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
(https://i.postimg.cc/nCXYM8Ww/6551380.jpg) (https://postimg.cc/nCXYM8Ww)
(https://i.postimg.cc/HVF0yZDW/6551382.jpg) (https://postimg.cc/HVF0yZDW)
(https://i.postimg.cc/Sn8676k6/6551383.jpg) (https://postimg.cc/Sn8676k6)
* 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.
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,
*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.
(https://i.postimg.cc/Sn8676k6/6551383.jpg)
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.
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,