I opened up the one that sounds muted - hoping that I wouldn't find a blown tweeter.
Found out why it is muted. The pot on the back has the wiper open-circuited and that is in series with the tweeter and its' cross-over cap. The tweeter was simply not getting any signal at all. The tweeter itself measures 3.9 ohm in-circuit. I measured the pot end-to-end and it returned 25.7 ohms. On the back is a number "25", so potentially it is a 25 ohm pot. It will be nice if I can get a functionally identical one or do some surgery on it to repair it. I'll draw up a circuit on paper and scan it in. Not a very complex cross-over. Tweeter is first order and so is the midrange (band-pass). The woofer is also first order.
Strange part of this is that the tweeter has a much bigger magnet than the mid-range driver although the mid might be alnico and the tweet seems to be ceramic. I visually had it backward until I looked at the cross-over. The cross-over is labeled as to which wire goes to the tweeter and which one goes to the mid-range driver.
Some observations on the construction and condition....
- It has all the original drivers which is good.
- The cross-over board is covered with a brown paper printed circuit drawing. It might be possible that this truely was a kit and that these were part of the diagrams and instructions on wiring the cross-over. It would be great if I could find the assembly instructions for a set of these speakers to confirm. I haven't seen such a basic diagram behind a factory assembled cross-over, but who knows?
- The speaker is an acoustic suspension design with no ports, but it is poorly sealed. This is both a design issue and a construction issue. This speaker cabinet is leaky! There is no sealant around the back panel and it is screwed in place, but ill-fitting. I won't need to be doing a push test to look for air leaks here. I'll have to put some thought into how to seal this cabinet.
- There were three 1" sheets of fiberglass as the batting that are the dimensions of the back panel. Not particularly inventive and I'm not sure it was good enough, but there isn't room for much more.
- The cabinet is so shallow that there is actually a 5/8" indendentation routed out of the back panel for the woofer magnet. I don't think the back panel would fit without the routed out depression in that panel.
- The corners of the cabinet are separating. Part of that is that the cabinet walls are starting to curve. It might be possible to use strong clamps and glue to pull it together again, but it will take some thought.
- Drivers are all screwed in from the inside. It is difficult to tell, but I don't think there is any sealant on the driver faces to the front baffle. (Just another form of air leaks.)
- I measured the two caps in the cross-over and my meter didn't return the labeled values. A 2uF cap measured as 0.7uF. The other 4uF cap measured as 6.2uF. They are both "Callins" caps with the red ends. These have to go away.
- Measured the DC resistance of the 3 drivers in-circuit. They were Tweeter=3.9ohm, mid=7.3ohm, and woofer=6.8ohm.
Here are some pics.