Roland JC 120 speaker dust cover VOLCANO Type / EMI RFI

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Hello everyone!


I started restoring a 1978 JC120, steel corners, original MUSICAL INSTRUMENT white label speakers, 6 connections at the back.


When I took it apart, one of the speakers had a damaged silver dust cap (in my opinion), while on the other speaker, the dust cap was nicely curved.


I sent both speakers to Maison du Haut Parleur in Paris for repair, and according to them, this dome is normal (photo attached), the dust cover has been cut/stamped/re-glued...
For them, this corresponds to a technique for modifying the high-frequency response. Many full-range speakers have bi-cones that look a bit like this.


When I look through tons of photos of JC120s (both vintage and recent), none of them have this difference between the two speakers.


At the same time, after recapping, I realize that the effects are only on one of the speaker outputs, the other speaker output is DRY, with no effect...


-Have you ever seen this type of "VOLCANO" dust cosver on JC120s ?

-Is this difference in effects between the two speakers a classic feature of the JC 120? (To create a kind of stereo effect?)

-with a phase meter, both speaker IN PHASE or OUT OF PHASE ?

-Have any of you ever tried placing an EMI/RFI filter on the power supply? The JC120 is clearly sensitive to radio frequencies.


Thanks in advance for your answers!

François

[J M Fahey]

* never ever saw that kind of dustcap before, on *any* speaker.

It would work similar to a Hi Fi speaker whizzer cone, not much use here.
If anything, it might be justified in an old school PA speaker, or an organ one.

* I have seen some Roland amps where effects were used in one of the Stereo channels , leaving the other dry.

* nominally same "name and model" but JC120 has changed a lot along the years.

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thanks a lo for your help JM,i will replace them with classic  domed one .

have to dig into the big DEFINITIVE ROLAND JC 120 for the dry/effect hp thing .

thanks again,

[J M Fahey]

*If you dare* you can replace that "dished in" aluminum dustcap by a new one.

Preferably Aluminum just to make it like the other speaker and for looks (does it show through front cloth?) but if unavailable, you can fit a standard cloth type one, sound won´t change much.



Aas I see it, reconer put dustcap in place "dry" (no adhesive yet) and then added a thin bead of adhesive around. "on the outside only" to keep it there.

Adhesive looks like standard contact cement.

If so, it can be softened/re-dissolved so you can remove dustcap with no damage.

Proper solvent is Toluene/Xylene/Benzene, sometimes sold as "contact cement solvent/diluent/thinner".

You put some in, say, a plastic Coca Cola bottle cap or similar and wet the glue joint around the dustcap repeatedly, using a q-tip.
After 10-15 minutes it will soften and you can pull it safely.

Warning: wait whatever it takes to FULLY soften adhesive.

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thank you so much for your help, a professional speaker tech will replace it with a similar one in aluminum, we can see them through the clothes and i want to have a similar sound behavior between the two speakers

all the  best,