b&w dm3 restoration...possibly

[sumday]

a quick but hairy blat down the M1 tonight (branches and junk in the road!!) and I've bagged a pair of dm3's from the bay for the princely sum of FIFTY QUID.

they seem unmolested and a quick check on my backup system tells me all is in order..all drivers working and no nasty noises.
the cabs are in amazing condition....no scrapes or knocks. they even have a nice dull shine and smell of beeswax!!

my question is what to look out for regarding a minor stripdown...the oval bass driver (hate the word WOOFER) is a unit I have no knowledge of...I'm guessing the level pots will need a squirt of cleaner and may upgrade the terminals whilst I'm inside.

also, the grille cloth, although fine is showing its age....can it be cleaned in situ as it has a layer of old foam backing...will this fall apart if I apply some erm..scrubology.(made up word).


all input greatly appreciated..

NIGE.

[walpurgis]

Nice find Nigel.

Classic, pre-plastic cone B&W speakers. Mordaunt Short made a rather similar model, the number of which escapes me at the moment. You should find these reliable as long as you don't go mad with the volume, although that EMI bass driver can go pretty loud. The tweeter is a Celestion HF1300 and the super tweeter is a Coles unit. 3001 or 4001 I believe. I'd take the grille cloth off and replace that crumbly foam. Plenty of thin foam sheet on eBay. As you suggest, a terminal upgrade won't hurt and cleaning the pots is a good idea. The crossover capacitors are likely to be a bit tired by now. If you replace them, use similar types. Paul at RFC (trade area) may be able to help with those.

[spendorman]

The bass unit in the DM3 is a bit special, looks like the usual EMI elliptical, but has a laminate cone that includes paper fibreblass and aluminium. It also has a rather large magnet. The part No. is 92390 DJ. The similar looking elliptical used in the MS 400 has just paper and aluminium, part No. 92390 GF. Hence the often confusion over these two units.

The DM3 used the Celestion HF1400, which is a larger magnet version of the HF1300, the Coles is the 4001G.

The MS400 is my view is not as good as the DM3, mainly because the cabinet is a bit smaller and not as rigid.

http://starchive.phxaudiotape.com/viewtopic.php?f=12&t=907&start=15

The most common problem with the DM3's is the bass unit surround that can shrink, affecting the bass, and eventually pull the voice coil off centre.

I have a contact that can replace the surrounds, he has done a couple of bass units for me, result is very good.

As you mentioned the switches can do with a bit of a clean, just sparing use of Servisol etc.

The binding posts are not bad quality, but it's worth cleaning and the contacts on to the circuit board. Although the board is quite thick, it's not fibreglass and is quite brittle, so do not over tighten anything on it.

The capacitors in the DM3 are all non electrolytic and I have never found one out of spec, although, of course it's possible. Inductors are of good quality, and the wiring used is pretty good too.

[sumday]

thanks...
I had the baffles off this morning, great old style engineering.
neat wiring and a simple connection to the xover via a plug, never seen this style before, allowing the baffle board and drivers to be extracted in one go...a bit like a pair of Cambridge R50s I had a while back....they were HUGE, very low WAF!!!

NIGE.

[spendorman]

Possibly better constructed than the R50, certainly better crossover components, in my view a better speaker. I class the R50 as similar to the KEF Concerto, which to my mind has poor midrange and not that nice treble.

Have you checked that the Coles supertweeters are working? Not much normally comes out of them, but something should. A good 4001G should measure 23 Ohms DC resistance. This seems a strange value, but is correct. If in doubt, the Coles site will confirm.

[sumday]

thanks again, the coles in both are I think fine.
did a check by ramping up the volume on a unloaded record deck input as this gives a nice white noise to check the highs.
the 14k switch on the rear allows cutting in or out of the coles and deffo allows one to hear it working or not.
I seem to have been blessed here as I've had a couple of speakers over the years with this model and as you hint, the poor delicate things are always the first to give out.

had them playing all afternoon and I think i'll leave them alone for now...assuming my ears adjust and I tune out the midrange honk from the bass driver.

by the way...having asked for some creative xmas prezzies in the past, I cannot wait to see the wifes face when I ask for some new bass suspensions!!!

NIGE.

[spendorman]

thanks again, the coles in both are I think fine.
did a check by ramping up the volume on a unloaded record deck input as this gives a nice white noise to check the highs.
the 14k switch on the rear allows cutting in or out of the coles and deffo allows one to hear it working or not.
I seem to have been blessed here as I've had a couple of speakers over the years with this model and as you hint, the poor delicate things are always the first to give out.

had them playing all afternoon and I think i'll leave them alone for now...assuming my ears adjust and I tune out the midrange honk from the bass driver.

by the way...having asked for some creative xmas prezzies in the past, I cannot wait to see the wifes face when I ask for some new bass suspensions!!!

NIGE.

I quite like the sound of the DM3, there is as you say "midrange honk", but it's not un-musical, better than the lack of midrange with some speakers.

The same person that does the surrounds has a mod that reduces the mid of the DM3. It is a little involved, but really does improve them. I guess that he would supply details if required. This guy's Dad was Senior Engiineer at Vitavox.

[walpurgis]

The EMI driver to tweeter crossover quite high. Somewhere around the 3 kHz region I believe, so the large bass driver cone will throw some colourations into the mid.