Quad ESL Cap mods

I have a pair of Quad 988 I'm considering replacing the electrolytic 220uF Capacitor for a film cap...

For those who have done it -- what caps give you the clearest extensions in the highs and improved mids without mudding the low???

Also, who makes a 220uF film cap? All I seem to find is lower values? Is it best to wire a bunch of lower value caps together for a total 220uF (if so, should you mix electrolytic caps with film caps - like some modders do) or is it best to have a single 220uF cap?

I do see Mundorf has 200+uF caps in their Tube Cap line -- are these Mundorf Tube Caps appropriate for the Quad ESL application? how do they sound?

Thank you guys very much for any and all input into this, as I'm just beginning my Quad mod journey...
19f9d02f 19dd 45d1 b5dd 0f012bafcd50grateful
Mundorf has a 220uf MCap, not the tube line.

Where is this in the circuit? I highly doubt a cap that large would be located anywhere near your tweeter circuit.

You can solder a few smaller values in parallel. Since you're looking to upgrade, I would avoid electrolytic caps entirely. Dayton, Solen, and Mundorf all have large values.
This is a input cap -- right after the binding post along with a resistor.
I believe it is a coupling cap, acting as a high-pass filter (filters out the deep bass), therefore it is very important for sound quality.
220uF in film caps will be very costly and occupy a lot of space, about the size of four soda cans. The least expensive option is four Solen 47 uF / 400 Volts caps plus one Solen 20-30uF uF / 400 Volts plus one Mundorf Silver/Oil 4.7 uF, all in paralell.
Since the original electrolytic cap has a tolerance range of +/- 10%, this combo will be in the ballpark.
You may want to do the full combo with Mundorf Supreme caps and use a 4.7 uF Mundorf Silver / Oil as a high frequency bypass cap.
There are many cap comparisons on the net, most agree that Mundorf Silver /Oil is one of the best caps regarding clean and extended treble.
I have experimented Mundorf S/O as compared to Kimber, Auricap, Hovland and Jantzsen film caps in the tweeter network of my dynamic speakers, all those caps sound good but Mundorf is ahead of the pack IME.
I hope this helps
I have a 220uf Mundorf here and it's only the size of 1 soda can.
Thanks for the forum input. Is the Mundorf 220uF cap you have film or electrolytic? What is the reference/catalogue number? Where did you buy? Whats you'r application and how does it sound?

Thank you,
Thanks for the great info. Any experimentation with the Mundorf TubeCap? Is the TubeCap appropriate for this type of application?

and When I replace the cap in the Quads -- does it have to be 220uF or can I go less?

Thank you,
It is a film cap and is a #71348 available from Parts Connexion.

I used it in a similar application, to filter out bass from a open baffle speaker. I didn't notice any negative effect on SQ, but it was only used on a woofer.

If you use a smaller cap you will lower bass response.

A Mundorf tube cap is not made for this application.
William, I have no experience with Mundorf Tube caps, because Tube caps are designed for high voltage tube gear power supplies, not coupling.

Regarding Face's Mundorf cap, it is a great choice if the original cap is rated for 250 Volts or less, however, I maintain my recommendation to bypass any large film caps with a Mundorf 4.7 uF silver / oil. This will provide speed and transparency in the treble range, which large film caps tend to lose due to high internal inductance (side efects of the soda can size).
224.7 uF as compared to 220 uF means that your speakers bass response will extend a couple of Hertz down, not really a factor given the usual 5% tolerance of large film caps and any resistors used within the speakers.
Good luck
I have been researching the same options for replacing the electrolytics in my Quad 2905s. FYI the 220uF cap is in series with the input and it does have a 1.5 ohm resistor in parallel with it (which could also use an upgrade to a Mills or something similar). I have read several of the capacitor comparisons including some comments specific to Quad 2905s over on the Audiocircle website. My observation is that for very little extra cost you can get 2-100uF Mundorf MKP 400V. And for a little more again you could get 4-47uF of the same 400V MKPs. With the 2905s any shift toward a heavier balance is not what I want. So with four caps instead of one I have the flexibility to try various values and see if the increased resolution and speed(expected from smaller values with higher voltage ratings) are what I like better. My big decision is whether to spend more on Auricaps which I understand have a slightly nicer midrange and apparently are a step up from the Mundorf MKPs. Their premium price at two to three times the Mundorfs would certainly indicate this. I would definitely appreciate comments from anyone who has compared these two. Another big plus with the Mundorfs is the availabilty of a nice selection of blend/bypass capacitors such as the Supremes or the Silver/oils as someone pointed out. I think its likely combining MKPs with their premium capacitor lines would really improve the sound. I'm not sure if the Auricaps sound would improve being bypassed for extra performance. From reading the Auricap datasheet, its actually discouraged. I would like to know if anyone has had good results doing this also.
I spoke to a moodder and he says F & T MPP caps with a Harmony cap bipass are the best way to go for the price. He feels the F & T MPP caps give a better result than the Mundorf MKP.

He sais the only appropriate cap for this (Quad esl) application is MPP type (Metallised Polypropylene) and of course they have to be of adequate voltage -- because of this, suitable candidates are limited and pricey...

Any comments?

Any experience or comparison of F & T MPP caps?

The Mundorf is rated at 250V, which should be more than enough.
I have Auricaps and Mundorf PP caps in my tube amps (not the same application, I know). I prefer Mundorf Supreme to Auricaps IMS.
Mundorf caps are metalized polypropilene.
Munforf silver and oil is a significant step forward in transparency as compared to M. Supreme or M. tube cao, but it is only economically feasible as a bypass cap.
Many high-end speaker manufacturers are switching to Mundorf caps for their top model's crossovers: BW 800D, Magico, ZU Definition and some others I forget.
Any cap smaller than 220uF will attenuate deep bass response.

Regarding the paraleel resistor, find out its value and power rating (usually written in the transistor body).
Mills MR12 is widely well regarded as a neutral, non inductive resistor up to 12 Watts per resistor. You can use several in paralell to reach the correct ohmic value and power handling. E. G. ten 80 ohms 12W resistors in paralell will add up to 8 ohms and 120 watts.
Dale makes aluminum-clad non-inductive resistors in 20W and 50W ratings.
DO NOT install the resistor flush with the cap(s), because it will get very hot on music peaks and may damage the caps.
Any PP or MKP cap will certainly sound much better than an electrolytic cap, but if you can avoid the cheapest ones, it will be a wise investment, given the Quad ESL's transparency and midrange naturalness.
Good luck

I hope this helps
I have upgraded the capacitor/resistor with 9-22uF Mudorf Supreme capacitors and a 1.5 ohm Duelund resistor on each 2905 speaker.

Here are my impressions at 100 hours:

Much more coherent, space around instruments sounds more "right". Timbres and brustling sounds from musical instruments are less smeared/obscured. For instance I can now hear Stan Getz taking breaths between notes on "Getz/Gilberto". Glare is reduced to the extent that some recordings which were bordering on unlistenable are back on my playlist. The 60 Hz bump in the low end has been tamed. My big concern that the highs would get edgy or sterile did not happen. Overall changes to the frequency response I would characterize as a slightly less warm/heavy balance which I prefer. The midbass is less round and smeared so a lot of new information is heard in this range. Textures in the mid to upper bass are easily discerned and transients on drums and piano are much more realistic. The low frequency rolloff seems to be about the same but I will probably still try an additional 22uF to evaluate the effects. I also going to try a 4.7uF Mundorf silver/oil on the capacitor bank and will report back on bypassing with other capacitors. Possible suggestions would be appreciated. I will probably also try a 1.35 ohm resistor.

I did make one disturbing discovery during the upgrade. One stock capacitor tested at 216uF and the other was 255uF. This would partly explain why there was such a big improvement in imaging and bass resolution. Both resistors measured 1.6 Ohms. Concerning the cost, I purchased the caps and resistor with a nice 20 percent discount that Partsconnexion was offering, so $1300 was a lot easier to swallow. Would I do it again? At this point, it's a no brainer especially considering how badly matched the stock caps were. I'd say, on the major characteristics I hoped to improve, all were easily met with the exception of the leaner bass which I would rate a draw although its seems to be improving every day with further break-in.

I appreciate all the information from other agon members. I will issue an update after the break-in is complete.
Hi, I have the Quad 988 speakers and I am also considering changing out the input capacitor and resistor. I find the upper extension to be somewhat lacking in comparison to my Magnepans, which was a real surprise. I am using Cary 805 Anniversary Amps. I have tried super tweeters, but they never worked well. I did pull the bottom panel on the 988's and measured the input resistor at 1.6 ohms (it has a rating of 1R5 at 5W), I was unable to measure the capacitor in the circuit because of the bypass resistor. The bipolar electrolytic is rated under 100v, so I would assume a film capacitor with a 250v rating should do fine. I was planning on using a Duelund 1R5 resistor, but wanted to get some feedback on using the Obbligato film capacitors (4x50uf) vs the Mudorf MKP (2x100uf), and then bypass with a 3.3 or 4.7uF Mundorf silver/oil. I have used the Obbligato and Mundorf silver/oil in my Magnepans with great success. I was wondering where you put the new capacitors and if you removed the old resistor/capacitor from the circuit board, or just bypassed them? Any follow-ups would be on your listening experience would be appreciated.
Hi Mksj
By all means remove the original resistors and capacitors if you perform this mod.
Also, make sure you buy the right wattage resistor. I suspect that a lot of power flows through the resistor at high volume level.
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.
I believe that the main reason why you are delighted with the sound quality after modding the speakers is the difference in distortion, clarity and speed between crummy electrolytic caps and high quality film caps.
I still can not believe that QUAD had the nerve to use lytic caps in series with the signal...Quad 988s are not exactly budget products.
A cap with unmeasureable ESR and DA is electrically pretty close to a straight piece of wire (within its frequency pass-band).

Also, your ground mods were a smart move. Many designers tend to think of ground as a zero volts, zero current wire, supposedly not important at all.
In practice, because of ground wiring resistance, voltage and current ride on the ground wiring and mess up the signal return path.
Grounding layout, wiring and connections are very important and are very rarely implemented correctly.

Agree with you, replacement of the Quad ESL speaker 220uF electrolytic capacitor probably accounts for the majority of the improvement. Probably replacing the electrolytic cap/resistor and eliminating the PTC gives the most bang for the buck and the simplest to install. You would not need to pull up the circuit board, just remove the stock wiring and snip out the PTC. I am surprised that even in the latest Quad 2805 and 2905 speakers that the electrolytic is not at a minimum bypassed with a decent film capacitor given their increased price. But then I have had Magnepan speakers through the years, and they are no better in the crossover department. Hardwiring the ground system and using a modified star configuration is easy to do and improves the safety.
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.
Hi, Johnsonwu. My Quad ESL 988 speakers measure around 2.8 ohms across the speaker terminals. The Quad ESL 988, 989 and the Quad ESL 2805, 2905 use the same circuit. If your resistance measures 0 and then goes to infinity, it probably is just seeing the capacitor and you have cooked (read open) the resistor that parallels the 220uF input capacitor. In either case they should not measure 0, this would suggest that your other resistor may be shorted. The protection circuitry measures the noise radiation that occurs with the onset of ionization. This triggers the triac clamping circuit to prevent arcing. The circuit operates by limiting the input, and when that fails, by short-circuiting the input. When the triac T1 shorts the amplifier see's just the 1.5 ohm resistor in the input filter. This leads me to believe that at least in one speaker the resistor is now open (resistance goes from 0 to infinity). The R15 input resistor on the Quad schematic is 1.5 ohms 5w, so it probably cannot handle a sustained overload. My understanding is that the combination of 220uF capacitor and 1.5 ohm resistor is a form of input filter, most of the mid and high frequencies pass through the capacitor, as the frequency goes lower it attenuates the signal and more voltage (LF signal) is passing through the resistor. If the resistor is open you would probably have no base. My other concern is that you may have damaged the triac if you are reading 0 ohms in the other speaker (it is not seeing the resistance of the transformers and 3.3 ohm resistors).

As far as capacitors, I have used Solen Caps in the past, but they are just OK. I have used the Mundorf's and also Obbligato films in my speaker crossovers and have been very happy with them. Obbligato's are inexpensive, but do not fit in the Quad. The 220uF Solen cap would cost about $60 and the 3x68uf + 15uf Mundorf MKP is about $100, with either one I would bypass with a high quality cap. So I do not think there is a big savings going with the Solen (not sure it would fit). Mundorf MKP series also has a 250V version of these caps that is a little less expensive (and smaller) than the 400V series, but were talking maybe 10% less cost. I tend to like the Mundorf Silver Foil Oil for a crossover bypasses, as the best bang for the buck without going into the stratosphere (V-Cap or Duelund). I also have heard that for crossovers caps, a smaller bank of equal sized capacitors sound better than one large capacitor. Ideally serial crossover capacitors should be 10uF or smaller, but the 3x68uF is the smallest I could go inside the Quad base. Bypass caps should be 1-2% of the total value. This is important if the capacitor is in series, if it is in parallel (i.e. across the - to +, or shunt) then size or number doesn't matter, use a single cap.

Regarding the resistors in speakers, most of the information I came by recommended the Duelund as the best sounding and then the Caddock MP820 / MP821 series. The stock Quad resistors look like they cost 5 cents. By increasing the wattage of the stock resistors, you would have a larger overload margin. The Duelund 1.5 ohm across the input capacitor is rated at 10 watts (stock is 5 watt and the Duelund can take much higher short term voltage because it is a chunk of carbon not wire), The Caddock resistors are films and are rated at 20 watts with a proper heat sink vs the stock of 7 watts. Some people use Mills resistors, but I think in this application they are a step down in sound reproduction.

I got most of my parts from Part ConneXion when they were running there 20% sale. If you are not adventurous, I would suggest that you consider sending your speakers to Kent McCollum at Electrostatic Solutions. I had spoken to Kent about this mod for the 988/989 series speakers and the 2805/2905. I believe he was going to fabricate an upgrade circuit board/kit with Mundorf capacitors. He has quite a bit of history on the different web forums, is very knowledgeable about Quad speakers and does great work.
Thanks Mksj2 a lot for the detailed and insightful response.
Guys - If this input cap and resistor are simply acting as a high pass filter then why deal with the problem at the speaker end? Wouldn't it be better at the line level stage? This way the power amps wouldn't waste valuable resources trying to reproduce the most energy demanding information only for it to be disposed of? Of course to do it digitally before the DAC of any system would be optimum. Another consideration is that many of the valve amps you guys are using will already use coupling caps between stages to prevent DC passing - why not chage the value here and do away with the second one? Far more efficient and one less cap in any audio chain has to be a winning solution....
Sugarspice, I think that protection at the speaker is safer.
Many well-regarded amps thump when powered up or down, not to speak of the absent-minded user that turns the amp on, then the preamp.
Hi Sugarspice, the RC does not limit the frequency range to the speaker, a characteristic of a dipole loudspeaker is that below the cancellation frequency, determined by the size of the speaker, the output will decrease with 6dB/octave. The series resistor of 1.5 Ohm compensates this effect that by increasing the Q factor. In parallel is a 220uF capacitor for bypassing the mid/high frequencies. So it would not warrant an external crossover/filter unless you were using a subwoofer. Removing the RC in the speaker would present the amp with a dead short if the protection circuit shut down the speaker. So I believe this is best dealt with at the speaker level, especially given the significant improvement with the changing out the parts as noted.
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.
Sorry to be tardy - I just found this thread.

I started modding my 2805 and 2905 nearly two years ago. First I messed about with caps and resistors - metallized poly all sounded the same; the big difference came with using more film and foil. When I got up to 70uF film and foil plus 150uF metallized, the sound was dramatically improved. Duelund resistors were better than Mills WW.

The big breakthrough came with changing the step-up transformers to the Vanderveen design manufactured by Plitron. I used the 75:1. To my ears, the best sound came with using NO signal shaping other than an input resistor (as Vanderveen suggests). Best resistor in my setup was obtained by using nichrome wire as speaker cables - I have a monoblock behind each speaker, so short wires are feasible. Each of the two wires is a bit more than one ohm.

Best improvement of all was making my own low power monoblocks with low voltage rails. With this setup, it is physically impossible for the amps to punch a harmful spike into the speakers - so I could safely eliminate all of the protection circuitry. Sound is quite magical.

I hope that you are experienced with ELSs, because they can kill the unwary in several ways. Please be careful. All the best!

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?
Hello Johnsonwu. I doubled the Duelunds to get the 20W.
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.
Howdy folks .

I have recently acquired a pair of ESL-989's and whilst mugging up on user tips and tweaks I came across this most informative series of posts, amongst others, recommending a number of modifications ,chiefly the replacement of the 220uf lytic with metallised or film/foil poly + associated resistor.

I was under the impretion that those componants would be reasonably easy to get at ala the 63's however I was kinda dismayed to see the components connected directly to the PCB

I'm pretty good with point to point circuitry however I tend to shy away from PCB work .

I would be grateful for any further tips regarding the safe removal of the 220uf + R15 together with any further info re securing the replacement polypropylenes into the circuit .
Any under the hood images post modification would be most appreciated.

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.
Thank you for the further information Johnsonwu.

I was also considering replacing the internal 220uf/1.5ohm with a section of wire to complete the circuit, placing the components in a separate box outside the base , connected via additional RCA's ~ Speaker cables > box > 989 speaker binding posts.

That way I could utilize several or large size Poly caps that would be difficult to shoe horn into the space available in the base!

Feasable Folks ? or a little Heath Robinson !
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.
Thank you once again Johnsonwu for your advice.
I would be very interested in viewing any Post Surgery images that you might dig out, most kind.

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.
My first Quad Mod was along the lines you suggest, Tsushima1. I simply removed the existing lines from the WBT connectors and insulated the ends. Then I routed new lines (15 AWG silver coated copper) from the connectors to the downstream side of the signal shaping network (cap and resistors). Then I put my caps and resistors in another box with WBT connectors on both sides, et voila! I connected amps to my new box, and the new box to the Quads.

To restore to original condition, one need only remove the new wiring from the WBT's, and re-solder the old, which you have carefully preserved in the black electronics box.

I found that the more film and foil, and the less metallized, the better. Changing to Plitron transformers was better. But, be careful, please. Since you are talking about modification, I assume that you know about the required safety practices. Otherwise, I suggest leaving it to a tech.
i have a photo attached to my Quad listing on my system page that shows my caps mounted - maybe this will give you an idea. Best mod
Hi Tsushima1, if you need some photos of the quad 988/989 mods/info, I can email photos of the mods I installed. I would leave the stock electrolytic cap/resistor in place and clip out the PTC that connects them on the PC board to the wire going to the step-up transformer, then reroute the speaker input wires. You need to wire in a separate bank of film capacitors/input resistor that are ideally mounted in the base (an not on the PC board).
My apologies for the Tardy Reply gents , I have only just returned to this project and thread.

Terry9 and MKsj2 , Thank you kindly for your assistance, I have emailed you both .Likewise Grateful , just off to admire your system.

Yes please send me pics of the 989 mod
Let me know how you want the information to be sent or posted, I have provided other forum members with more detailed information. I am not aware that the information can be uploaded to this forum. Newer Quad models after the 988/989 present more of a challenge because of their interior space, and have had more quality build issues.
I'm thinking of getting a pair of 2905s and really interested in this mod discussion. But a) i am rubbish with a soldering iron and b) don't fancy messing with the high voltages involved.
is there anyone in the UK that can provide an upgrade service along the lines of Plitron transformer substitution or caps swap etc ?
Sorry, I'm across the pond.