[sdiy] Re: ARP Axxe popping..is this normal??

[Bob Weigel]

Hey greg,

Yeh sadly the vca cvr seems to work fine.  There are two kinds of pop 
troubling me here.  THere's a key pop that goes off at an annoying 
yet..possibly considered tolerable level given that I can't see another 
thing I can do about that.  Even with all of the sliders down  it's 
still coming out of the filter!  You can turn all the vco/noise inputs 
down, all CV's down and still you get a sharp spike when a key is depressed.

The other pop is resutling from the fact that we can't calibrate the 
filter and get any sound out.  I had qualitatively determined that but 
running the formal proceedure I see something else.  The resonance 
frequency calibration...it goes nicely down to 40msec...but you are 
supposed to adjust to 62.5msec.  After about 40, the sin wave falls flat 
and dissipates no more to be seen.  Absolutely can't sustain a resonance 
lower than that at the moment. hmm.

Anyway doing that the best I can, and moving onto the cvr adjustment on 
the vcf, you minimize the signal with just the lfo up 3/4 and the adsr 
up all the way (on the filter I assume...I mean..there's be no reason to 
adjust the lfo speed then turn the filter adsr up :-)  They didn't say 
that but one *would* assume they meant the filter lfo control) .  
Well...the signal drops to a few tenths of a volt transient...and stays 
there.  However...nowhere along that range will the synth actually 
produce sound.  It only produces sound when you bump it up to where it's 
making more pop. -bob

klosmon wrote:

> I may as well be the one to ask the stupid question ("go with your 
> strengths", they tell me):
> have you checked/adjusted the VCA offset trim?
> I haven't worked on one of these for a while, but I seem to recall 
> that there WAS such a trim, and that it made a radical difference.
>
> ~G
>
>
> At 09:08 PM 1/1/2007, you wrote:
>
>> Even with a slow attack, it's got all the charm of a machine gun at 
>> the moment. :-)  No didn't see need to employ any energy discharge 
>> techniques on this one. hehe. I had resonance down. This *is* the 
>> ladder with three 3086 arrays; all replaced in sockets now. Nothing 
>> changed like I said with all new transistors...and cant' see the 
>> signal on the inputs to the op amp..just output.  Really bizarre as 
>> always if I bring it up :-) -Bob
>>
>> harry bissell wrote:
>>
>>> Hi Bob...
>>>
>>> you didn't short the caps with a screwdriver did you ???   :^P
>>>
>>> ~seriously~
>>>
>>> Is this the ladder filter or the state variable ??? Usually the pop 
>>> is a sudden DC level shift. If you use a slow envelope
>>> does it ~not~ pop ???
>>>
>>> In the ladder case I'd look for bad matching in the two halves of 
>>> the ladder...
>>> that will cause a big DC swing...   I was able to look at the bottom 
>>> of the ladder
>>> and the DC level was changing a lot. Matching both halves cured that 
>>> problem...
>>> Maybe one transistor is shorted there ??? try with no resonance, 
>>> measuring the
>>> DC drops on both sides of the ladder. They should be equal...
>>>
>>> In the SVF maybe there is a bad offset voltage on an OTA stage...
>>>
>>>
>>> Bob Weigel wrote:
>>>
>>>>
>>>> I've never played one of these things but..I don't recall this much 
>>>> pop when I hit a key on the odyssey I repaired.  It was a later one 
>>>> I guess..but anyway... I can't figure out where this is coming from.
>>>> It originates, it seems at the output of the vcf op amp.  the other 
>>>> half of which is in the vca, a 1458.  I replaced it thinking it had 
>>>> gotten cross talky.  Didn't help much.  Maybe a little.  btw the 
>>>> sudden filter respond I talked about before was due to an open 
>>>> resistor.  Got that solved and everything else in the vcf replaced 
>>>> or verified except the dual fet and transistor pair.  All new 
>>>> filter caps including local ones. -Bob
>>>>
>>>> harry bissell wrote:
>>>>
>>>>> Usually the glitch is from the triangle that feeds the sine shaper.
>>>>> Its usually generated from the sawtooth, which causes a discontinuity
>>>>> where the reset occurs. Best way around it is to use a triangle 
>>>>> wave core...
>>>>> which usually has its own problems to solve...
>>>>> I did a little study on how to eliminate the glitch from the 
>>>>> triangle... I'd predict
>>>>> the reset time (perhaps with a comparator that is set just below 
>>>>> the normal
>>>>> reset point, then use a track-hold circuit to ride out the glitch.
>>>>> Woah its pretty
>>>>> complex but looks workable. Don't know if the distortion would be 
>>>>> worse than
>>>>> a small cap to cover the glitch... at least this method would keep 
>>>>> the effects of the
>>>>> glitch reasonable constant with frequency
>>>>>
>>>>> H^) harry
>>>>>
>>>>>
>>>>> Paul Perry wrote:
>>>>>
>>>>>> The $64 question is, where does the blip at the peak come from?
>>>>>> And, does it appear symmetrically at the lowest point of the sine?
>>>>>> ......and does it matter what diodes are used?
>>>>>>
>>>>>> paul perry melbourne australia
>>>>>>
>>>>>>
>>>>>>
>>>>>
>>>>>
>>>>>
>>>>
>>>
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