Polk SDA 1a Is This Wire Wound Part Of The X-over Smoked?

SeleniumFalcon

No problem, glad to help.

Benjisan

Hi,

So I measured the other 1.82 inductor as it sits still soldered into the other crossover dimensional board, but all speaker leads are disconnected and it reads 03.6 ohms. Not sure what part of that reading is taking in for the resistors, capacitors and other 0.4 mh inductor still in the circuit as I'm not skilled enough in electronics to account for their overall effects on the design of this circuit board.

Benjisan

Well I pulled the inductor from the board to measure it as a stand alone device and it reads almost exactly the same @ 03.7 ohms.

If I deduct the DVM variance of 00.7 from the inductor value it looks like a 03.0 ohm inductor should do the trick.

The 1.820 mH Jantzen 26AWG with the DCR value of 3.087 George recomended as a direct drop-in should be perfect.

Now the question is should I replace all 4 of them or is the slight variance between the originals and the replacement Jantzen insignificant enough to not affect the overall performance of the speakers as a collective sum of its parts.

Gardenstater

I would test the others too, but I'm thinking that he probably used that non common ground amp for a while and some damage was done to the coils in the Dimensional array. To test some of the others may require unplugging MW's and tweeters.

If it is only this one inductor you could probably get away with just putting the one Jantzen in, since you were lucky enough to find what is basically an exact match. You may get a little better bass by getting a pair of the 24awg ones, either Jantzen or Mundorf, or it may upset the balance. I'm really not certain. They seemed to be talking about the later model SDAs in the SDA Handbook, where the bass driver in the Dimensional Circuit was stereo in the very low frequencies.

The mystery is still there regarding why you got 11.5 and 11.8 Ohms, when one of the Inductors was shorted partially. Should've been much more of a difference. The game is still afoot, as Sherlock Holmes would say.

I would do the rest of the resistance checks in the SDA Handbook.

Benjisan

The other inductor in the same position for the right speaker is most likely toast since it appears to have been over-heated too.

At the minimum I will swap it out so they're both the same.

Oh, since I'm going to replace all the caps and the resistors, but I haven't seen any mention of those little brown ones attached in parallel on top of the stock capacitors.

Any ideas about their purpose and whether I should just reuse them.

audioluvr

Don't reuse the little brown caps. A good quality cap doesn't need them

Benjisan

audioluvr wrote: »

Don't reuse the little brown caps. A good quality cap doesn't need them

Okay.

Let me ask this as a general question here.

I need a capacitor value of .5uF and my only replacement options are .46uF or .56uF.

Which direction should I lean towards?

The higher value capacitor or the lower value capacitor?

Gardenstater

Yes they are silver mica bypass capacitors, 750pf. They used them on the mylar high frequency capacitors. Most people here say they are no longer needed when you go to high quality Polypropylene film capacitors like the Sonicaps , or other high end audio caps. I honestly never detected a difference, with or without, in my 7Bs.

pitdogg2

They seemed to be talking about the later model SDAs in the SDA Handbook, where the bass driver in the Dimensional Circuit was stereo in the very low frequencies.

This SDA woofer and tweeter do not work without the cable connected in those speakers. It is pre sub bass drive circuit IIRC. I'd replace both inductors that got hot. The inductors are cheap enough. I know from experience that they (jantzens) can vary pretty good so if they offer matching pay for it so they are close.
I've bought inductors that possibly were mis-tagged. Should have been 18mh and came almost 22mh. That would make a big difference.

The Polk factory inductors were also well known for being off as well.

Usually the Polk inductors were very well coated with hot melt glue. Did you notice any or is that what was melted down into the wire?

Benjisan

pitdogg2 wrote: »

They seemed to be talking about the later model SDAs in the SDA Handbook, where the bass driver in the Dimensional Circuit was stereo in the very low frequencies.

This SDA woofer and tweeter do not work without the cable connected in those speakers. It is pre sub bass drive circuit IIRC. I'd replace both inductors that got hot. The inductors are cheap enough. I know from experience that they (jantzens) can vary pretty good so if they offer matching pay for it so they are close.
I've bought inductors that possibly were mis-tagged. Should have been 18mh and came almost 22mh. That would make a big difference.

The Polk factory inductors were also well known for being off as well.

Usually the Polk inductors were very well coated with hot melt glue. Did you notice any or is that what was melted down into the wire?

Yes, in the first picture from pg-1 you can clearly see the coating around the inductor had heated up enough to form a long tear drop style drip.

The hot glue holding the inductor in place (along with the 2-sided tape) didn't seem to be affected from the excessive heating.

By the way, do you have an opinion on using a higher value capacitor or the lower value capacitor if the exact same numerical value is not available.

audioluvr

Use the one with the closer value. What cap are you considering?

http://www.soniccraft.com/product_info.php/gen-sonicap-047-uf-600vdc-p-413

http://www.soniccraft.com/product_info.php/mundorf-mcap-evo-047uf-450vdc-p-4142

Benjisan

The Sonicraft capacitors look like pretty reasonable replacements.

I noticed that the SDA 1a schematic lists a 2.5 ohm and 2.7 ohm resistors as part of the crossover build.

My SDA 1a crossovers use 3.5 ohm and 2.5 ohm resistors.

What would the reason be for that and what kind of aural effect would be gained or diminished for the difference in the two resistor sizes?

Gardenstater

I'm sure we don't have all the intra model revision schematics. The one we have is dated 11/26/84 and says that the 3.5 Ohm was 4.0 Ohms. Also they deleted the fuses and added polyswitches #RDE050A. It appears to show 3.5 Ohms and 2.7 Ohm resistors. Yours has 2.5 instead of 2.7. That is a minor difference. It could even be from a different model of polyswitch in yours or they may have made a minor tweak at some point to the tweeter circuits. Your speakers had the SL2000s?