Note: Working on Quad speakers is inherently very dangerous due to their high voltages, although you can do these mods at your own risk (and could result in death), I recommend that you send your speakers to someone that has experience in working with these speakers. This way you get to enjoy the music.
It took me a awhile to figure out a capacitor configuration to replace the stock 220uF 50V electrolytic, and the low voltage signal resistors. I have used Mundorf caps, and have been very pleased with their price/performance ratio. The MCap dimensions given at Mundorf's web site are incorrect as they listed a length of 66 mm for their 56uF-100uF caps, the 100uF are 72 mm. The stack height is an issue (66 mm max length) and the diameters are limiting factors of what will fit in the Quad base. Most other high end caps just won't fit and I am not sure of the benefit with such a large capacitor value. The MCap 100uF would not fit, unless you use the 250V version, per Mundorf, they say their 400V version sounds better then the 250V. I finally settled on the MCap (400V) 3 x 68uF + 15uF and a MCap Silver Oil 2.7uF, this configuration just fits stacked vertically in front of the step up transformer on the side opposite the power transformer. This also keeps all the signal wiring away from the AC. I used some 4 mm acrylic sheet, drilled/milled and glued the capacitors on one side, I hardwired everything with Cardas wire and it bolts to two of the studs that hold the upper electrostatic panel. Thin circuit boards may have been easier. With the MCap Silver Oil cap., one could go up to 3.9uF given it is the same dimension as the 2.2 or 2.7, I just went for the one that was available. MCap's have very tight tolerances, so both banks of caps measured at 221uF, they had unmeasurable ESR & DA. I replaced the stock 3.3 wire wound resistors with Caddock MP821 3.3 ohm resistors on the main board, they fit perfectly in the stock holes when you bolt two to the same heat sink (back to back and I needed to offset the heat sink I used). They are rated at 20W each as opposed to the stock 7W. I attached them first to the modified heat sink and then mounted them to the circuit board. I used a Duelund 1.5 ohm 10W resistor at the input, seems to smooth out the sound and costs only slightly more than the Caddock. I replaced the stock 1.5uF cap 250V cap. with a 400v 1.5uF Mcap bypassed with a ERO1837 10nF. Removed the stock wiring and pulled the PTC (FS2) . I left the stock electrolytic/1.5 ohm resister on the board (no really reason to pull them with the PTC/input wiring removed). Replaced the brass binding posts with some decent Cardas CCBP S posts. I wired the Duelund resistor (from the + binding post) and capacitor bank directly to the IN1B terminal (they are long). The negative binding post was connected directly to the OUT1 terminal (moved the bottom plate ground lead to the chassis ground circuit). This made for very short direct signal runs, O resistance in the runs and was easier to wire.
In the stock configuration, I did find some grounding issues (increased resistance between the chassis/HV transformer grounds and the circuit board ground/- terminal), this was because ground connections where screwed together. So I soldered the black step up transformer ground taps to the ground tab screwed in at the power transformer and continued the same wire and soldered it directly to the board OUT2 ground plane with 14g silver Teflon wire. The resistance in the ground circuit dropped to 0 ohms. Before this the step up transformer ground taps were grounded mechanically at the power transformer and the screw was not very tight. As mentioned the ground for the metal base cover was soldered to the ground tab that connects to the step up transformer near the new capacitor bank.
I lined the complete base cavity and cover plate with thin sound deading material, this significantly reduced the cavity resonance. I changed out all the 8 mounting washers that bolt the electrostatic speaker panel to the base with larger/thicker steel washers, and added lock washers to the bolts that did not have a ground tabs attached. This made a significant difference in the speaker flex between the base and the panel. Ideally I should have probably used washers with an even larger area or a small fender washer. The stock washers are no larger than the nut and were dug into the plastic (i.e. loose, especially those without a lock washer).
Fired it all up last night and frankly my wife and I were floored at the difference. I have had the 988's for several years, initially listened to them infrequently and then they were in storage for several years. I never was really satisfied with the sound, great mids, but lacking at the extremes. I replaced the amps, replaced the preamp, tried different wiring and even tried supper tweeters to no avail. Never could figure out what I was missing until now. Well, with the mods they are completely different speakers. Much better base and articulation, vocals sound like vocals, better air at the upper end and improved balance across the music spectrum. Since they only have a few hours on them, I look forward to them getting better as they break in. I am using Cary 805 anniversary amps that are good for 50W, I also have a Mac275 that I want to try. The changes made a significant difference, but I cannot really say the contribution of each change. Great addition for the 988/989 speakers, not sure of the space in the 2805/2905 Quads but the mods bring these speakers to whole new level.
