Loudspeaker repair: Tweeter problems, damaged crossover?

Benjamin Albrow

Estimable
Sep 24, 2014
3
0
4,510
0
I would greatly appreciate any advice on repairing my lovely vintage Mission 752 Loudspeakers on ebay. I bought a 2nd hand amp to drive them but after a few days it died. I then bought a new amp which started to switch itself off intermittently when driving them. so bought a multi-meter to hopefully find the problem before any further damage. Initially I thought the tweeters were blown as this model has a reputation for easily damaged tweeters if stored in damp conditions etc: These were in storage 5+ years before I bought them... However after testing I think it could be the HF crossover components (possibly?)

Speaker Specs:
4 BiWire connnecting posts: HF/LF
General resistance: 8 ohms
Tweeter resistance: 4 ohms

Here are some test results:

Low frequency binding posts: approx 7 ohms
High frequency binding posts: infinite ohms
Direct tweeter coil reading: approx 4 ohms

Does anyone have an idea what could be the problem? I'd ideally like to repair them but don't want to sink more limited funds into them without an expert opinion.(

I've seen original replacement crossovers on ebay for 30GBP (+shipping)

Many thanks!
 
I think your amps have reacted to a problem in the speakers.
You can remove the tweeters and connect them directly to the amp. Play them at modest volume since they are now getting a full range signal. If they seem OK then the crossovers may have a problem. You may be able to check some parts in the crossover without desoldering them from the others. A shorted or devalued capacitor or resistor could cause the amp to go into protection.
 
The resistence values seem correct for a 2 way crossover.

Frankly I think you need to try new speaker cable and another amplifier - borrow one to check them under a low load (low volume levels).

I think you will find you have the weird case of two dead or damaged amps in a row ... I know that sounds odd ...
 
I think your amps have reacted to a problem in the speakers.
You can remove the tweeters and connect them directly to the amp. Play them at modest volume since they are now getting a full range signal. If they seem OK then the crossovers may have a problem. You may be able to check some parts in the crossover without desoldering them from the others. A shorted or devalued capacitor or resistor could cause the amp to go into protection.
 

Benjamin Albrow

Estimable
Sep 24, 2014
3
0
4,510
0
Thanks for the advice Reynod and Americanaudiophile. I bought the 2nd hand loudspeakers with some high quality biwire speaker cable (one length was about 5m). Turns out this was the source of the problem. The insulation is clear polymer and on closer inspection there was some discoloration (yellowing) at some points. Seems unusual that a wire could stop operating as the wire looks intact inside but not risking the amp and buying replacement cable. Relieved as I was about to sell the amp for 20€ as defect!
 
Great result.

Ditch the biwire idea ... the theory and practical application of that idea is about as noticeable as a 20FPS difference between two gaming boxes running above 200FPS.

Negligible as tone wood.
 

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