DAC90A speaker (later model)

[Helder Crespo]

Hi
This is the speaker from my 1957-ish DAC90A.
Earlier models don't usually have a screw on the back of the magnet, but this one does. Does anyone know what this is for?
Thanks
Helder

[Lloyd 1985]

I would think it will be securing the pole piece, maybe other ones are glued instead? I must say I’d not noticed! I just looked at the one that I’d borrowed for my Ekco A22, and that has a hole where the screw should be! It works fine and doesn’t rattle or run, so I don’t think it ever had a screw fitted.

Regards
Lloyd

[Helder Crespo]

Originally Posted by Lloyd 1985

I would think it will be securing the pole piece, maybe other ones are glued instead? I must say I’d not noticed! I just looked at the one that I’d borrowed for my Ekco A22, and that has a hole where the screw should be! It works fine and doesn’t rattle or run, so I don’t think it ever had a screw fitted.

Regards
Lloyd

Thanks Lloyd. You are right. I checked this by removing the dust cap. The corresponding nut is right in the centre of the pole piece. Having glued the spider some time ago, I'm still trying to eliminate some rattle that only occurs for low frequencies (large excursions), which can either be due to a slightly off-centre voice coil, or perhaps intrinsic to the speaker (resonance; distortion).
Helder

[Sideband]

There might also be rust particles between the voice coil and pole-piece. You might be able to shift them with an airline (carefully) or worst case, remove the centre bolt and pole-piece, clean any debris away and use paper shims to reassemble.

[Helder Crespo]

Originally Posted by Sideband

There might also be rust particles between the voice coil and pole-piece. You might be able to shift them with an airline (carefully) or worst case, remove the centre bolt and pole-piece, clean any debris away and use paper shims to reassemble.

Thanks Sideband. The area between the voice coil and the pole piece is actually very clean. I realised the problem was a partially detached spider. My previous glueing didn't held and I could easily detach the whole thing. I have applied new layers of contact adhesive, inserted 3 paper shims, and the glue is now drying with the spider held by a 1.5V D battery, which gives around 500mA. The speaker was originally in a poor condition and the cone has required some repair too (small tears and holes) but I really hope this will work as I would like to keep the radio as original as possible. The radio has some mods on it (kalee20's mods around the output valve and Colin Boggis' bluetooth w/ a microswitch activated by the tuner pointer), but they're all done on separate boards attached to the chassis via original holes and screws, and hence are perfectly reversible.
Helder

[Helder Crespo]

Here are a few photos. A very important improvement, especially because the cream urea-formaldehyde cabinet is particularly prone to cracking right above the dropper, was the addition of an aluminium dissipation plate - inspired by a photo on Yannis Karalis' website. The radio now runs only slightly warm. Bluetooth operation is indicated by a white LED mounted on the switching relay+BT board (by Colin Boggis), which gives out a soft yellow through the translucent cabinet.
Helder

[Helder Crespo]

Glueing the spider seems to have fixed the issue. I still have some distortion/light vibration but only for very low frequencies and high volume, which I guess is normal as the speaker is probably being overdriven.

[dj_Keet]

Very neat restoration Helder Crespo

[Helder Crespo]

Originally Posted by dj_Keet

Very neat restoration Helder Crespo

Thanks bj Keet. This is actually my very first vintage radio and restoration. I got it also to learn and practice prior to working on my first Bush TV22. For some time it was the ugly duckling in my small collection (2 DAC90As and 2 TV22s), but it's looking and working quite well now.