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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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In Reply to: Fisher - Hot Chassis? posted by Burke on March 1, 2006 at 01:45:56:
The classic Fisher tube receivers all have two bladed 120VAC connectors. To reduce hum, there is a high voltage capacitor to the chassis from one of the 120VAC lines. The owner can choose which way to plug the unit into the wall; whichever produces the least amount of hum. Unfortunately, sometimes these caps leak...just a little.But on the hot side, most classic Fisher tube receivers used a voltage doubler HV power supply. That's a standard solid state rectifier and capacitor trick that enables a manufacturer to use a power transformer with a lower voltage, high current winding, but unfortunately is also a scheme which invites one capacitor to beat another matching capacitor to hell. And when one of them starts to show evidence of the battle, it starts to leak which can result in the power transformer putting out a lot of extra current, hence the heat.
In addition, both the 500C and the 400 have fixed bias. Because the owner cannot adjust the bias, and bias can drift over time, sometimes the receivers can run hot.As most folks who buy vintage receivers usually have to rebuild the power supplies on these 45 year old components, this is all normal and not particularly a big deal. It's nice of the seller to mention it, though.
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Follow Ups
- Re: Fisher - Hot Chassis? - corerosin 05:21:34 03/01/06 (0)
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