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Cutoff and Filter Envelope issues
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Cutoff and Filter Envelope issues

by Crabby on Tue Nov 22, 2016 1:15 pm

Hi, I've just replaced the resonance pot as the old one was getting very scratchy/dropping in and out of resonance at the top. Replaced with an ALPS 50k linear x 2 from Mouser that fits perfectly into the mainboard, and the resonance is working great, but now:
1. No audible sound when cutoff fully down, sound only starts when it's at about 9-10 o'clock, quite suddenly. I tried adjusting TM3, but no joy.
2. There's a slightly slow attack now on the filter, so that it sounds bubbly.
I've checked my soldering and all seems well, so I'm a bit baffled by this behaviour. I'm worried maybe I damaged one of the transistors or caps maybe?
Any advice would be greatly appreciated, my Xox is in almost daily use and I need it back working as normal asap!
Thanks!
C

Crabby
 
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Re: Cutoff and Filter Envelope issues

by antto on Tue Nov 22, 2016 1:40 pm

can you record a sound clip demonstrating the issue?
what's the exact part number of the pot you used?

if suddenly you don't appear to get sound when the cutoff knob is below a certain point - that probably means that the cutoff frequency is going very low, this can happen if the voltage at R72 goes significantly below 3V (i had this issue myself, the two halves of the resonance pot were internally shorted)
..it may also happen if the offset voltage at R63 or R73 are (for some reason) lower than normal

to check if you have the same problem as i had - measure the voltage at R72 (make sure you're not playing anything with accents and keep the accent knob low) .. you should have about 3V (a few hundred millivolts up or down isn't a problem)
also check whether physically moving the potentiometer shaft up/down/sideways causes the behavior to change

antto
 
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Re: Cutoff and Filter Envelope issues

by Crabby on Wed Nov 23, 2016 6:09 pm

Hi, thanks a million for the assistance!

The Mouser part no. is 688-RK12L123000E, page here:
http://eu.mouser.com/search/ProductDeta ... 12L123000E

Here's a short sample of how the filter's sounding:
https://www.dropbox.com/s/bzr2ht1krl0rf ... 1.mp3?dl=0

I measured the voltage across R72, I'm assuming it's operating on DC? It got nowhere near 3v, it was peaking at about 0.6 but jumping around a lot from 0.01.
Made no difference with cutoff closed or open, and I tested a flat pattern, no accent, slide etc, accent knob fully down...
Thanks again for the assistance!
C

Crabby
 
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Re: Cutoff and Filter Envelope issues

by antto on Thu Nov 24, 2016 2:55 pm

oh boy
if this x0x has no "modifications" - things are quite wrong
the lack of ~3V around R72 would explain why the cutoff frequency is uber low, however, that's not all
it seems the filter envelope is gone abnormal, it mimics an "attack" which is not supposed to be happening (this however could be actually caused by some other component, i can't say)

so, if you really think you might have damaged some components (maybe with the iron) - check them out, also look closely around the solder joints for anything suspicios like bad joints, blobs of solder or shorts, etc

if you can't find anything obviously wrong - i suggest you desolder the resonance pot carefully, take it out of the board, and see if the x0x will behave properly then
- measure the voltage at R72 again
- you will not have feedback in the filter, thus no resonance ... you can short the two outter-most pins (from the row of 6) to simulate max resonance, then check if it sounds "normal" (or record another clip if you're not sure)
while the potentiometer is out, you can also check its resistance with the multimeter:
the pins are described in the datasheet in the mouser link you posted, basically it contains two variable resistors, which should not have any electrical connection - if they do, the pot is busted

antto
 
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Re: Cutoff and Filter Envelope issues

by Crabby on Thu Dec 01, 2016 12:05 pm

Ok, so I checked all the joints, all seem ok...then desoldered the res pot and checked to see if it's shorted, which it isn't, although I was getting some pretty big variations in resistance, but maybe within the 20% spec.
I bridged the outermost pin holes for the res pot on the board, and the filter sounds fine, cutoff begins normally, audible all the way from 7 o'clock upwards, and no weird inverse envelope with the resonance.
I tried to measure voltage again across R72 but couldn't get anything?!? Maybe because the res pot is out? I would've thought there'd be a difference anyway though.
I dunno, I'm baffled at this stage. At least, it doesn't seem to be an issue with damage to the board and components, maybe the replacement pots a dud....

Crabby
 
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Re: Cutoff and Filter Envelope issues

by Crabby on Thu Dec 01, 2016 2:48 pm

So, after much blood, sweat and burnt fingertips, it seems the replacement pot was faulty. I managed to find another 50k x 2 linear pot in my box of odd components, and did a fair bit of microsurgery on it to get it to fit the board.... and the filter is now working fine, no weird attack on the resonance, sound still audible with cutoff all the way down....so I'm back in action :)
Thanks for all the assistance, really really appreciate it!
C

Crabby
 
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Re: Cutoff and Filter Envelope issues

by antto on Fri Dec 02, 2016 1:36 pm

yeah, so you've had the same issue as me
i replaced 5 resonance pots in the same x0x in a short period of a few months, they were all "alpha" pots
maybe a bad batch, who knows
the last pot i placed there was an ALPS like the one you linked to (but i got it on ebay) and so far it's working fine (but the pcb traces around the resonance pot are in a very bad shape from my impatient desoldering)

btw, with the resonance pot removed, you should've checked if you get about ~3V a bit earlier in the circuit, for example at R46, i should've been more clear

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