[sdiy] Octave Cat modulation weirdness

[Gordonjcp]

On Wed, Sep 30, 2015 at 01:58:27PM -0400, Travis Thatcher wrote:
> I recently restored an Octave Cat SRM and it was working pretty well.
> I replaced nearly all the CMOS and recapped somethings. The only thing
> I couldn't perfect was the scaling when you switched from mono to poly
> mode but that didn't bother me - just meant I had to adjust the tuning
> of osc1 by a step.
> 
> The weather here in central VA as well the rest of the Eastern US has
> been a bit humid to say the least. Yesterday I fired it up only to
> find that osc 2 is constantly getting an amount of modulation from the
> LFO, even with the modulation pot turned completely off. I checked and
> leg 1 of the pot still has continuity with ground. I'm guessing a cap
> some where finally bit the dust, maybe egged on a bit by the humidity
> in the house. Just curious if anyone has seen anything like this. I'm
> going to open it up tonight to investigate.

Outside of cheap crappy switched-mode power supplies, electrolytic capacitors mostly don't fail.  Disc ceramics do, and tantalum bead capacitors fail but you'd know about it from the bang and terrible smell.

In nearly 30 years, I've replaced maybe about five or six electrolytics that weren't in a switched-mode PSU.

What I have done is recover the aftermath of people "re-capping" things, to the point that when someone brings me a piece of gear that they say has been "re-capped" I just tell them to take it somewhere else, like the municipal tip.

Go over it carefully and check your soldering, really carefully.  Clean off every bit of flux.

Once you've done that, dig out the circuit diagram and try and work out where the signals are going.  Don't just blindly rip out and replace components and hope that this will fix it - more damage is done by the "HERP DERP NO WORKY MUST REPLACE CAPACITATOROSZZZ" brigade.

-- 
Gordonjcp MM0YEQ