[sdiy] Chroma Polaris strangeness

[Bob Weigel]

On 9/19/2012 7:37 PM, Bob Weigel wrote:
> Hi all... I had some dirty sockets etc. that I think we got all 
> resolved...but ch5 still turns off during auto tune.  Viewing the tune 
> buffer it's signal is if anything a hair hotter than the others at 
> that point.  When I turn the channel back on it plays fine.  What 
> gives with that I wonder? I'm out of reasonable ideas... Already sold 
> it and then this.  Any help appreciated..
>
Well.... this is bizarre.  I recall someone saying prophet 600 
filter/vca chips could be very picky too...(maybe that's the issue I 
have with a P600 here that has to warm up a bit to tune?)  But didn't 
recall the Polaris was picky.    So I noticed there was some level 
discrepency in the auto tune.  yet waveforms coming in to the CEM3372 
looked fairly identical.  Most are near the same but ch4 is actually 
lower by 5% maybe and 5 the one that won't tune higher by about the same.

I swap the chips.  Four AND Five fail now!  (And yes the levels have 
swapped.  Now 4 is high and 5 is low so it is being caused by vca 
discrepancies apparently) . Now I swap in a pair of chips from a scrap 
polaris with the same lot number.  ch4 is back up..still no channel 5.  
And I look on the scope and I see that 4 is actually right in there now 
but 5 is back up high again.

So I finally swap 4 and 5 again and this time they all work!  Wow.  So I 
guess there are some fairly fine tolerences there for whatever reason.  
I thought we're like..just dealing with a comparator in the tune buffer 
that chops the signals like usual and gives a nice square wave that can 
be counted.  Anybody got an explanation?

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