[sdiy] despicable delta destroys detective's day (VCF insanity)

[Bob Weigel]

Thanks Magnus and I played it a bit more and all ok so far!  The strings 
sound nicer on this one than any I've heard for some reason.  It's a 
beautiful thing now.  I've got one in the studio that just sounds a 
little different.  Unless someone wants to pay the bucks for this one I 
think I'll use it for a while after all that. lol.

Anyway I figured sharing the story might be a good insight for some like 
me who tend to analyze rather than just replacing everything in cases 
like this.  Sometimes it's quicker to just replace everything :-)   But 
it's good to know that a cap can fail in that way.  I moved it.  
Stressed it...no response.  It looked great.  Measured great... no leak 
on diode checker.  I didn't bother running it to voltage... but anyway 
when trouble goes....you don't ask where right? lol

-Bob

On 7/10/2011 5:36 AM, Magnus Danielson wrote:
> Dear Bob,
>
> On 07/10/2011 09:10 AM, Bob Weigel wrote:
>> This tops 'em all. Ok... I'm using a Delta I repaired months ago the
>> other night to give it a test and avoid taking the one out of my studio.
>> It crackle-pop-fizzles within a few minutes of use. I opened it and
>> wiggled/tapped and it seemed to have some mechanical correlation but I
>> couldn't isolate it. One of *those*....
>>
>> So back at the ranch..er...bench...I look and find that the signal is ok
>> up past the first ota stage. Noises happen in the signal at the inv. in
>> of second stage..which..means nothing since there's feedback there so we
>> can't tell where it's originating from that of course w/out some deeper
>> evaluation. I *assumed* it's probably just the 13600's and changed them
>> both as I've done on several units with success. This time..uhuh.
>>
>> I re-soldered everything in the area. Still no result.
>>
>> Now when it's failing...it sound kinda like a resistor going open type
>> effect. And indeed when i finally get it to cut in and out regularly
>> enough to see what is happening... the offset voltage at pin 5/7 I think
>> it is...before the buffer...is zigging from 1.1V or so offset to near
>> zero when the signal gets attenuated to a noisy low level. NOW what
>> resistors could affect that? Well the 10K feedback, the 10K input or
>> maybe the 15K or the 220ohm ones. Replaced them all....no result! Still
>> exactly the same. Usually plays a few seconds when you turn it on as you
>> hear the noise gradually coming and then BONK....or it would go into a
>> rapid 'switching' between working and not working ... anyway....
>>
>> I'm like..well..there's only one component (after changing the OTA a
>> second time...) that we havent' done ..the .001 uF mylar cap which
>> showed NO particular corrleation while squeezing or twisting it.
>>
>> I pulled it..put the diode checker on thinking it would pick up a leak
>> if it was leaking since mine goes up to 3. some volts. Showed no issue.
>> Anyway I put a new cap in anyway... and it is working now. I haven't
>> heard it crack up once in hours now. Has anyone EVER seen a failure mode
>> of this type with what appears to be a mylar cap? -Bob
>>
>
> I so recognice the frustration you had.
>
> I've found that open up feedback loops can be an aid in fault 
> analysis. That way you get a waterfall analysis which can be simpler. 
> You can also observe the input offsets of op-amps easier.
>
> Anyway, as for the fault, it could be that the mylar to pin 
> interconnect is flakey... so you have quite a different capacitance 
> and all you balances are way off. Consider it flakey-fluke when 
> sitting in the negative feedback over an op-amp... the gain of that 
> circuit will jump like mad.
>
> Thanks for sharing this story. We all end up with some strange bug 
> every now and then. I've felt pretty darn stupid myself at times. Ah 
> well. That would be buissness as usual then :)
>
> Cheers,
> Magnus
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