edition is here:

http://launch.groups.yahoo.com/group/oberheim/files/OB-xa%20files/

3rd edition: http://www.synthfool.com/docs/Oberheim/Oberheim_OBXa/oberheim_obxa_service_manual_3.pdf

You don't need a scope to tune the boards. It is very straight forward if you follow the manual.

To do this right, you need to perform the full service. The 5 and 15 volt piece always comes first followed by all of the adjustments on the bender assembly. Once those are finished, then you tackle the cards.For your first time, you might want to plan on 2 hours for the whole calibration.

James


Show quoted textHide quoted text
From: walter bonacci <walterbonacci@...>
To: oberheim@yahoogroups.com
Sent: Tuesday, January 7, 2014 5:51 AM
Subject: [oberheim] Oberheim OBX-a voice tuning specifics

Hi,

Someone has the specifics for the voice of an oberheim obxa?
I would like to know which trimmer does what.

In theory is it possible to do the tuning by hear and without an oscilloscope?

Thanks,
Walter





Re: [oberheim] Oberheim OBX-a voice tuning specifics

2014-01-07 by James

Just run the pot to the extremes and the most hollow/square sound will become obvious.  Make sure you are plugged into an amp and have it turned up a bit!

The hardest part of tuning on 4 and 2 pole is getting the noise to match the pitch of a note (initial frequency calibration). That one can be tricky but do as the service manual suggests i.e. setting the osc to half and full. I like to do one more step. I toggle through all 8 cards and if the pitch of the noise does not match the other cards, it will stick out like a sore thumb. Repeat on both 2 and 4 pole settings.


Sounds like the button might be going out. Doug @ synthparts can help. Easy to replace them.

Have you found IC #64 yet for the DAC adjustment. It is actually IC 166

CHeck out my blog on my Oberheim experiences. I have some calibration and restoration stuff in there!

https://sites.google.com/site/jlonanostuff/home/oberheim-ob-xa



________________________________
 From: walter bonacci <walterbonacci@...>
To: oberheim@yahoogroups.com 
Sent: Tuesday, January 7, 2014 8:57 AM
Subject: Re: [oberheim] Oberheim OBX-a voice tuning specifics
 


  
Thanks guys!

Ok, I will start with the power supply and then follow the service manual. Yes I tried it before and the pulse weight was the hardest part... (do you have any tip? Searching for the most hollow sound doesn't tell much).

Can I leave the modulation at the end if I disable it?

I know I have a  problem with -1 octave transpose button (down), sometime it works more often it doesn't... seems to be random.
The light turns on always but the transposition seems to work only in certain circustances, I couldn't realize when, maybe only when I select certain programs... or sometime after clicking it 20 times or moving down from up :/

Is the mod for enabling self resonance on the filters hard to implement?

Thanks!
Walter



On Tue, Jan 7, 2014 at 2:42 PM, Les Lambert <les_lmbrt@....uk> wrote:

 
>  
>I have done it without any testgear at all, but if you don't have a good ear, you're in for a rough ride.
>The pulse width adjustments and tracking are particularly hard, and there are many interactions of course.
>It all hangs off the accurate adjustment of the power supply as mentioned above, and the bender wheel adjustment is crucial. 
>Previous answer is the correct one, do what they did at the factory, if you can get the info.
>At the time I did it, the info wasn't easily available, and I'd done my OBX a short time before, so felt confident.
>Mistakenly some might say.  : o
>
>
>
>
>
>
>On Tuesday, 7 January 2014, 12:34, James <fl_799@...> wrote:
> 
>  
>Walter,
>
>First, download the 1st and 3rd edition of the service manual. First edition is here:
>
>http://launch.groups.yahoo.com/group/oberheim/files/OB-xa%20files/
>
>3rd edition: http://www.synthfool.com/docs/Oberheim/Oberheim_OBXa/oberheim_obxa_service_manual_3.pdf
>
>You don't need a scope to tune the boards. It is very straight forward if you follow the manual. 
>
>To do this right, you need to perform the full service. The 5 and 15 volt piece always comes first followed by all of the adjustments on the bender assembly. Once those are finished, then you tackle the cards.For your first time, you might want to plan on 2 hours for the whole
 calibration.
Show quoted textHide quoted text
>
>James
>
>
>
>
>
>
>________________________________
> From: walter bonacci <walterbonacci@...>
>To: oberheim@yahoogroups.com 
>Sent: Tuesday, January 7, 2014 5:51 AM
>Subject: [oberheim] Oberheim OBX-a voice tuning specifics
> 
>
>
>  
>Hi,
>
>
>Someone has the specifics for the voice of an oberheim obxa?
>I would like to know which trimmer does what.
>
>
>In theory is it possible to do the tuning by hear and without an oscilloscope?
>
>
>Thanks,
>Walter
>
>
>
>

Re: [oberheim] Oberheim OBX-a voice tuning specifics

2014-05-13 by walter bonacci

Hi Guys,

I started yesterday doing the tuning, following your advices.

I'm having trouble when doing the tuning of the filter, in particular when I have to compare the noise to the note played by half of the VCO 2 (initial frequency calibration). It is not so straight to do it by hear and even using an external tuner in my DAW it doesn't get precisely the pitch of the noise.
This is true for both 2pole and 4pole filters.
The next step, volts per octave calibration of the filters, is affected by the same problem, I know how to tune an instrument but I'm in trouble finding the pitch of the noise or comparing it to the sound of the VCO.

Then I have another question, when it comes to tuning the resonance, I was surprised to see that both the filters can actually self resonate, I think they are supposed not to without a mod, but actually I can clearly distinguish the self resonance from the sound of the VCOs.
Why the manual tells to tune the resonance just before hearing something, so before making them selfresonating when resonance is turned fully clockwise? (The sound of the self resonance get's strong only when track is selected, is quite without).

Thanks!
Walter

Re: [oberheim] Oberheim OBX-a voice tuning specifics

2014-05-15 by ipc.de@...

Dear Walter,

I had troubles with the tuning of the filters by noise. I therefore always do it in way I do it on all other analog synths VCFs.

See, the first goal of the tuning is a sychroneous tracking of the filter frequency on all voices. no matter what the absolute frequency of the cutoff actually is, I should in any instance be the same for all voices. The absolute cutoff for all filters can easily be adjusted in the patch via the filter pot on the panel.

The first step of the tuning is to find the absolute offset frequency. Be aware that filter tuning can be properly performed only when the KCV (tracking) is present. Do not forget to activcate "track". This is actually the second step. to first step is to allow the voice boards to warm up. Depending on you local conditions 20 mins will do.

Now switch off all signal to the filter (OSC1 off, OSC2 off, NOISE off). Then crank up resonance on all voices, but only to a level where self-oscillation just begins, but is stable. Not more! Since heavy resonance might lower the resonance frequency and thus the apparent cutoff frequency of the filter which would then lead to misalignment. Resonance occurs quite easily in the range of more than 250Hz.

Check whether self-oscillation can be accheived in the same manner in all voices.
(If not, one of the CEM3320 may be out of range (replace). In most cases I had here which showed trouble with the filters, the problem was in the 741 OP-amps processing the incoming CVs.Or even in the TL084 OP-amps generating the CVs on the CPU boards. I don't know, what is the reason for this, since 741 and TL084 normally are pretty robust. So if a voice board is bad, do not always blame the CEMs for it.)

Now take a chromatic tuner, to tune all voices corresponding to the key pressed on the keyboard. Take care to adjust the voices to each other rather than to the absolute note value.

Then proceed with the slope adjustment. Do not expect a perfect tracking! This is not a digital synth and the design of the OB-Xa in this respect is poor. Acceptable tracking can be acchieved over two octaves only - at most, if not less. For your filter tuning use the note range you are using most.
Note that offset substantially changes with the adjustment of the slope.

I quickly alternate between C2 and C4 on the keyboard (activate 1 voice only, of course) and simultaneously adjust offset and slope until a two octave jump is heard.
This is laborous and tricky, but you can manage it with the filters in self resonance. It would be impossible with the noise method described in the manual.

Repeat for 4-pole.... ;-)

Then, before switching on the oscillators, play a little tune with the resonating filters on the keyboard. And make a break.

I recommend to lower the resonance setting back to "slighly below/before self-oscillation". Verify the absence of heavy self-oscillation at higher frequencies/notes to rule out any surprising "resonance attack" from one or more of the voices during playing. IMHO, the OB-Xa is not the right synth to shine at self-resonance, one should not go for it. Besides, the factory patches may not sound like they were meant to with a wrong resonance setting.

(Speaking of authetic factory patches, consider to adjust both LFOs on the CPU board to about 3 Hz with LFO pot twelve o'clock. SPLIT setting (of course with the same patch on upper and lower). I experienced heavily misaligned LFOs in my OB-Xa s which has considerable impact on the sound of some of the patches and in particular in DOUBLE mode using the same patch on both parts of the machine.)



Another alternative method for filter tuning I had once used, does not require a full self-oscillation (but high resonance!) and no chromatic tuner: Swith on HALF OSC2, pulse wave 50% (square) and adjust the filter for maximum amplitude of the sine wave signal leaving the filter. At high resonance settings the low pass acts somewhat like a band pass an the cutoff frequency, i.e, the (basic) frequency of an oscillator signal is let through at maximum when the cutoff frequency corresponds to the frequency of the oscillator.
This method works nice, but you have to be aware of that the oscillator's signal somewhat shifts or influences the filter's actual cutoff frequency. That is why Oberheim originally recommends to use noise as the input signal instead of a tuned oscillator signal.



Any comments, please share your personal experience with the filter tuning process.


Cheers

Mondo




Re: Oberheim OBX-a voice tuning specifics

2014-05-15 by richtrix@...

I don't really know the best method for the filters but I had to give up on the method for volts per octave given in the service manual as it was getting me nowhere when I replaced a CEM3340 recently.

I finally busted out my peterson strobe tuner and tuned it the same way I'd inntonate a guitar. First I would put the master in tune for the lowest C. Then I'd go to the Volts per octave pot and put in tune the highest C on the keyboard. This will throw the master tune out of whack. So repeat. You should see a closing of the gap with each repetition until you are seeing the low C and the high C in tune. It might take 3-10 times doing the cycle.

Re: Oberheim OBX-a voice tuning specifics

2014-05-20 by ipc.de@...

@richtrix:
This is close to the method I use (explained in this thread) But I experience that V/oct. linearity could not be accheived over the whole range of the keyboard (or even the extended reange including HI and LOW transposition). When "offset" and "slope" on the voiceboard are set such that a "jump" from C0 (lowest C) to C5 (highest C) on the keyboard appears correct on the chromatic tuner, it might not be correct for octave jumps in the range of C2 and C4. Please consider to concentrate your tuning efforts to that keyrange you are intending to use most.

Please share your tuning experience with me.

Re: Oberheim OBX-a voice tuning specifics

2014-05-20 by ipc.de@...

@mvrzo:
The way of filter tuning I had described here I use for basically all analog synths on my bench that allow for and require tuning/adjustment of filter and/or keytracking.

I also works on a OB-X: look for the "init Freq." (T9) = "offset" and "V/Oct." (T10) = "slope" trim pots on each voice board.

Mondo

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