ST70 chassis hum

My ST70 has just recently developed a mechanical chassis hum (not from the speakers, but from the chassis itself).  I have tightened all transformer screws in addition to other socket screws, etc. to no avail.  I have swapped tubes with no improvement.  The hum appears to be 60 cycle in nature.

Interestingly, when I pick the amp up and hold it a few inches off the cabinet shelf the hum disappears.

The hum is very noticeable when the unit is muted, but masked when operating the system.  

Is there a solution?   Isolation pads, etc.?

Thanks for any suggestions.

Bruce

It'll be mechanical hum from the power transformer being transmitted through the chassis.
See Bob Latino's answer to this question in this link:

https://dynacotubeaudio.forumotion.com/t3524-mechanical-hum-power-transformer

mijohn wrote:It'll be mechanical hum from the power transformer being transmitted through the chassis.
See Bob Latino's answer to this question in this link:

https://dynacotubeaudio.forumotion.com/t3524-mechanical-hum-power-transformer

The power transformer, which I purchased from Dynakitparts, has the isolation washers so that is not the source of the problem. Though I thank you for the suggestion.

Bruce

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"Interestingly, when I pick the amp up and hold it a few inches off the cabinet shelf the hum disappears."

Breading, you could try isolating your amp from the cabinet with the sort of thing in this link, others may have recommendations of similar products.

https://www.audioadvisor.com/products.asp?dept=66

Breading,
I'm going to take a SWAG here, that your rubber mounting washers have shrunk a bit.  And when you're picking the amp up you're flexing it -- not much flex to one of Bob L's 10-gauge chassis, but it will flex, no matter minutely.
One thing I would suggest is just retorquing the transformer's hold-down hardware. Tighter isn't always better; sometimes backing off turns the trick, let those isolation washers puff out a bit. I would loosen all four bolts and (assuming you're working from the top and not the bottom of the top deck), turn the bugger on and torque the bolts down in a pattern until the hum quits and when it does quit, you're noise-free. Don't tighten them any further; that big tranny ain't going anywhere.

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What if you placed each leg of the amp on a small rag to stop the vibration from being amplified on the table and then with a mechanic's stethoscope (carefully!!!) poke around to locate the noise. But yeah, you don't want 500V going through your head so be careful.