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Mission 770's rides again!

Arkless Electronics

Arkless Electronics

Trade: Amp design and repairs.
Or will soon anyway! One done and other not far off.
I've been promising myself to do this for ages and had said a few times here on pfm about round tuits....

Too busy with customers work to spend much time on them but thought it would be a quickie job anyway... An hour should do it, just got to re-fit the tweeters...
Famous last words.... I'd forgotten that I'd converted them to active... to reach the crossovers and put everything back as it should be means removing all the foam pieces... which are a tight fit and only only go in one way.. crossovers have nuts on inside and have to be removed "by feel"...

Then I found that the crossovers couldn't possibly work! They were just not electrically correct and could not work without either a wire link or a resistor being added in order for any signal to reach the treble part of it.

Much head scratching....

Found a couple of photos of the crossover on google in the end and the penny dropped about "ah yeah I removed those big black wirewounds to fully isolate the passive crossover when I went active!". Then recalled them being on top of an oscilloscope ready to go back... 20 odd years ago in another house...

No idea what happened to them other than I WILL have put them somewhere safe... so safe I can't find them... Anyway a thread on DIY audio mentioned them being 3.9R and that fits with what I remember.

I'd long been using better tweeters with them (Son Audax TW025A1) and recalled being not 100% happy with the HF output level, although damn close and better than standard 770's. I'd fitted an "L" pad to help things but this time I'm making it adjustable, temporarily at least. I have a bag of 20R 3W wirewound presets (rather like the ones in the JR149 crossovers) so I'm extending the wires where the resistor would be in the crossover to a couple of feet long and have them dangling out the bottom of the speakers where they will connect to the preset, with a further 10R or so fixed resistor across it. Hey presto... adjustable HF level! When I'm happy with it I can use a fixed resistor in the normal way of course.

So, spiders rescued and webs hoovered out, foam hoovered, white "alloy mould" removed from bass unit chassis and hoovered up, cones checked for free motion with no rubbing etc.. One done and the other not far off:) I'm hoping to get a listen before I go to visit a mate around 9 tonight...

Gawd knows how much "burning in" the woofers may need after not seeing elecktrickery for over 20 years.... or if 54 year old me will be as bowled over by them as 17 year old me was... Obviously I've heard a hell of a lot of other speakers, some definitely better than the 770's, since then!

I only actually owned a pair when I was more like 24 and they were my reference that beat all other speakers they came up against for at least ten years...

More to follow:)
 
Original* white-face version? A friend had a pair and I never quite made my mind up about them. Certainly good in some areas!

*IIRC there were actually two, they changed the tweeter position on the baffle slightly.
 
Tony L said:
Original* white-face version? A friend had a pair and I never quite made my mind up about them. Certainly good in some areas!

*IIRC there were actually two, they changed the tweeter position on the baffle slightly.
Click to expand...

Yep original white face ones. Apparently the very first ones had Chartwell bass units but mine have the SEAS units that they quickly moved on to... with "Patented dynamic damping motor system" according to the stickers on the bottom of them... I believe all sorts of detail changes were made without them getting a "mark II" status... which could explain the wildly differing reports one hears about them.

FWIW my only grumble with them was a bit of a "sting" to the top end around 6-8KHz ish, nothing that could be called blatantly too bright or poor treble overall but that could make a bit of a meal of poor early digital! On really good recordings it was basically not noticeable/added to the "air" of it.... The Audax tweeters much improved but didn't completely remove it and was always a bit concerned that it could be a break up mode/whatever from the bass unit as only a single inductor is the entire bass end crossover and a fair bit of HF must get through... the bass units are meant to have a built in roll off, Robin Marshall style.

I hope to have more easy control of the HF level with the "left dangling" HF level control I'm going to try them with now. I never got round to anything like trying Zobel networks on either tweeter or woofer back in the day either.
 
I had them many years ago - fantastic speakers at the time, though many reviewers did not think the same..
 
zippy said:
I had them many years ago - fantastic speakers at the time, though many reviewers did not think the same..
Click to expand...

News to me! I recall them getting rave review after rave review and one reviewer, Paul Messenger IIRC declaring them "the best speaker in the world"... mind you his previous champion was the Linn Isobariks!:eek::confused: There was all sorts around that would have thrashed both 770 and Briks...

I can't recall a single review that rated them any less than "4 out of 5 stars" or "seriously worth considering".
 
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Well I just managed to get them up and running before going out last night... although it made me 20 mins late. Only made sure all drivers were working. I got a bit of a listen late at night for half an hour or so and first impressions, with Leak St20 powering them (Beast of a MF A370 was used with them for most of my previous experience with them years ago), were kind of "not bad but no cigar"...

M Spendor BCII's are the speaker that supplanted the 770's years ago and on first listening I can see why, and will be expecting the Spendors to be back soon...

I'll give them rather more of an outing and try with SS power amps etc then report back...
 
It seems the tweeters I used all those years ago are still available! https://www.falconacoustics.co.uk/audax-tw025a1-hd12x9d25-hd12x8d25-tweeter.html

Yes they are ferrofluid and getting on a bit now... I bought them from Maplins in around 1993 IIRC...

I just blu-tak-ed them in place to avoid any cosmetic damage etc if I ever choose to replace the original tweeters to sell them... and that's how I put them back this time also...
 
Hmm... It seems the original SEAS tweeters were 6R... this explains why I used an "L" pad years ago which I presume I used to both drop the HF level a bit whilst presenting the crossover with 6R...

My suspicion is that the ferrofluid may have at least partially dried up and reduced the sensitivity of the Audax tweeters as when I tried them last night I set the HF level to a position where I expected it to need turning down a touch... but if anything I was left thinking it needed turning up rather than down from that point!

I've just adjusted the added level pots to match the bog stock original crossover, which I recall being rather "hot" in the top end years back when I fitted the Audax ones and to the extent that I made up the "L" pad after only about half hour listening to it.. I'm wondering if reduced sensitivity will mean it is about right now!? I'll have a listen...

I can see me installing REW and hours of frustration trying to learn REW and then optimise this.... and digging out my home made measuring mic from years ago...
 
Well even after half an hour listening I can report that basically that did the trick...or at least massively in the right direction... They're sounding very much like I remember now... a touch too bright but fast, dynamic, and "open" sounding (no doubt artificially helped by slight HF excess) plus easily go loud without any complaint. They seem a tad more efficient than the Spendors and even with the Leak St20 driving them I'm thinking that you'd need to be quite a headbanger to want much more power than this... says he who once used to drive these so hard with an A370 that the woofer dust caps were slightly warm to the touch!:eek::rolleyes:

Early days yet but I'm thinking that to have the correct output from the tweeter for it to properly blend with the woofer in the critical midrange then it has to be rather too "hot" in the 7-10KHz range... which would explain my memories of it having a touch of "sting" to the treble from yesteryear.
Image seems just a tad pulled to the right so maybe the tweeters have not aged evenly... lucky I "temporarily" fitted treble level controls...
 
I have the mission bug too - there are no less than 6 of 760 (4 special edition!) hanging around me. These do look the business though, not much in it between them and my triangle comete at first glance. The main driver looks the part too.
Thanks for the info on the 770s.
 
Arkless Electronics said:
News to me! I recall them getting rave review after rave review and one reviewer, Paul Messenger IIRC declaring them "the best speaker in the world"... mind you his previous champion was the Linn Isobariks!:eek::confused: There was all sorts around that would have thrashed both 770 and Briks...

I can't recall a single review that rated them any less than "4 out of 5 stars" or "seriously worth considering".
Click to expand...

Paul Messenger is quoted saying the Spendor BC1 is probably the ‘Best Buy’ in used speakers.
As for the 770, I found it rather ‘relentless’, but then I’m a BC1 owner...
 
Martyn Miles said:
Paul Messenger is quoted saying the Spendor BC1 is probably the ‘Best Buy’ in used speakers.
As for the 770, I found it rather ‘relentless’, but then I’m a BC1 owner...
Click to expand...

That must have been years later in Messengers career... But as you know I'm a big fan of Spendor BCI, BCII and SPI (all very similar). Back in the day.... he used Brik's in his own system and for a while moved to 770's and declared them better... I believe he moved away from the 770's after maybe 6 months to a year... it was along time ago to be recalling precisely what a reviewer said in 1982!

Yep I can go with someone finding the 770 a bit "relentless" due to that slight brightness I mentioned... They are actually quite similar sounding to Spendors, more alike than different if that makes much sense!? Going from one speaker to another can often lead to some head scratching due to VERY obvious differences but there is quite a similarity overall... They are more "ballsy", dynamic "PRaTy" than the Spendors but less refined and rather more ragged I would say. Certainly at the level I now have the tweeters set to I think they would be rather "in yer face" with a hotly recorded violin! Which is basically why after comparing the 770's and BCII's at length years ago I gradually moved to the Spendors as my main speaker. It's making a nice change and very nostalgic for now though!

If I can't fix that "tweeter needs to be set to give a sting in the top end for it to be right in the midrange and crossover area" issue then I reckon I will be going back to the Spendors... early days in experimenting as yet though....

Yer need to try them Spendors of yours with some less "pipe and slippers" amps to hear them properly!:D;)
 
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