Oscillation problems WAS Re: C-Audio schematic?
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From: Denny Strauser on 2 Jul 2008 20:08 Eeyore wrote: > > Denny Strauser wrote: >> On Yahama 3K, 4K & Midas Venice, I've experienced something that sounds >> like an oscillation problem. On these consoles, when an EQ gain is >> turned up the whole way & Frequency is swept while passing a signal, >> there seems to be some sort of oscillation. >> >> What is this? > > Pot mistracking due to lack of regular use. One side of the frequency circuit goes > nearly open and it creates an oscillator around that EQ section.. > > Whizz the Freq pot back and forth a few times and it'll go away. > > Graham > Thanks for your insight. But, after hearing your opinion, I doubt it's lack of regular use, but lack of cleaning the pots. Am I right? All of these consoles were used constantly, but not serviced regularly. A friend of mine (my former repairman, who now installs & repairs electron microscopes) told me that there my be electromagnetic crosstalk in the system. Due to poor shields and/or high power speaker cables near mic cables. Any thoughts on this? -Denny
From: Phil Allison on 2 Jul 2008 21:27 "Denny Strauser" > > Thanks for your insight. But, after hearing your opinion, I doubt it's > lack of regular use, but lack of cleaning the pots. Am I right? All of > these consoles were used constantly, but not serviced regularly. ** The pots used in the Midas Venice are near impossible to clean - they are fully sealed types. IME, they all go bad after only a few years and cause major problems, then there are the dodgy ribbon cables. A desk to avoid. > A friend of mine (my former repairman, who now installs & repairs electron > microscopes) told me that there my be electromagnetic crosstalk in the > system. Due to poor shields and/or high power speaker cables near mic > cables. ** Wot ridiculous BOLLOCKS !!! ROTFL !! ....... Phil
From: Eeyore on 3 Jul 2008 00:19 Denny Strauser wrote: > Eeyore wrote: > > Denny Strauser wrote: > > >> On Yahama 3K, 4K & Midas Venice, I've experienced something that sounds > >> like an oscillation problem. On these consoles, when an EQ gain is > >> turned up the whole way & Frequency is swept while passing a signal, > >> there seems to be some sort of oscillation. > >> > >> What is this? > > > > Pot mistracking due to lack of regular use. One side of the frequency circuit goes > > nearly open and it creates an oscillator around that EQ section.. > > > > Whizz the Freq pot back and forth a few times and it'll go away. > > > Thanks for your insight. But, after hearing your opinion, I doubt it's > lack of regular use, but lack of cleaning the pots. Am I right? All of > these consoles were used constantly, but not serviced regularly. Unusual. Freq pots don't see THAT much service usually and therefore aren't likely to be physically badly worn. The above method has always worked for me. Any *decent* console should NOT need 'pot cleaning'. Mostly these days they are moulded and sealed types. You might also want to whizz the +/- pot back and forth a few times too for good measure. > A friend of mine (my former repairman, who now installs & repairs > electron microscopes) told me that there my be electromagnetic crosstalk > in the system. Due to poor shields and/or high power speaker cables near > mic cables. > > Any thoughts on this? Sounds like complete bollocks I'm afraid. Graham
From: Phil Allison on 3 Jul 2008 00:58 "Eeysore Criminal LIAR" > Any *decent* console should NOT need 'pot cleaning'. ** WRONG. Mostly these days they are moulded > and sealed types. ** WRONG - plus irrelevant to the need for cleaning, ....... Phil
From: Eeyore on 3 Jul 2008 15:05
gwatts wrote: > Eeyore wrote: > > Denny Strauser wrote: > >> > >> On Yahama 3K, 4K & Midas Venice, I've experienced something that sounds > >> like an oscillation problem. On these consoles, when an EQ gain is > >> turned up the whole way & Frequency is swept while passing a signal, > >> there seems to be some sort of oscillation. > >> > >> What is this? > > > > Pot mistracking due to lack of regular use. One side of the frequency circuit goes > > nearly open and it creates an oscillator around that EQ section.. > > > > Whizz the Freq pot back and forth a few times and it'll go away. > > I've seen/heard it on brand new Furman PQ-3's, implying I don't think > it's a worn or dirty pot, more the filter circuit going unstable due to > the frequency related topology* changing while the gain is high. I've > only had it happen while sweeping and with the Q set for less than 1/2 > octave width. Just keep the gain lower until you find the right > frequency, then adjust the gain to suit. > > * If you want to go in to poles and zeros I will, but that gets a bit > pedantic. Must be very poor track matching I'd imagine then. Higher Qs would exacerbate the problem. Graham |