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Username: mica

Post Number: 2249
Registered: 6-2000
Posted on Friday, February 04, 2005 - 10:26 am:   Edit Post

The F-2B has been in continuous production and largely unchanged since 1969. Aside from the superficial changes in the chassis and decoration, only two electronics changes have been made.

1. The first couple hundred units used a pilot light with a jewelled cover for a power indicator. After that, we've used an LED.

2. Filter capacitors. The high voltage power supply of the F-2B uses a voltage doubler followed by two stages of decoupling. Originally, we used Sprague "Atom" capacitors of 20uf. Later units used the premium type "Compulytic" type (39D) in 35 or 40uf. When Sprague discontinued these, we changed to Cornell-Dubilier WBR series. The capacitors are axial-leaded aluminum electrolytic type and the negative leads are spliced longer so they can return to the printed circuit board for connection. The ends of the capacitors away from the circuit board are supported by foam tape attached underneath to the chassis, and above to the lid of the enclosure. If the F-2B is regularly used, the capacitors will have a very long life, more than 25 years, so virtually all units that are in use will remain in good functional condition. However, if the units are stored and
left powered off for perhaps 5 or 10 years, the capacitor life will
be shortened, and they may need replacing. The foam tape also will deteriorate after 20 years or so, and will need to be replaced so that the connections to the circuit board will not be stressed by vibration during transportation. At some point in 2005 we
plan to change the capacitors from the axial-leaded type to a so-called "snap-in" style where both leads exit the same face of the capacitor and solder directly into the printed circuit board. This will eliminate the need for the foam tape.

Lots of folks ask about changes in the knobs. The earliest units had black plastic knobs with two set-screws spaced about 120 degrees apart, tapped into a metal insert that was not visible when the knob was installed on the unit. They had a smooth red cap that was super-glued onto the main body of the knob. They were manufactured by Knobex in a factory in Burlingame, California. This company eventually stopped producing the parts, so for a few hundred units, the knobs were all-aluminum with two set-screws and could be found in both black and natural aluminum colors (both colors were anodized). The supplier was Alco. Since about 1990, the knobs are a collet style plastic knob with a skirt glued onto the bottom to conceal the nut mounting the pot bushing. The collet knobs have no set-screws visible, and you pry off the cap with your fingernail or a knife to reveal the collet screw, which is loostened with an 6mm nut driver or alternately with a large flat-blade screwdriver turning the slots milled into the collet nut.

Other than that, things are pretty much the same inside. We're still even using the original printed circuit board artwork.

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