[sdiy] power supply (?) wierdness

[anthony]

One of my reverb things has a weird problem that I've been wracking my brain 
trying to figure out what's going on.
The problem is this. When powered on it makes a slow fwump fwump fwump. So I 
thought there's a bad capacitor somewhere (because that's what always seems 
to happen with my EV amp - I should just change all of the electros in that 
thing and I won't ever have to open it up again.). I replaced just about all 
of the bigger electrolytic capacitors and even some of the smaller ones. But 
no. So I put the output on a scope. It's about a 4Hz ramp that goes both 
positive and negative - almost rail to rail. There's no DC offset. Just for 
the heck of it I put the scope between ground and the chassis and it's on 
that too. Just to test something I put a 470uF capacitor between ground and 
the chassis and now it's not there or on the output. And there's a normal 
looking signal at the outputs. BUT if you hook ANYTHING up to the outputs, 
the signal goes away on the scope and the fwump fwump is there again. I knew 
that capacitor wouldn't actually fix what was happening. But that's kind of 
weird. I'm guessing it's something bad in the power supply. I changed the 
regulator to no avail. The supply is kind of interesting. It's a +/-7.5 
regulated with a 15V regulator so it has a virtual ground. There's also an 
anit-pop delay from a comparator in the power supply to a 4016. The noise 
isn't everywhere in the audio path either. My instinct says, duh there's a 
short somewhere or there's something that I overlooked in the power supply. 
I didn't change the bridge rectifier but I don't think that's it. The BBD's 
are functioning fine. Nice solid clock going to them too. Actually I just 
looked and I can see the square wave throb ever so slightly with the fwump 
fwump. I also replaced every single op-amp - with the same exact kind. I 
think there might be a short to the chassis in back but it looks like I'll 
have to disassemble the whole thing to find it. Still the fact that it goes 
rail to rail makes me doubt this. I guess it depends on what exactly 
shorted. Also weird that it goes to both rails.