Most of you that know me personally realize I’m an audiophile nut job. During the colder months I tend to stay inside and do less photography to concentrate on other hobbies. I’m also a sucker for a good deal so when I saw these DCM Timeframe 350 speakers advertised in the local classifieds for a giveaway price I had to snap them up. I knew they would need a little tender loving care due to their age. With vintage loudspeakers most problems arise from either a blown tweeter, leaking capacitor in the crossover network and the most common issue deteriorated foam surrounds on the woofers. The DCM Timeframe 350 is what they classify as a transmission line speaker. It’s a lot different than a sealed or ported box design. Transmission line speakers are typically larger boxes but they can be best described as acoustical labyrinths where the rear wave of the woofer travels through a maze like enclosure and ports through a slot either in the rear bottom or front of the enclosure. Most older DCM speakers are a little more difficult to work on because instead of featuring a removable grille they use a removable back, top and bottom panels. The grille cloth can then be removed or folded down like a tube sock.
Day 1. After disassembling the speaker the foam surrounds on the woofer were rotted away so my first step was to order new speaker foams and adhesive. While I was waiting to take delivery of the new foams I carefully inspected the crossover network in each speaker for any cold solder joints and leaking capacitors. Luckily everything looked good so it was just a matter of waiting for the new foams.
Day 2. The new foams have been delivered so I carefully removed the old foams and any left over residue of old adhesive from the speaker cones and frames. When replacing speaker foam you have to take great care to make sure the voice coil is perfectly centered or the end result will sound like a scraping or tapping while listening to music. There are two schools of thought on how to center voice coils when re-foaming. The first is to remove the center cap on the woofer and use shims. The second way is to use a signal generator set to 50hz (cycles) fed into an amplifier set to a very low volume which is connected to the woofer you are working on. The signal generator doesn’t have to be an expensive piece of equipment. You can download free signal generator apps for iPhone, iPad or android phones and connect it to an amplifier with a cheap mini headphone to twin RCA connector cable.
The first step is to glue the inner portion of the foam surround to the woofer cone. You could center it visually. Let the glue dry for one to two hours before proceeding to the next step. The next step is to use either your fingers or in my case two 3/4 inch short dowels to prop up the woofer cone from underneath so that you can apply glue to the frame of the woofer. The next step will require a little patience. Remove the dowels and set your signal generator to 50hz and turn on your amplifier. The woofer cone will start to vibrate. Make sure you don’t have your amplifier volume set too loud. Now move the woofer cone around until you hear a clean tone (that means no scraping of the voice coil against the voice coil magnet). You will know when the voice coil is scraping against the magnet because it will sound like a tapping noise. Once you get the voice coil centered tack down about four sections around the perimeter of the foam surround. When you are sure that the speaker cone is vibrating without scraping on the voice coil you can turn off your amplifier and tone generator then finish securing the perimeter of the foam surround. Thats it now proceed to the next woofer. Be sure to let the glue totally cure before reinstalling the woofers back into the speaker enclosure.
Day 3. Now that the glue has fully cured I was able to reinstall the woofers and pull up my socks (err the speaker sock that is). I was able to reassemble the speakers in about two hours taking my time to insure no wrinkles in the grille cloth. Now all that was left to do was to hook the speakers up to my system and give them a listen. I really wasn’t expecting much in the way of bass response being the DCM timeframe 350 speakers are a 2 way design featuring a 3/4 inch dome tweeter and a 6 1/2 inch woofer but I was pleasantly surprised when the music stated playing. I think DCM was really onto something when designing the transmission line enclosure because the bass was clean and punchy. The midrange and highs are what these speakers are known for but I’m quite impressed with the bass response.
So there you have it. A couple of days in the life of Joe’s head and workshop. I hope some of you fellow audiophiles whose woofer foams bit the dust will think about undertaking a similar project.