SDA SRS "Upgrade Story"

trumpeterherman

After reading so many forum posts about various SDR SRS upgrades, etc. I'd post my own little anecdotes, in case someone might benefit. I'll try to be brief, but the story isn't!

My beloved SRS SDA (blade-blade) from ~1987 started making "fuzzy nasty sounds" in the left-channel tweeter section about six months ago, getting progressively worse. I tried some tweeter swaps, but to no avail. After reading/studying, and also receiving some very helpful advice from a David Shirly (DHS Speaker service), I decided to bite the bullet and dive into an upgrade of tweeters to the RDO194 models and replace all the capacitors, bypass the polyswitches, etc.

Well, after buying/installing 8 new tweets, NO FIX (honestly, didn't expect there to be, since I'd already tried swapping tweeters from other speaker).

After buying/installing capacitors (Solen and Sonicaps on the high frequency circuit, equivalent electrolytics from Parts-Express on the dimension+stereo array), NO FIX. Incidentally, working on this crossover is a BUGGER - had to rig up a makeshift "desk" in front of the speaker cabinet since I didn't want to cut all the tweeter leads and/or disconnect them all and re-route when done. I spent about 3 hours hunched over, removing/installing caps. Not Trivial!

So now I'm getting exasperated.. In desperation, I grab a known-working speaker from different system, thinking I'd swap it out just to "make sure" that it is absolutely the speaker and NOT something coming from "upstream". As I loosen the binding post on the rear connection on SDA SRS, I notice that it was not tight - even "jiggly". The fuzz sound (root cause) was intermittent connectivity from the main posts, fed through the "jumpers" to the high-frequency section. ARGHHHHHHHH! Note to future, smarter-self: CHECK ALL EXTERNAL CONNECTIONS BEFORE DISASSEMBLING CIRCUIT BOARDS! (reminds me of the Microsoft customer service logs where rep has to remind people to plug the PC in to power...)

So I completed the capacitor upgrades to second speaker, cussing myself all the while. Plugged everything in and gave it a listen - sounded fantastic, but something seemed "missing" - the SDA effect seemed "subdued". Honestly, I think it has been "subdued" for a long time, but I never investigated. I googled some more, and read about the "tests" for SDA (FM static, L-R sweeps, etc.) and sure enough - SDA array was NOT working. I got out my multimeter and started taking resistance readings on the pin-post combinations and comparing it to the great collection of info found in the "SDA Compendium". A few of the readings were "off"- red to black was about 2X too high at about 8 ohms vs. ref 4.3 and more concerning, one of the values (pin 2 to red) that was supposed to be ZERO was showing ~10 ohms. SO, I tear the whole bloody thing apart AGAIN (!!!) and start checking why a supposed direct connection was giving 10 ohms on BOTH speakers. Long story short - the direct circuit board connection between L1 and L2 blades was faulty- something w/ the blades or the board copper or both - I installed a copper jumper across the top of board on those pins, and voila, all resistances were "as desired". Weird thing is, SAME EXACT PROBLEM was found on the other speaker! ! So if your SDA SRS is not functioning, check out those reference resistances!

I reassembled, plopped in a test CD, and slipped in to auditory nirvana SDA immersion - all the SDA was back, and the speakers have brought back that "amazement factor" that I had when I first heard them! Stuff coming from the sides - even behind!

Final bit of interesting info - everyone says "Electrolytic Capacitors go bad after ~25 years", so I thought, "Let's just see about that..". I checked the 28 year old caps out with a good multimeter. Here are the results:

Polk SDA SRS Crossover Circuit Electrolytic Capacitors, 28 years later…

Capacitor Location Intended Measured
TSI-8548 USA PXML130PY75 130uF +-20% Dimensional array 130 uF ACTUAL= "NA" (shot)
TSI-8548 USA PXML130PY75 130uF +-20% Dimensional array 130 uF ACTUAL ="NA" (shot)
TSI-8548 USA PXML130PY75 130uF +-20% Dimensional array 130 uF ACTUAL =73 uF
TSI-8548 USA PXML130PY75 130uF +-20% Dimensional array 130 uF ACTUAL ="NA" (shot)
TSI-8539 USA PXH20MS50TF 20uF+-10% Dimensional array 20 uF ACTUAL= 22.0 uF
TSI-8539 USA PXH20MS50TF 20uF+-10% Dimensional array 20 uF ACTUAL= 20.0 uF
TSI-8539 USA PXH20MS50TF 20uF+-10% Stereo array 20 uF ACTUAL= 22.4 uF
TSI-8539 USA PXH20MS50TF 20uF+-10% Stereo Array 20 uF ACTUAL= 22.5 uF
T1 E-0012-1 12J100V mexico (yellow) Tweeter 12 uF ACTUAL= 12.4 uF
T1 E-0012-1 12J100V mexico (yellow) Tweeter 12 uF ACTUAL= 12.4 uF
T1 4.4J100V mexico (yellow) Tweeter 4.4 uF ACTUAL= 4.5 uF
T1 4.4J100V mexico (yellow) Tweeter 4.4 uF ACTUAL= 4.5 uF
TSI-8533 USA PXM92PY75T 92uF+-10% Tweeter 92 uF ACTUAL= 100 uF
TSI-8533 USA PXM92PY75T 92uF+-10% Tweeter 92 uF ACTUAL =100 uF

So the 3 out of 4 of large caps on the dimensional array were by and large "shot". All the rest, FINE!

But I feel better knowing that all the caps are "good for another 30 years"..

skrol

Somewhat similar story with my RTA11TL's. I thought they were sounding a little harsh and slightly scratchy. I decided to do the XO upgrade and RDO198 tweeters. When I pulled one of the XO's I found that where they had attached one cap across the leads of another, they forgot to solder it. The lead was just twisted around the other and a little loose.

westmassguy

Glad I could help Peter.

PolkieMan

Sounds like a job well done and always nice to have everything is as it should be. After 30 years, replacing the caps even with just standard caps, is needed. Everyone should read and know this.

K_M

All the caps should be replaced, well before 28 years.

Even with standard quality caps, there will be a very noticeable difference.
They develop resistance. Even when the Capacitance value appears to read correctly.

Our one vintage set of Polks, caps read pretty close, but the sound from the tweeter was dull and lower than what seemed normal.

Used entry level couple dollar caps, and it was transformed sound wise.

The ESR changes over time.

MarkCianciolo

I know this is several years late.....but my experience with these speakers, and I’ve owned several, still have a set in my main system is the cheap wiring. You should have replaced it. It literally turns green because it wasn’t air tight. It’s a disaster. Plus the earlier models like you know were sodered to the crossover....which is a deterrent for most to change. I’m sure after all of these years you have found the problem. I’m better this was it.