Flap or fart sound at heavy bass

[bubba]

I have 2 Dynaco A-25s. When I put on a Radiohead LP or something with hard techno drums or sub bass I hear a crappy bass flap sound out of one of the woofers. Is the Woofer done? Voicecoil or cone? When I push on the cone I might feel a faint rub but barely or not much hard to tell. It is subtle. Woofer looks brand new.

 I am assuming this is a common problem. What can I do to fix? Can it be repaired? Crossover caps are original so are the woofers in both I believe. If I listen to the average LP with Bass UP I do not hear this. Just when I put an LP on with Harsh Electronic drums. THe other side does not do this and I have swapped speaker so I know it is not the amplifer. 2 dynaco Mk4's and Pas Preamp. Any advise appreciated.  

Also Side Question, Is there any benefit to changing the 5uF caps in the crossover? I have heard of electrolytics drying up but do these unpolarised caps really do that? Tweeders seem to sound fine.

 

thx

[RTally]

Often, a flapping or farting sound when playing heavy bass is due to the surround coming loose. I suggest inspecting the surrounds to ensure that they are still firmly attached all the way around to the cone and the basket. 

As for recapping, it depends. Generally, 40+ year old electrolytic caps have degraded. It is their nature. But the degree of degradation will vary.

I recapped one A25 in my pair and could not tell the difference when I A/B the speakers. So I did not bother recapping the other speaker (the woofer was also stuck to the baffle firmly and I did not feel like fighting it). The cap in my A25 was a silver can type and not one of the Temple caps known to fail.