Power Supply Filter Caps

 


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Last Edited:
12-Sep-2004

 
Troubleshooting Electrolytic Filter Caps

Filter caps are usually in the form of a cylindrical "can",  (metal or paper-covered) and usually house two (or more) "multi-section" capacitors that utilize a common (positive or negative) ground. Look for the pointing arrow or + or - signs indicating polarity.

When caps are in good working order, they're likely to look good, as well (Caps have a dirty tendency to spew their guts). Look around the bottoms for tell-tale signs of degradation: leaking slime, brown pimples or dirty-looking dots.

'Lytic caps need exercise to feel good and be well "formed." (see How to: Reform, below). When an amp has been stored for a number of months or years, the critical elements inside a cap can dry out and break down.

Anytime your vintage H.H. Scott unit exhibits a higher then normal amount of low-frequency AC hum, suspect the filter caps. Typically the the most vulnerable filter cap is the one that is connected to the power supply rectifier tube, or the first filter stage. What you are hearing is the AC ripple (hum) leaking into the audio circuits. Not good.

Also, any filter capacitor physically located next a heat source will tend to have a shorter life. A capacitance checker is not always a reliable way to find out if a capacitor is leaking. Often filter caps will bench-check as OK, but will fail in-circuit, as real-world voltages and heat make their working environment tougher. Large-value filter caps will initially look like a short when bench-checked on an ohmmeter; a good cap will then build up a charge and look like an open circuit (high resistance) to DC.

Finding exact-value, new-replacement, filter caps can pose restoration challenges. You may find can-type replacements that will not physically mount in the same location and permit the cabinet to be reinstalled. A possible solution is to mount new tubular (axial) caps below the chassis. Preserve the original wire routing (and farad and voltage ratings and polarity) to avoid introducing ground loops or additional hum.

The original can caps are almost always mounted on wafers insulating the metal cans from the chassis. The original H.H. Scott designers were careful about grounding, and generally a well-thought out ground buss runs throughout the chassis. Each filter cap's can will generally be connected to this common ground bus.

How to: Reform or condition Electrolytic Filter Caps

The electrolytic material inside any cap (new or old) that has not be in use (for months or years) needs to be "formed" slowly. Failure to do so could damage the cap, it might even blow up in your face!

To re-form the filter caps in your vintage H.H. Scott gear, use a special tool called Variac. A Variac is an auto-transformer that lets you adjust or choose the AC supply from 0 to 130 VAC.

The idea in reforming the filter caps is to SLOWLY AND SMOOTHLY bring the unit up to the normal operating voltage of 117 VAC. This process should take between 20 to 40 minutes and should be done without large jumps in voltage. This is very similar to the procedure described under 1st SOFT START.

 

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