April 2, 2024
Someone took me up on my offer and I've just rebuilt a ML Theater panel. I had
never built or rebuilt a curved panel so this was a learning experience for me.
Step 1 – Sound check:
I connected the panel to one of my power supplies and found that it played at
usable volume, and I found no obvious faults other than a small dead zone at
one corner.
The owner had recently acquired this speaker used and noted that it didn’t play as
loud as the conventional main speakers it was paired with. That, and the dead
zone in one corner were suggested the panel had lost some of its original volume.
I measured the panel output vs input using my DBX Driverack’s RTA & mic, and a
1kHz sine wave tone. The diaphragm’s drum-head resonance was determined by
tapping on the panel and noting the response on the RTA, which was a peak at
200Hz.
Driven by my transformers and bias supply, the volume seemed pretty good to me,
but since I only had the panel, I couldn't determine its balance to the woofers in the
speaker, and it did have the dead zone which I later traced to a non-conducting
area on the diaphragm.
Given my inexperience with ML panels and the high potential cost of the materials, I
consulted and chose to proceed.
Below: Panel sound check (before and after rebuild)
Below: Before removing the original diaphragm, a still-intact area was measured for
deflection / tension using this shop-made beam & cantilever deflection gauge, and
its tension was marked on the gauge for reference (red toothpick pointer and mark).
The new diaphragm was tensioned to the same deflection (reference mark).
Step 2 - Disassembly:
Plan A was to pry the stators apart by inserting a butter knife between them at a
corner, and working around the panel. * I had hoped to save the diaphragm but
could not.
Plan A worked for much of the panel but the remainder was stuck so well that I
risked bending a stator, so I moved on to Plan B and cut through the remaining
bond line using a string saw (garrote) made from 50lb braided fishing line.
* If the stators separate cleanly, leaving the diaphragm intact, merely cleaning
and re-coating the diaphragm and replacing the charge ring will fully restore a
dead panel.
Step 3 – Post Tear-Down Inspection:
- A still-intact area of diaphragm was measured for tension using a simple DIY
deflection gauge.
- I probed the diaphragm with my "poor man's megger", which measured the
conductivity as 0-3.5 volts, typically 1.5-3.0 volts (vs Licron Crystal @ 0.9 volts).
One downside of higher conductance is that is shunts more engergy into
any arcing events, allowing the arc to burn hotter and cause more damage.
Note: The 0-volts/ zero-conductance area explains the panel's dead spot, but
higher conductance doesn't give proportionally higher output. Think of the
diaphragm as a leaky jug and the coating as a hose filling it with water. The
hose must flow enough to keep the leaky jug filled, but once it's full, making
the hose larger can't put more water in it.
- Tape/spacer thicknesses were measured using the depth gauge on my Vernier
caliper.
- Front stator spacer thickness was 0.025” (edge tape, spars + tape-adhesives).
- Rear stator spacer-thickness was 0.045” (edge tape, spars + tape-adhesives).
- The transparent tape bonding the diaphragm to the spars was 0.005" thick.
- The spars remained adhered to the stators and were not removed so I could not
access and measure the tape bonding them to the stators.
- The bias contact was a full-periphery charge ring made of some metal-powder
coated tape (not a foil) sitting atop the diaphragm on the rear stator.
At the end of this write-up is a material list for a Theater panel complete rebuild,
replacing all materials (about $275 + shipping). I estimate the min-buy
materials for a pair of larger panels would run about $435 to $505, depending tape
thicknesses.
The Theater panel’s spacer-tapes were thinner than anticipated, and the tapes I
had on-hand were not compatible. Fortunately, the diaphragm remnants peeled off
cleanly, leaving the original periphery spacer-tape intact and viable, so the owner
saved some bucks and opted not to replace it.
Disassembly ruined the transparent tape on the spar inner surfaces so it had to be
replaced. This tape was probably 4.8 mil 3M VHB, which is only available as a large
quantity min-buy ($$$), but I found a single-roll substitute tape on Ebay for $20.
We would also substitute 6-micron Mylar in lieu of the original 12-micron diaphragm,
coat it with Licron Crystal, and install a copper foil charge ring, all of which I already
had, and donated, so the owner ended up spending just $20 on materials.
It’s best for longevity to replace everything but the stators, but the costs add up and
the panel will sound just as sweet reusing still-viable materials so… you decide.
Step 4 – Cleanup
No doubt, many new curse words originated from cleaning up acrylic adhesive
residue, which never peels off cleanly and no solvent that’s safe to use will
dissolve it. Acetone softens it and makes it easier to “roll off”, but it’s a major pain.
In this rebuild, the periphery spacer tapes were saved (not removed), which saved a
lot of cleanup. Only the transparent spar-to-diaphragm tape had to be removed.
Acetone can damage some plastics, and I didn’t want to risk acetone seeping into
the spar-to-stator bond lines, so I didn’t use acetone. I placed an old cotton sock
over my hand and used finger pressure to push and roll the adhesive off the spars.
The rear stator cleanup was easy, as it only involved removing the diaphragm. The
diaphragm film can’t be pulled off the adhesive slowly— you must break the bond in
peel using quick snatching movements in plane with the bond line.
The charge ring comes off with the diaphragm. Since the underlying spacer-tape
would not be replaced, I immediately re-covered it with a lightly-applied strip of
Mylar to keep it clean and tacky.
Step 5 - Apply spacer tapes to stators:
*The edge-band spacer tapes on both stators were undamaged / not removed.
- Apply new 4-mil clear d/s tape on spar inner surfaces.
- Since the edge-band tapes were not replaced, cover them with 4-mil d/s tape.
- On the back of the rear stator, apply 1/2" wide 4-mil d/s tape along the periphery
to secure the wrapped-over diaphragm film.
The Diaphragm Tensioning Jig:
A curved plywood jig (shown below) was built for tensioning and installing the
diaphragm. The diaphragm will be pre-tensioned and attached to the outer frame and
then lowered onto the stator for bonding.
Below: The Jig shown with its outer frame raised, and the stator resting on the
inner frame, below diaphragm level.
Below:
The jig is shown with its outer frame now lowered, and the stator resting on the
inner frame, at diaphragm-level. The diaphragm has already been tensioned over
the stator, measured for deflection between the spars, and adhered to the outer
frame with adhesive tape.
The backing remained on the stator adhesives to allow repositioning the film and
adjusting its tension, without the tacky adhesives grabbing and possibly tearing the
fragile 6-micron film.
After tensioning the diaphragm, the jig's outer frame will be raised to allow sliding
the stator out of the jig from underneath. After the adhesive backing is removed,
the stator will be inserted back into the jig. The jig's outer frame will then be lowered,
which also lowers the tensioned diaphragm onto the stator for bonding.
The jig worked perfectly! (the technical term is “tits” ) 😎
Step6a: Initial diaphragm placement:
- Apply double-sided adhesive tape to the JIG contour-surfaces & remove
the paper backing.
- Set the jig’s outer frame in the DOWN position (level to stator surface).
surfaces.
Step 6b – Measure and adjust the diaphragm tension:
- Measure the diaphragm deflection across the center spars using the
shop-made gauge. Adjust the tension by picking up and repositioning the
diaphragm as needed to achieve uniformly correct tension. Verify tension
- Press along tape lines to ensure the diaphragm is firmly adhered to the JIG.
- Place the JIG in the UP position (raise the outer frame above the stator).
- Remove the stator from the jig by sliding it out one end.
Step 6d – Bond the diaphragm to the stator:
- Place the jig in the DOWN position to lower the pre-tensioned diaphragm
onto the stator.
- Use a soft cloth to press and rub the diaphragm into the stator’s tape
adhesives.
- Rip up the excess film from the JIG surfaces to release the stator.
Step 7 – Coating the diaphragm:
- Solvent wipe/clean the diaphragm with denatured alcohol (91% IPA is fine).
to sputter if the can is held horizontally, and the coating tends to run on
vertical surfaces, so I compromise and orient the panel and can at 45
degrees.
- Hold the can at 8-10 inches from the diaphragm and apply one mist coat
humid day, it will blush (turn cloudy). If you screw it up, just wipe it off with
alcohol and re-apply.
- Solder the bias supply wire to the copper foil tape. Since the wire will insert
- Apply the copper foil around the panel periphery, centered on the spacer tape
(the foil has a conductive adhesive so either side can charge the diaphragm)
Below: The rear stator is sitting in the jig, complete with a new diaphragm and copper
foil charge ring installed. The front stator is shown inverted, with 4-mil d/s
adhessive tape added over the original spacer tape (backing still on) to ensure the
stators will bond when mated.
Step 10 - Final Assembly:
together.
together. I used small spring-clamps to hold the stators in contact.
- Connect the panel leads to a power supply and do a preliminary sound check to
There really is no burn in but it’s smart to crank the volume for an hour or
so to make sure no gremlins pop up (i.e. buzzing, popping, sparks flying).
diaphragm eliminated the aforementioned dead zone, and the net result
was a measured 1db gain after the rebuild.
Mission accomplished J
Regarding the Jig's Contour Surfaces:
The Jigs’ contour surfaces should closely match the curvature of the diaphragm as installed in the speaker.
It’s best to make a template from the panel mounted in the speaker, but I only had the panel, so I sat the panel upright on a piece of plywood and traced the curve of the rear stator on the plywood, then offset that line about 1/8” (thickness of rear stator + spacer) to define the curvature at the diaphragm. I then cut out the jig's contour boards on a band saw.
If you’re wondering why I’m so anal about this curve, consider the following:
If the panel’s curvature in the speaker does not match the curvature at which the diaphragm was installed (curve closes or opens when installed back into the speaker frame) the diaphragm’s tension and shape will change accordingly.
If the panel curvature closes, tension parallel to curvature will increase and pull the diaphragm closer to the rear stator.
If the panel curvature opens, tension parallel to curvature will lessen and create radial winkles in the film.
What concerned me about this rebuild is that I could not know the panel’s curvature mounted in the speaker frame, so I had to assume its relaxed-state curvature was the same as in the frame.
Below- A “Poor Man’s Megger”:
If you’re going to be testing diaphragms for conduction, you might want to make yourself one of these. Basically, it’s two probes (nickel coins sanded smooth) with leads soldered on and plugged into a DVM. A 9-volt battery supplies a voltage potential between the probes and the DVM measures the portion that conducted across the surface probed.
This rig can't accurately measure surface resistance/conductivity because it wasn’t calibrated to any standard but it's still useful for showing whether one coating is more or less conductive than another, and if you can determine a threshold conductance where volume drops below it but doesn't rise above it, then you've established the perfect standard for future reference.
Rebuild Parts List for Martin Logan Center-Channel ESL Panel
(shipping cost not included)
3M VHB 4929 Mounting Tape, Black 0.025 x 1.00 x 15ft,
McMaster-Carr #75935A13 $57.76
3M VHB 4949 Mounting Tape, Black 0.045 x 1.00 x 15ft
McMaster-Carr #75935A19 $66.36
*3M 415 Bonding Tape, Clear 0.004 x 0.50 x 65ft, Ebay $20
Copper foil tape 1/4" x 18ft McMaster-Carr #76555A711 $12.46
Licron Crystal ESD coating 8 Oz. aerosol spray
Mouser #577-1756-8S $53.63
Diaphragm film, Mylar 12um x 25” x 10m **Ebay $35
* Substitute for 4.8 mil, 3M 9832 bonding tape, which is not available in 1-roll quantity.
** Ebay purchase from China, expect 4 weeks for delivery