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Dynaco A-25 Tweeter Cuts Out


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Hi, 

I bought a pair of A-25s. They seemed to have never been opened before because when I was sussing the issue, the glue was all in place. I run a ST-70 and a PAS-3 (and Hafler 101).

One of the tweeters cut out. Thought it was blown. Opened cab. Tested all resisters / caps. They all are good. I tested re-soldered connections to speakers. Used the battery test. Everything checked out. Reconnected speakers. Tweeter worked. Then cut out. I repeated the process. Tweeter worked. Then it didn't. In fact, as I was typing this, it just happened again (thought it's been stable for a few hours).  Very frustrating because I can't tell what's causing the issue. Everything seems to work until it doesn't, and there's no obvious problem/weak spot. Then, one time, when it went out again, I did the battery test by disconnecting the speaker cab cables from the ST-70 to check that tweeter wasn't blown; it wasn't. When I reconnected the cables, it worked again. So now when it happens; I've been doing that process: disconnect cable from ST-70 > Battery Test > Reconnect Cable. And it always works. Until it doesn't. It was stable all day today, but as I typed this, it's happened 2x. This last time, as I was disconnecting the ground cable, the tweeter worked. So then I sat down type, and it went out again.  So I just disconnected and reconnected the cables without the battery test, but that didn't work. So I did the batter test, and, yep, the tweeter works again. For now. 

 

With my other speakers (KEF Q series), this problem never arose, so that confounds me further. 

 

Anyway, thoughts at all on what this could be? 

 

Thanks!

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On 2/6/2021 at 11:03 PM, Martin said:

A connection is probably loose either at the tweeter or crossover. Why not pull the tweeter out & see if it works properly outside the cabinet?  Just don't put low frequency signal through it. 

Thanks for this. That was my first thought, too. I haven't pulled it out as proposed, though that's a good idea, but I have re-soldered the connections at the crossover and at the tweeter end to no effect.

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12 hours ago, RickB said:

I would swap the left and right speaker Wire at the amp and see if the  problem follows the amp

You know, it's funny. I've not tried this, but I should've probably tried this first, rather than doing the more complicated tests as I've done! I think it might be the speaker wire. I just never tried it cause it didn't have that issue with the KEFs.

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On 2/7/2021 at 9:04 PM, RickB said:

I would swap the left and right speaker Wire at the amp and see if the  problem follows the amp

So I've done that now, and it does follow the speaker, so that means now I simply can't think of anything else it could be: all the resisters test fine. The cables test fine. The Cap tests fine. The wires from resisters/cap test fine. The speaker isn't blown, since it returns to full function after it cuts out. It seems to be a volume thing, but other than that, I simply can't think of anything else!

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On 2/7/2021 at 9:04 PM, RickB said:

I would swap the left and right speaker Wire at the amp and see if the  problem follows the amp

So I've done that now, and it does follow the speaker, so that means now I simply can't think of anything else it could be: all the resisters test fine. The cables test fine. The Cap tests fine. The wires from resisters/cap test fine. The speaker isn't blown, since it returns to full function after it cuts out. It seems to be a volume thing, but other than that, I simply can't think of anything else!

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For posterity's sake, in case anyone down the line runs into this issue, after testing all resistors and caps and wires and cables and swapping l/r connections, I simply couldn't figure out what was causing this issue where the tweeter would cut out for perhaps volume reasons but would return once I simply connected the speaker cable connection from the amp to a 9-volt battery and reconnecting the cable to the amp. The tweeter wasn't blown in any normal sense of the word; it would just simply cut out. And then restart like normal. Repeat. I still don't know what the issue is, but I resolved the problem by replacing the tweeter with another tweeter (found the same SEAS on ebay).

If anyone has any thoughts on this, I'd still love to hear it.

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  • 9 months later...

The problem is almost certainly the voice coil wire in the tweeter. I have had this, well a very similar problem with one of my A-25's. I have also had this problem with a few Klipsch speaker diaphragms as well as the Polk audio SL 2000 tweeter. I am not exactly sure how to relay the fix to you though. It involves taking the tweeter apart and finding the broken wire and soldering a small jumper across the broken voice coil wire or simply a tiny bit of solder. It requires very strong magnification (for me anyhow) to see where it is broken or finding the cold solder joint on the terminal wire that the voice coil attaches to and re heating it to make the connection good again. This does not always work and can ruin the bad tweeter, get my drift... Maybe you fix it, maybe you totally destroy it. What you did was the best approach. Replace the tweeter. Keep in mind that a cold solder joint on the crossover board can cause crazy stuff like this. When I check a speaker in its resting position with a meter I am measuring DCR or DC resistance. The speaker receives an AC power from the amplifier as it has to move in and out. When it moves in and out that is when this can happen making this hard to troubleshoot. My two cents worth anyhow. Oh to edit this one more thing. Speakers do not like DC power. It causes them to sit still and gets the voice coil very hot eventually burning the coil. be careful with the battery check thing.

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  • 3 months later...

I had this exact same problem with an A25 tweeter not working and found the broken connection in the voice coil wire where it was soldered to the tinsel wire. Dynaco glues the tinsel/coil wire connection down at bottom of voicecoil and it is a pain to get to and resolder/fix. I ended up damaging the voicecoil wire so much I had to unwrap the coil a revolution for each solder joint on the tweeter and feed the wire through dust cap onto front of tweeter to get a good solder connection. Now my tweeter has less voicecoil wire and lower ohms (about 5 ohms), but it works!  

Anyone else have experience with unwrapping a revolution or 2 of voicecoil wire and how much it affects sound?  I ordered a replacement used tweeter just to get the speakers back to original condition. 

9C0B0CFE-ACDC-4F5E-943F-AD6E2E28CC9B.jpeg

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