Refurbish bass panel

Diagnosis: old panel vs refurbished panel

Measured in same conditions, bass are boosted around 60Hz on a refurbished panel whereas on an aged one, bass boost occurs around 100Hz. This is the main audible consequence of ageing on panels.

Old panel in pink, refurbished panel in green

On the drawing above one can see that ageing of treble cells is not significant at all. Main trouble that can happen with treble cell is arcing, due to too high power/voltage applied on it.

Measuring old panel (black)
Measuring refurbished panel (gold)

And what happens when comparing 2 refurbished panels ? Same trend (boost around 60Hz). You will see some differences in drawing below but do consider that internal passive components age as well (original ones used in this case). Besides, vertical position matters at lot for highest part.

2 refurbished speakers (passive components in audio transformer not changed)


Bass panels should be refurbished using SARAN film (PVC). Unlike MYLAR (PE), it can easily be heat shrunk. In all cases, use 12um thickness. If using Mylar, annealing might be required (heat film under tension), or at least additional heat shrinking once in place and glued.

I use a jig, with loads of 1.5 lbs spread around, hooked with grips (see jig chapter).

SARAN is not MYLAR !
After 8 years, still glued thanks to ‘3M 467 MP’
Elvamide is still visible after 8 years. Applied on both sides

Once bolted, use soft PVC tape like 3M super 33+ to isolate borders. Do not use too rigid tape – like packaging tape – otherwise the full panel will be thicker and you will have difficulties to screw the back grill.

Prepare stators

Opening cell takes time: 45 minutes to remove about 66 rivets (vs 56 for treble cell). Do not remove rivets used for connections !

3 wires are needed:

  • 240mm from center connector to left then right solder points (diaphragm)
  • 105mm from left connector to rear stator
  • 180mm from right connector to front stator (going through the full panel)
10kV wires

Tip: take care not to overheat connectors on stators when soldering.

Check connection between the 2 internal stripes and the external ring. Use aluminium tape for external stripes and conductive paint for connections in between, to repair if necessary. Example here with spray paint:

The lower part was re-painted with spray paint

Take care that cheap conductive paint is not … conductive. Make trials !

Tip: check now before gluing a film that resistivity between each stator and metal strips is higher than let’s say 10GOhm. If below, stop there and track for leakage.

Insulation check. Target: few tenths of GOhm on one stator alone

Here below, a true example of leakage that could be measured to as low as 100MOhm. Enough to have full restoration fail. Solution: water & soap, the dry and clean with alcohol.

Leakage !

Once re-wired, check again leakage of both bass cells. Here just above 500MOhm which is not so good. Below let’s say 10GOhm, the EHT cannot cope with it, resulting in drop of sensitivity of … treble cell.

Measuring leakage of both bass cells. ‘+’ probe on PSU/6kV. Any of the 4 stator terminal can be used for ‘-‘ probe

Attention !: take care to apply ‘+’ probe on PSU/6kV and not ‘-‘ probe. Otherwise you will measure as low as few tenths of MOhm, as you would measure on the PSU itself alone, unsoldered (see chapter on checking PSU). For ‘-‘ probe, you can use ground as well.

No leakage for this one: >20GOhm
This one leaks: 400MOhM

Stretching film

PVC film (12um) is stretched using the jig. Some tape is applied on borders where the clamps grip the film to avoid tearing it. Which happens with some fragile PVC films. Let the film be tensioned for a while (1/4h) before gluing it to check that there is no isolated weakness somewhere. Worth trying: clean with alcohol to check if this triggers some changes in the tension of the film (because Elvamide is diluted in alcohol). Then you can place temporarily a stator and mark borders with a pencil or even masking tape.

In all cases, clean first the tensioned film with alcohol before applying coating, and let dry.

(fragile) PVC film stretched on the jig
(strong) PVC film stretched on the jig

Then Elvamide is applied on full surface corresponding to stator except borders, to reduce leakage.

This simple tape will prevent applying coating on borders
Elvamide under preparation
Strips of “3M 467MP” glued on stator

Now stator can be glued. “3M 467MP” is for sure ideal here: it just works great.

Stator glued on film
Ultimate basic check

Optional: check surface resistivity (after applying coating). 1010 to 1012 is ok. Reference is   

Optional check

Then coating on the other side, then bolts and nuts, then dust covers, et voilĂ :

Full bass panel ready (almost: before cleaning leakage !)

Tip: use soldering iron to melt the PVC film before inserting nuts through the panel.

New connection terminals for this one (spacer M3 10mm or more)
Double sided scotch to fix foam rubber on dust cover