Polk Newbie, need help with 7B's

Charles218

I have been collecting and refurbishing speakers for the past ten years or so, mostly AR, KLH, Advent, etc. I just came upon a pair of 7B's, they are my first Polk speakers. One of the speakers works well, while the other does not put out any sound. After opening it up, I was immediately stuck as the crossover is quite different then any that I have ever worked on. Could someone help me with getting into the crossover? It appears that there are a couple of circular boards that are snapped together, do I just pull up on the upper board until it snaps free? I just want to make sure that I don't cause any damage to it while trying to get at it.

F1nut

At the top of the standoffs are two angled pieces, pinch them together while lifting up on the board slightly until all the standoffs are free, then lift the board completely off.

westmassguy

F1nut wrote: »

At the top of the standoffs are two angled pieces, pinch them together while lifting up on the board slightly until all the standoffs are free, then lift the board completely off.

I sometimes find it easier to remove the standoffs from the bottom. This is especiallty true with the newer, larger SDA boards. Some of the standoffs are inaccessible from the top, due to being sandwiched between other components. Use a pair of needle-nose pliers, and gently squeeze the bottom prongs together and push the stand-off upwards.
Charles218, be very carefull with the crossover, when the two parts are separated. Theres a single solid wire coming up from the coil that's soldered to the bottom of the circuit board. If you move the two parts around too many times, the wire and solder point can become weakened, and sometimes breaks off. If that happens, you'll have to clean a new section at the end of the coated coil wire, and resolder the connection.

StantonZ

The standoff's are the easy part! You need to decide if you want to remove/desolder the six wires attached to the bottom; otherwise you have to go in from the front and unhook them from the tweeter/driver. In either case, I don't think you can get around at least removing the (+) and (-) wires from the PCB if you want to do any real work on it (like remove a bunch of parts).
Good luck!

Charles218

Thanks to each of you for your timely and helpful responses. I wanted to make sure that I didn't mess things up, but after I got into it I could see that it was quite straight forward.

Peeking inside didn't help me along with the problem of no sound. I did the battery trick on each of the drivers and they appear to work. I was hoping that I would find something obvious that would cause both drivers to be silent. I have always replaced the capacitors when I work on vintage speakers as I know that they go bad. Is it possible for a bad cap to cause both drivers to not function?

I'm not very experienced when it comes to circuitry, but can check for continuity, resistance, and a few other basic procedures. Does anyone have any suggestions on what I might look for?

Charles

F1nut

Start at the binding posts and trace the circuit step by step until you find the problem.

westmassguy

F1nut wrote: »

Start at the binding posts and trace the circuit step by step until you find the problem.

^^^^^^^^^^
This
If the tweeter AND woofer are not functioning, it should be quite easy to find the break in the circuit

StantonZ

It's got to be something where the (+) or (-) inputs come into the board. I had a "no bass" problem after I rebuilt mine and it turned out I had lifted the (+) pad when I was removing parts. Since you can't really "replace" the pad, I soldered the lead to another wire/pad on the same node.

Charles218

Thanks for the help with this, though now the issue is becoming murky. I am beginning to get mixed results and need to continue working on it. Earlier today, I was able to get the woofer working in the problem speaker, this after I replaced the 34 mfd capacitor. So this is a work in progress, made frustrating as I have so little time right now to devote to it.

Charles

StantonZ

Do you have an Ohm-meter (they can be had on the cheap @Radio Shack)? The circuit is simple enough that you can "Ohm out" each component/node. You've already demonstrated that either:

1) you had a bad cap
2) something wasn't making contact or you had a bad connection (solder)

Continue through the circuit (as F1nut suggested) "Ohming out" each node; if that all makes sense, then you have another bad component (probably one of the other capacitors).