Hum....whats normal in a st70

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Hum....whats normal in a st70

Postby mrED » Mon Sep 14, 2015 3:18 pm

I just picked up an ST70 with an after market driver board. Look like tubes4hifi after a little internet surf.

It hums a bit more than I can handle.

Any tips on logical trouble shooting steps? I am not a super technician by any means, but can muddle thru with some coaching.

Thanks!
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Re: Hum....whats normal in a st70

Postby Geek » Mon Sep 14, 2015 6:48 pm

A few things come to mind:
- Bad filter caps.
- Output tubes woefully un matched.
- Bad dressing of the signal input leads.
- Oscillation.

Cheers!
-= Gregg =-
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Re: Hum....whats normal in a st70

Postby 20to20 » Tue Sep 15, 2015 6:51 am

mrED wrote:I just picked up an ST70 with an after market driver board. Look like tubes4hifi after a little internet surf.

It hums a bit more than I can handle.

Any tips on logical trouble shooting steps? I am not a super technician by any means, but can muddle thru with some coaching.

Thanks!


Is it humming without anything connected to the inputs? Many times the hum can be coming in on the input cabes from the source or even the jacks because of light corrosion or poor internal connection. First thing to do is rule that out by shorting the input jacks with a paperclip on the end of the cables or the jack itself.
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Re: Hum....whats normal in a st70

Postby mrED » Tue Sep 15, 2015 7:51 am

Thanks for all the tips.
I will be lucky to get to test some of these by the weekend.
Work gets in the way of fun all the time.....
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Re: Hum....whats normal in a st70

Postby 20to20 » Tue Sep 15, 2015 8:07 am

mrED wrote:Thanks for all the tips.
I will be lucky to get to test some of these by the weekend.
Work gets in the way of fun all the time.....


If the amp gets quiet with shorted input jacks then you can rule out the power supply causing it and can then concentrate on the front end picking it up from external source getting in somehow. Another possibility is PS transformer physical vibration making its way into the driver tubes and being sensed and amplified (microphonics).
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Re: Hum....whats normal in a st70

Postby skidave » Thu Sep 17, 2015 6:44 am

Not sure if the unit still has the multi-section filter cap (big tall silver can behind the driver board and between the power transformer and the right output transformer). If so, make sure the cap has a solid attachment to the chassis. If the can is mounted, but none of the mounting tabs are soldered...solder one for a solid connection.
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Re: Hum....whats normal in a st70

Postby jgf » Fri Sep 18, 2015 8:49 am

With the floating (ac grounded by caps) heater supply you'll get 60hz hum on the input tube sometimes, if that sounds like what you're hearing, try different input tubes.
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