SDA@ polyswith replacement

[mimarrero]

I am trying to replace the poly switches in my SDA2's because the tweeters turn off after awhile. Ken sent me 2 switches and an instruction sheet but the instrutions do not seem to match. I purchased mine in early 1986. I am facing the following problems:
1. After removing the sub-bass radiator I do not see the square circuit board mentioned in the instruction instead I see 2 identical circular boards. One connected to a large coil and one connected to the speaker wire input so I cannot tell which is the crossover network
2. The intructions talk about removing the wire harnesses but there aren't any. There are a few clip ons but that is all. Plus some of the other wires are soldered in so i am not sure what goes where.
3. Finally, I am not sure which item is the poly switch needing replacement. Does each speaker require 2 poly swiiches?

I would appreciate any advice and/or diagrams

Thanks

[F1nut]

Look at the new polyswitches, study them, then look at both circular boards and look for the matching device, that's your polyswitch. Only one per speaker.

[mimarrero]

Originally posted by F1nut
Look at the new polyswitches, study them, then look at both circular boards and look for the matching device, that's your polyswitch. Only one per speaker.

I've studied the new switches in detail and there isn't anything that remotely matches. The only differences between the circular boards is what look likes an extra capacitor and resistor on one of them.

[F1nut]

Which SDA 2 do you have? Pull out one of the mid drivers and look for the MWxxxx number.

[mimarrero]

Originally posted by F1nut
Which SDA 2 do you have? Pull out one of the mid drivers and look for the MWxxxx number.
MW6501

[F1nut]

Ah, that would explain why you can't find the polyswitches, there aren't any. You have a fuse on the binding post plate. Try removing them, cleaning the fuse ends and contact posts, then see if that fixes the problem. If not, try some new fuses.

Have you changed any of your gear lately? Are you turning the volume level up past halfway?

[mimarrero]

Originally posted by F1nut
Ah, that would explain why you can't find the polyswitches, there aren't any. You have a fuse on the binding post plate. Try removing them, cleaning the fuse ends and contact posts, then see if that fixes the problem. If not, try some new fuses.

Have you changed any of your gear lately? Are you turning the volume level up past halfway?

First, let me thank you for all the help.
Now, when you say binding post are you referring to where the speaker wires connect to the speaker? If so or not, how do i get to the fuses? I have looked where the speaker wire comes in but did not find any fuses. I even took off teh back plate. Please forgive my ignorance

On your other questions I have changed euipment but this problem has been around for many years. I am just now aware that I may be able ot fix it. It occurs at less than half the volume

[F1nut]

Hmmmm....The SDA 2 is suppose to have an external fuse mounted on the binding post plate, which as you quessed is where the speaker wires connect to. Your speakers should have either a fuse or polyswitch. Can you take some pictures of the binding post plate and the crossovers? Also, do you have 3 mid drivers and two tweeters or some other configuration?

What are the specs and brand of your power supply? Exactly at what point, using a clock dial, does this happen?

[BlueMDPicker]

Originally posted by mimarrero
3. Finally, I am not sure which item is the poly switch needing replacement. Does each speaker require 2 poly swiiches?

I would appreciate any advice and/or diagrams

Thanks

The SDA II crossover was modified (apparently on 7/1/85) to incorporate two changes:

a.) 3.5 ohm resistor on each board was replaced with a 4.0 ohm; and
b.) fuse protection was replaced with one polyswitch per board - part # RDE050A (two per speaker)

Tracing the circuit from the positive binding post, the first component should be the polyswitch on each board, followed immediately in series by the 4.0 ohm resistor. On the stereo array board that resistor has a 750 uF silver mica and 0.5 uF mylar capacitor in parallel with it.

If your speakers were manufactured before the boards were physically redesigned/remanufactured to accomodate the crossover design changes, the polyswitch might possibly be external to the boards - but the first component in series from the positive binding post.

Send me a PM with your email address if you'd like me to scan the schematic and send it to you as a file attachment.

Edit - BTW my 2B's have the circular PC boards as well. They are "sandwiched", with plastic connectors separating them, off the large coil which is secured with a through bolt in the cabinet back.

[BlueMDPicker]

While not your crossovers, here is a picture showing what the original polswitches looked like for color and shape:

[dorokusai]

The old ones can also be yellow.

[F1nut]

Good job Mike and looking at the schematic now I can see why they would need two polyswitches per speaker.

[ardvark1]

Since the polyswitch topic has come up, I have a question.
Why did polk move from fuses to polyswitches in the first place?
I would think they would be more likely to degrade the signal than a fuse would.
I am thinking of revamping the cross overs in my first gen SDA-2's that use the fuse set up.

Can anyone give me a good reason why I should change to polyswitches?
Also did they go from a 4 ohm to a 3.5 because of the polyswitch?

Mike, STILL WAITING FOR TWEETER'S TO GET HERE!

[F1nut]

Polyswitches can reset, fuses can't and I believe the polyswitch reacts faster. I'd have to quess that's why Polk went with them.

[BlueMDPicker]

Originally posted by ardvark1
Can anyone give me a good reason why I should change to polyswitches?
Also did they go from a 4 ohm to a 3.5 because of the polyswitch?

Bill,

If your early model 2's have fuses, I'd leave them intact.

Actually, the change was from a 3.5 ohm to a 4.0 ohm resistor when the polyswitch was added. Since the rest of the circuit remained unchanged, I can only assume the resistance change was for the polyswitch addition.

Mike

[ardvark1]

Thanks for the input guys, the fuses will stay!

I have another question, when I have them torn apart for awhile
I will be going to headphones to hold me over.
Doe's anyone have any personal recommendations?
I am currently looking hard at the Sennheiser HD 650 and the
Beyerdynamaic DT 880 which looks like a good set of cans for the money.