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regal 28th August 2011 07:58 AM

Heater Power supply
I have the LT1085 heater supply board, 12.6V AV in- 12.6V AC out. The output voltage is fine but the heatsink for the LT1085 vibrates. It doesnt get hot and is only supplying 600 mA. With this kit the input and output caps are 470uf electrolytics, the datasheet calls for much smaller values, would this be the cause of the buzz/vibration on the LT1085?

John Caswell 29th August 2011 08:10 PM

Heater power supply
I'm sorry I don't quite understand this.
You say "12.6V AV in- 12.6V AC out"
As the LT1085 is a Low Drop out DC voltage regulator there should not be any AC near it.
The AC supply would need to be rectified first then applied to the regulator. As it is an active device there would need to be at least 14.6V for it to regulate to 12.6V.

John Caswell

regal 30th August 2011 11:55 AM

Sorry typo 12.6 AC in, 12.6 DC out regulated with the LT1085.

After rectification I have 17V's feeding the LT. This is Quanghoa's board which I can't find a schematic of.

But the entire heatsink for the LT1085 is buzzing. I do have an T0-220 isolation pad.

Regulation seems fine, but I haven't a scope at the moment.

The odd thing is the PCB has me with a 470uf cap on the output instead of the typical small tant.

The heatsink is fairly close to the filament transformer. I also have the ground lifted 25V taken with a voltage divider from the SSHV prefilter (with a 1uf cap in parallel to ground.)

I have never seen a IC voltage regulator buzz like this. It isn't hot, only supplying 300 ma's.

John Caswell 30th August 2011 06:45 PM

Buzzing heatsink
This may be a silly question but is the heatsink aluminium or steel? As it is near the tx if steel it could be magnetically excited whereas if aluminium (aluminum) then it may just be vibration from the tx setting it off. The isolation pad will not make any difference to this.

John Caswell

regal 31st August 2011 11:39 AM

Its actually an aluminum heatsink and none of the transformers are stressed or vibrating.

But I am using a steel chassis, and I am coming to the conclusion that this was a bad idea.

My separate amp chassis OPT's pick up 60hz up to -80dB when it gets closer than 2 ft to the steel powersupply chassis. My guess is the steel chassis is just ampliifing all the emi from the transformers.

Going to switch to aluminum and hopefully the issue goes away.


John Caswell 31st August 2011 11:50 AM

Vibrating heatsink
Transformers do not need to be stressed to cause hum/vibration, but certainly a steel chassis will aggravate the situation and may vibrate in sympathy. You could try temporarily isolating the power tx on rubber.
You will never eliminate the vibration from transformers due to a physical phenomenon called "magnetostriction" q.v. on wikipedia.

John Caswell

regal 31st August 2011 12:37 PM

Thanks, the thing that is odd to me is none of the transformers feel to have any vibration or buzz, its just that damn LT1084's heatsink.

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