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.
