HIgh 5V rail on LM2575 after recap and voltage regulator change

  • Hi,

    I recently acquired a Virus B for parts that wasn't turning on and the screen was flickering that I ended up bringing back to life after recapping it and changing the old voltage regulators for new ones. I'm currently having a weird issue with that where it's outputting 5.1V on the lm2575 chip ONLY when the screen is plugged in otherwise it seems to be outputting correctly. It seems I have a short? Or dead chip somewhere but without a real schematic (I have part of one but its incomplete) it's hard to figure out. When the unit is on the controls randomly change to the effects section whenever I touch any of the controls and a few random LEDs are staying lit up which I'm guessing is caused by the high 5.1V rail feeding to the DSP. On the front panel board I also changed a few chips to try and fix the issue I was having with the controls randomly changing when I touch stuff since I read about that somewhere but I'd need to find it again. Anyway, if any more info is needed or if anyone has any idea where to look I'd definitely appreciate it!

  • I wouldn't be concerned about the extra 100mV. I would however advise against changing ICs for no good reason because depending on where you source your replacements from, you may end up replacing a working good quality IC with a Chinese fake without realizing it.

    With the current information, the issue with the controls sounds like an issue with the external power supply introducing an unwanted current path through ground. Have you tried a different external power supply, ideally one with an earthed plug?

    Also, does it make a difference whether you run the Virus plugged in only to the power supply vs. also having it connected to the rest of your audio chain (audio/midi)? The grounding situation may be different in both cases.

  • Yeah I know all about fake IC's. I've only sourced my parts through mouser and digikey for this so far though, I also should of added this has been going on for serval months. I've tried 2 separate power supplies including one that I have that works fine with my TI. I genuinely don't believe it's the power supply though, the power supplies I have have a ground plug. Haven't really checked it with the rest of my audio chain but I can definitely give that a look over. I can give a more detailed rundown of the IC's I've changed too. There's some high pitch noise that happens when the unit is on that I also should of added.

  • Do you have a scope you can use to observe the 5V rail? Maybe there's excessive noise on the rail that causes these issues.

    Does the high pitch noise come from one of the coils or is it emitted through the audio out? Either way it suggests unstable regulation, so I wonder whether the cap used for the LM2575 has sufficiently low ESR. Can you show me which cap you ordered/used? Also, do any of the caps become warm?

  • Yeah I have a few scopes, and I can definitely try that. It might be from one of the coils? Not from any of the audio out's no. Yeah I read up on that and purchased low ESR caps anywhere I could, https://www.mouser.com/ProductDetail/710-870135675003 here's what I ordered and replaced the 100UF cap with. Yeah, I've already tried replacing two of the lm2575's (second time mainly listening to my partner but I didn't think that was the problem to begin with.) But yeah, still kind of confused as to why this is happening.