Linear Fans?

[Shockwave]

Got some Cetron 572B's for my old Yaesu FL-2100 only to find the fans inside the amp are due for replacement. One runs at half speed no matter what I do and it's not mechanically bound up but runs hot. It was also under the tube that went flat in the last pair. So, I figured I'd just replace them with some new DC fans using the typical bypass cap and choke on the DC feed to keep RF out of the fan motor. This has worked well in the past but failed miserably here. After getting both fans mounted, they proceeded to instantly stop the moment the amp was keyed.

There is enough RF radiating right of the anode of the tubes 1 inch away, that it seems impossible to keep them running when RF is present. My next thought was replacing them with some 60mm AC fans. However, I now find that all of the AC fans available today do not use an induction motor and are actually DC motors with electronically commutated brushless motors. Even the AC ones convert to DC inside the fan today. I'm thinking this will create the same problem. The AC ones run from 110 to 230 volts so I imagine that I cannot slow them down with any resistor if they run too loud. I'm having a hard time finding something that will be quite (low speed), fit and work when the amp is operating. Has anyone run into this before and found a fan that fits inside the FL-2100 and works when RF is present?

 

[sp5it]

Why not to get 110/220V fans?

 

[Shockwave]

Why not to get 110/220V fans?

Anything I can find that fits here (60 to 70mm) in an AC version, is an "EC" type motor. Which means AC gets converted to DC inside and the motor is actually an "Electronically Commutated" DC motor. Same thing that's failing here but just at 120 volts, instead of 12. I think they did this to make them universal so they can run on anything from 110 to 230 volts, without a change in speed.

 

[nomadradio]

If you had the original fans you could disassemble, clean and lube them. The porous bronze bushings have a felt doughnut around them that serves as the oil reservoir. Wicks the oil slowly from the felt, through the bushing to the shaft. Cleaning the schmoo from the shafts and bushings and oiling the felts will either fix the problem or reveal that the junk clogging it up hides a clearance issue, and the shaft will rattle around rather than spin smoothly.

No way to tell before removing decades of stuff from the shaft and bushings.

And if this amplifier came with the new brushless fans, this is not an option

A faraday shield made from quarter-inch or 3/8-inch hardware cloth will probably cure the problem with the brushless DC motors stalling. We take the two power leads to this kind of fan and pass them into a half-inch toroid. The wires each get wrapped in opposite direction one on each side of the core. Five or six turns seems to be enough.

Not sure if the term "hardware cloth" has become obsolete yet. This is a coarse square metal screen that appears as of it's soldered where perpendicular wires meet. A standard hardware-store item for as long as I've been alive and probably before. I hear folks call it "chicken wire". But that's really a different design with larger openings. Just cover the fan opening below each tube and use flat washers large enough to span the opening when you bolt it to the deck. Shouldn't reduce airflow enough to matter.

Those fans are so close to the anodes of the tubes that RF is getting rectified by the transistors inside the fan. Shuts down the oscillator circuit driving the motor. A shield is the only trick I know to cut down a case signal rectification like this.

73

 

[Crusher]

I’ve ran into something similar to that before. I used aluminum tape and covered over the wires and the non moving center of fan. This stopped rf getting into the DC fan.

 

[nomadradio]

More than one way to skin a cat.

73

 

[bob85]

I once smoked a 6" aluminum sunon fan,
It was choked & caps close to the fan motor & had worked perfect for weeks then something happened when we were out foxhunting & it let the smoke out big time,

had to jump on the brakes & bail out of the car it was so bad, lots of acrid poisonous smelling smoke,

odd thing is the fan still works OK, whatever it was that went up in smoke has not changed how the fan performs..

 

[Shockwave]

I’ve ran into something similar to that before. I used aluminum tape and covered over the wires and the non moving center of fan. This stopped rf getting into the DC fan.

That was the perfect solution.

 

[Crusher]

Good deal. Glad it worked out.