Mission 720 refurbishment advice needed

[Tom mish 720]

Cheers Alan. So I must have to remove the secondary cap to access this gap? Seems to be a sealed unit?

[ajgriff]

The only relevant video I can find shows someone replacing ferrofluid in a KEF tweeter:

https://www.youtube.com/watch?time_c...&v=wtfqarUiFhw

From my memory of dismantling a SEAS H Series unit with a defective coil the process was quite similar. However, I'm not sure I'd risk it with your H211s unless the performance has very clearly degraded.

I may have posed more questions than answers in which case please accept my apologies.

Alan

[Tom mish 720]

Cheers for vid Alan. Actually might have answered my question. I've opened up the seas as attached and you can see its similar to the tweeter in the vid. My tweeter is definately bone dry but there is residue that needs cleaning and the gasket on the cover plate seems intact so I should be able to clean and refill. I'm thinking of putting a direct signal from my amp at low volume and not bass into it and check the before and after sound. Also will be a bit of guesswork as to how much fluid goes in although you can see a reservoir on the magnet around which the gasket meets the edge of. All a bit trial and error...

[Tom mish 720]

H211 opened up

[ajgriff]

Well done indeed Tom. Glad my ramblings weren't too wide of the mark. Audio grade ferrofluid seems to be quite expensive stuff and I have to say that I'm not entirely convinced of the benefits. It might be interesting to find out how the tweeters perform after the old residue has been cleaned away ie, without any ferrofluid.

Alan

[Tom mish 720]

Will do AB tests with cleaned and non cleaned and see how that sounds. Prob will get fluid when I order foams for mids so can do another AB test then...

[Diabolical Artificer]

Interesting, so some 720's have tweeters with ferrofluid in them, mine didn't having as Alan says H107's in. You can still get all the datasheet's for Seas vintage tweeters from the Seas website, it might have something about ferrofluid replacement.

Andy.

[Tom mish 720]

Im looking at 720 Foam kit from Greg Barnes in the UK. The foam for the big driver looks spot on measurement wise and the kit includes two foams for the KO40 mids too so each foam works about £7.50 each which is OK. Only issue is the mids foams are convex against the originals concave ones. Andy – you got convex ones from the NLs didn’t you? Anybody any views on concave v convex or is that Audiophool territory? Cant see any physical issue with fitting concave foam on the K040 as there should be clearance even with the covers on but I can imagine some slight sound difference with the way the nature of the foam allows the cone to move. Doubt I will be able to discern any diff though! Suppose I could do a test with a new convex foam on one and a repaired old foam on the other (one of the KO40 only has a slight split in the foam). Am I over - thinking it too much?!? I probably ought to spend more time on the PCB where a dodgy Cap could influence the sound more?

[ajgriff]

I very much doubt that the profile of the foam surrounds will make any noticeable difference to perceived performance. I'm not a fan of fancy (ie, expensive) crossover capacitors. As long as the existing caps are not leaky or significantly out of spec I'd leave well alone. If you concentrate on the physical condition of the drivers and the structural integrity & rigidity of the cabinets you won't go too far awry.

Alan

[Diabolical Artificer]

Mine are convex (KO40's) and sound fine. When i got mine they were in a crap state, so not having much cash I used watered down PVA wood glue to hold them together, they sounded ok to me. When I refoamed them I could hear a discernable difference, (improvement? They sounded "clearer") in sound for a day or two.

As Alan says check the caps with an ESR meter/cap meter and replace any out of spec ones with decent quality but don't spend mucho £'s on caps containing fairy farts. I have an ESR meter and cap bridge if you want to pop over and measure em to save buying said meters, your welcome, we could even listen to some music : )

A.

[Tom mish 720]

Thanks for your advice chaps - convex Mid range foams it is then. The PCBs are proving a pig to remove even with hot knife and hair dryer – I am worried I am going to snap the things or at least damage some components. What about if I just leave PCB in place then A/B sound check each type of speaker in isolation across the two cabs as I put them back? That would tell me if the caps involved in the path to each of the drivers are relatively OK across the two PCBs? Its a bit of a subjective test but presumably one speaker would sound “worse” than another if the caps (or other components) were compromised in one of the PCBs? (I guess if both PCBs caps could have gone equally out of spec and both produce a “poor” sound I wouldn’t notice!). On that subject, what do I need to be careful of when testing speakers? Are there any dangers using a single LH or RH channel signal from my amp direct to one of the speakers to test out the cab? (taking care with the tweeter of course). Also If I attach one speaker at a time back to the PCB and test are there any issues there?



Andy – “Listen to music”?!?!? Dont have time for that what with all the record cleaning/filing/fiddling about with components/adjusting decks/tracking down more records. Something not right there..

[Diabolical Artificer]

They do need some brute force to get them off, I would have left mine if it wasn't for the resistors being burnt out and the inductors hanging off.

If a cap is really way out of spec you'll hear it and if a cap is just a bit old and tired replacing will result in a subtle improvement probably. One way to check that all is well without ripping the Xover PCB's out is to use a sig gen and scope to check the cutoff frequencies. A PC based tone generator and analysis software like Audacity or Soundcard Scope will be able to run a pretty reasonable check if you don't have the test gear.

To run a check look up the old 720 thread of mine it has a PDF of the 720 brochure in it with the cutoff frequencies. Plug the line out/headphoness at a push from your PC into the back of the speaker cab then clip a test lead (3.5mm mono/stereo jack with bare wires/croc clips on tother end) onto the woofer wires say, plug this into your line in socket on your PC.

Enable/play a test tone sweep whilst monitoring the OP. If the filter cuts off at the right point, all is well. Or bring the cabs over and we'll bung em on the bench.

I pay two big lads to come round everyday at 4 o'clock to drag me away from knob twiddling and dial watching, whereapon they tie me to a chair and switch the stereo on for an hour : )

Andy.

[Tom mish 720]

Thanks Andy, that seems like a good plan. Ive spent the day faffing around touching up the cabs and cleaning up the drivers so am ready to go when the foams arrives. 5 way switch on the blink on daughters amp so that's my next job. Cleans up ok for a few days then iffy again. Will start another thread as i think it needs a new one. Actually sat down and played some music today too...