H. H. Scott 420A integrated amp weird buzzing issue

[harrisxr650]

Hi everyone.

Im having a weird buzz issue with this, other than that, great vintage amp.

I'm getting a buzz from phono input, its not the turntable, i tested it on another set up,
Here is the weird thing..
When i dont connect anything to aux, tuner & phono inputs turning the input selector i get a sligt buzz from aux and tuner input and a hiss from phono, all getting slightly louder as i turn the volume up. Also i can hear it on headphones too.
When i connect my TT to phono input all other inputs are 100% buzz free except from phono but no hiss this time just the buzz which gets louder as i turn up the volume and headphones output are buzz free except when i turn selector to phono which results in buzzing.

Ground loop on the phono stage?

I'm attaching the schematic if anyone willing to have a look and point me where to look.

Thanks

[Enzo]

On phono, we must not only have the grounded shielded cables, but there must also be a separate ground wire from the TT chassis to the amp chassis. You have that?

[harrisxr650]

Hi Enzo,
Thanks for looking into this.

TT is a stanton St-150 which is internally grounded.
It has a switch for ground lifting which only makes things worst.

[Enzo]

How can it internally ground without a ground wire? I may be missing something, but that would seem to mean it can short its chassis to its cable ground or not. For what it is worth, my experience is I never had a stereo preamp or receiver that didn't have the separate chassis ground screw on the rear for this purpose, and I never had a TT that lacked a ground wire. Having been in the pro audio game for decades, I have services countless Technics SL1200 for DJs, and they have the separate wire.


Touching a length of wire from one to the other would answer the question.

[harrisxr650]

The amp does have a ground screw for TT on the chassis. I did use a cable to touch TT and amp chassis but the buzz it's still there. I also hooked up my old technics sl bd20 TT which have a ground wire with no luck. Both TT works flawlessly on my other integrated amp. The stanton it's the one in the pic attached

[nhbassguitar]

40 year old amp => possibility of corroded or loosened ground points. Unscrew and re-tighten every ground bolt/washer/nut you can find. Even better, remove them and clean up the chassis mating surfaces with something like Caig DeOxit D series in the tiny bottle with the brush in the cap. That stuff's for metal that isn't gold, isn't plated, and doesn't use copper as a base metal. It's really strong. If you have doubts about the metals you're going to apply it to, check out https://caig.com/wp-content/uploads/...h-info-new.pdf for guidance on which product to use. Also check any internal cables that contain ground wires and are detachable. Un-mate them and clean those as well. I've seen Molex connectors whose pins/sockets were almost white with corrosion. Verify the power cord is to spec all the way back into the amp. If the power plug is non-polarized, try plugging it in the opposite way. If it *is* polarized, verify the socket is wired to spec, including no funny stuff like ground swapped with neutral, or tied together... all that stuff.