Quad ESL Cap mods

Greetings, may I ask if anyone has tried Solen 220uF film caps bypassed with something, or perhaps those Obligato 100uF 500V caps?

I'm running into a problem with my 989s recently and I suspect its protection behavior might be the reason why my power amp fried. Stereophile wrote that the inputs become SHORTED when a 989 goes into protection. Can any fellow member pleaseclarify that?

Also I notice that when the speakers are not connected to an amp, I have one speaker whose terminal measures almost 0 ohms using a DC Ohmmeter and the other starting from 0 and slowly going to infinity as if measuring across a cap.
So which speaker is behaving normally? Thanks.

Thanks Mksj2 a lot for the detailed and insightful response.

It's been almost a year since I applied a duct-tape fix to the 989 by using Nichicon bipolars (I thought they'd be better than the orig. Bendix) bypassed with a film cap.

The fix only marginally improved the threshold before protection kicks in and the sound was best described as phasey.

Finally I got around to getting a big Solen (for convenience in mounting and cost) and rewired the input filter with 12 gauge wires, replaced the 3 Ohm resistors with heatsinked Caddocks, and replaced that nasty 1.5uF cap with a Solen (again slightly compromising quality for the sake of space). I also bypassed the FS2 which doesnt make any sense unless I m using monster amps. I did keep the triac clamp circuit in place for safety and protection of the panels.

The result is stunning, I can play it at dynamic-speaker volumes with absolutely no distortion or stress, and the sound is much more transparent and coherent.

If I ever get around to making the mounting I might eventually go with paralleled Mundorf or VALAB 47uF caps but for now it's a huge step above what it was ever capable of.

Thanks again to all contributors esp mksj2.

Hi Terry9, did you end up doubling up the Duelund resistors to match the orig. power rating? My experience with Duelunds are outside of Quad mods but were you able to fine 20W+ Duelunds for the job or was your mod only for the input R and C before the split-tranny?

Good for you then cos I had to change out Duelunds on my friend's JMLab Micro Utopia in favor of Mills MRA12s. The Duelunds caused a major saturation in the sound when overdriven.

The PCB is one sided, so you will need to carefully unscrew the mounting screws and flip it around and unsolder the orginal sand resistor and lytic.
You will need special nylon tie wraps to tie the new (HUGE) lytic and bolt the tie wrap onto the large screw that the panel is mounted onto the base.
You will need to use some 16 to 14 gauge wire to extend the leads of the cap to reach the appropriate connection points.

YOu can still use several AXON 250 90uF available for cheap @ Partsconnexion if you dont want a single cap.
SPend some $$$ on resistors, the cap does a lot but its not the be all end all to the mod.
Placing them outside the box may involve drilling which WILL make the speakers unsellable.
I'd think twice about it.
I can dig up some old pictures of how mine looked like after the mod and send it to you.
It might take me a few days to find them though.

Sadly I could not find any photos of the inside showing how I mounted the cap. I remember I took a bunch of them but I could not find them anywhere now. Mine were sold in Sept.
Take a picture of yours and I can guide you on how to easily mount them.