Weird problem with Rocktron Vertigo (leslie sim)

[nordine]

Hi again, im currently servicing a Rocktron Vertigo and while i cleared most of the issues, theres one remaining issue thats bugging me and in fact driving me crazy

the effect works fine now, every parameter does as its supposed to and everything.

HOWEVER theres a serious problem in the startup. If i left it unplugged (power plug) for say 30 min, when i plug the power i get a loud HUM like something its shorting inside badly.. but, 3 seconds later, the hum ceases, still theres no sound, 2 seconds later the guitar comes but theres no modulation, 3 seconds after, one can hear the modulation and everything works perfect, EXCEPT for the Rate pot, which when in slowest position (about initial 20% of traveling) it ceases to oscillate.

if i unplug and replug instantly, theres no hum, theres clean guitar and 3 seconds after fades in the modulation and everythings fine

there was a 7805 that did output 5.9V, so i changed it and now it outputs 4.9V ..but this problem was there before and after the regulator change
every IC reads as it should

for me, a gradual everything on pedals means a capacitor is misbehaving, but where? cant point it..maybe at the very first stages of filtering... but the HUM? any ideas?

[nordine]

extra info:

from what i gathered this is an all analog circuit, a kind of mxr phase 90/45 type on steroids
has 6 phase shift stages, a stage for rate speed up (leslie thing), also a 4016 that does the bypass

[GibsonGM]

Does it run off a wall wart?  Have you measured the wall wart's output?  Any way to replace the wart with a battery and see if it still does it? Just eliminating a bad/over-voltage PS....

I hate instantaneous, transient BS like this!  Tough to find it.  Ultimately, it does sound like a cap/filter issue, though...

[nordine]

Does it run off a wall wart?  Have you measured the wall wart's output?  Any way to replace the wart with a battery and see if it still does it? Just eliminating a bad/over-voltage PS....

I hate instantaneous, transient BS like this!  Tough to find it.  Ultimately, it does sound like a cap/filter issue, though...

yeah, running it from a regulated/filtered source... 8.9V, feeding 8.4V to the audiopath, after all the filtering

will try the battery way, cheers

[nordine]

just tried it with a fresh battery

the same behavior, just as described 

to me its like it takes a while to "charge" the voltage bias of everything, but why?

[Quackzed]

i'd guess a big cap / large value resistance to ground, maybee the 'rate for speed up' stage bit could do this? i'd look for the big caps that have high value resistors to ground near 'em.

[nordine]

i'd guess a big cap / large value resistance to ground, maybee the 'rate for speed up' stage bit could do this? i'd look for the big caps that have high value resistors to ground near 'em.

i too think theres a big cap involved ..in the speed up stage there are a pair of 10uf caps...but could they make sound everything shorted at first, the loud hum? i still cant explain myself how can it go gradually from sounding "9V to ground" shorted to normal sounding ...more than a local cap, i think its something in the first filtering stages... but then again how does that explain the stop of oscillation in the first 20% of rate travelling?

[nordine]

bump

[Quackzed]

look for puffy or oxidised caps, or just replace all the big ones...
could be that the cap needs to charge up in order to bias some stage or active component, and because its dying it's taking a lot longer to charge up and stabilize... so uncharged, it seems shorted - no sound-... the oscillation 'could' be a different issue probably not , but skin the biggest cat first...