How to Rebuild Dynaco A25 Speakers

Written by jan benschop
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How to Rebuild Dynaco A25 Speakers
A rubber-surround woofer was used in the Dynaco A-25 (A Grey Loud Speaker. image by daseaford from Fotolia.com)

Over 1,000,000 Dynaco A25 loudspeakers were manufactured in the 1970s and are to this day highly revered for their smoothness. A25s had a 10-inch woofer with a rubber surround, a 1.5-inch tweeter and an aperiodic resistive slot loading system that kept the impedance at 8 Ohms at system resonance. The SEAS tweeter has been succeeded by a better design and the SEAS woofer can be re-coned to original specs. The phenolic terminal board on the woofer should be replaced.

Skill level:
Moderate

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Things you need

  • Phillips screwdriver
  • Soldering iron
  • Solder
  • Liquid solder flux
  • Re-coned woofers with new terminal boards
  • 2- SEAS Prestige 27TFFC (H0881) Textile Dome tweeters
  • 2- Solen 5.1uF 400V Polypropylene Capacitors
  • Automotive felt underlayment
  • Large scissors
  • 18-thread count grill cloth
  • Brush-on contact cement
  • 200-grit sandpaper
  • Danish rubbing oil
  • Heavy-duty hand stapler with short staples
  • Hot glue and gun

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Instructions

  1. 1

    Pry the grille frames out of the speakers, exposing the drivers. Unscrew the nameplate from the front of the grille, and remove the cloth from the frame. Using the old cloth as a sizing template, cut new cloth and attach it by wrapping it around the frame edges and stapling it in place.

  2. 2

    Unscrew the woofers and tweeters and unsolder their connections. Send the woofers to a reputable reconer. Ask for new woofer terminal boards, original equipment cones, voice coils, dust caps and butyl rubber surrounds.

  3. 3

    Cut the leads of the capacitors protecting the tweeters and pull them out. Hot glue new 5.1 uF capacitors in place and solder the leads back in place.

  4. 4

    Solder the tweeter leads onto the new tweeters, observing polarity. Screw the tweeters back in place. Cut a rectangle of automotive felt to fit inside the tweeter part of the grille frame. Cut a 3-inch round hole in the felt where the tweeter will fire through. Apply contact cement to the felt rectangle and the area on the front baffle where it will fit around the tweeter and inside the grille frame. Dry to tack and apply.

  5. 5

    Resolder the woofer leads, observing polarity. Screw the woofers back in place.

  6. 6

    Lightly sand scratches and apply heavy Danish rubbing oil to restore the finish.

Tips and warnings

  • Phenolic terminal boards on the old SEAS woofers got brittle with age. Have them replaced when reconing the woofers.
  • Although replacing the woofers seems like a good idea, it can seriously degrade the system sound: the boxes were designed around the original woofers.
  • The felt around the tweeter will enhance focus and imaging. After applying the cement on the felt around the tweeter, lay the front side inside the grille and temporarily lay the grille in place to position it. Remove grill, press down.
  • Do not have foam surrounds installed on your woofer rebuilds: butyl rubber sounds better and lasts longer.
  • Under no circumstances remove the resistive plugs under the woofers: that will ruin your speakers.
  • Make sure the reconer uses 8-Ohm voice coils.

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