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Old 1st Jun 2020, 6:17 pm   #1
Tom mish 720
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Default Mission 720 Xover help.

Hi. Replaced the foams and tweeter oil on my 720s with help from this forum about a year ago however.... One of the cabs has just stopped working and a bit of basic testing and swapping fuses narrows it down to the xover (I think). I've no experienced with xovers but understand I can only test components if I desolder one end? Looking at the photo is it a good bet that's it's the resister (?) in the foreground that has gone? Looks Brown/leaking underneath? Any tips on what I should try next would be appreciated. Thanks. Tom.
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Old 1st Jun 2020, 8:14 pm   #2
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Default Re: Mission 720 Xover help.

The coils are virtually indestructible.
The power resistors are robust and easily checked with an ohmmeter.

It is those &^%$!!! bipolar electrolytics which are the most likely problems.
Maybe you want to check them with an esr meter, Or maybe you just replace them anyway. You might choose non-electrolytic types, at least for the smaller values. Depends on space.

Have a check for dry joints on the print side. It's not unknown, but the electrolytics do have a hard life and do have a limited lifetime.

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Old 1st Jun 2020, 9:21 pm   #3
Tom mish 720
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Default Re: Mission 720 Xover help.

Cheers David. Suppose those capacitors are 45 years old so yep, should prob all be changed anyway. I assumed if one had failed then only one of the speakers would have failed? I got an abrupt nothing from any of the 3 speakers so assumed it was that first resistor coming off the fuse. Will get a meter and do some testing...
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Old 2nd Jun 2020, 8:29 am   #4
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Default Re: Mission 720 Xover help.

The green 4R7 resistor drops voltage to the mid and tweeters, it runs hot so burns the board, see pic of mine . It's odd that you have no sound out at all, no one component on the Xover AFAIK will do that, only a loose wire or ground will.

If you get stuck am not far away, your welcome to pop over. It's about time I had my Xover out and checked the caps ESR and capacitance, Andy
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Old 2nd Jun 2020, 9:26 am   #5
Tom mish 720
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Default Re: Mission 720 Xover help.

Thanks Andy. Interesting that i should be getting some sound if a cap or resistor has failed. I will check connections, solder joints and try bypassing the fuse carrier first then..got to be the problem surely...?
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Old 2nd Jun 2020, 10:09 am   #6
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Default Re: Mission 720 Xover help.

No sound at all would focus attention on common connections from the input terminals to the point where the signal fans out to the different sections of the crossover.... and the matching ground returns as well.

UNLESS there is more than one fault. Could a tweeter have failed and not been noticed for a while until something else packed in?

Have you tried other speakers on the end of the cable from the amp?

It's always worth double checking things before tearing anything apart, but those electrolytics are due for some attention just from expected life and the slow chugging along of the calendar.

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Old 2nd Jun 2020, 10:27 am   #7
Tom mish 720
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Default Re: Mission 720 Xover help.

Yes, swapped the speakers over to test so it's definitely this Cab thanks. It's possible that I would not have noticed the tweeter or mid drop out on one of the cabs previously but less likely. Agreed, caps prob all need changing as will be way beyond normal life span. Am researching but seems to be a limited range of electrolytics available to match the existing and modern caps are much larger and the big ones expensive.
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Old 2nd Jun 2020, 10:42 am   #8
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Default Re: Mission 720 Xover help.

As David says, it is highly unlikely that capacitor failure would stop all sound output. If you do not have a Digital Multi-Meter then I suggest getting one as you need to do some continuity checks. If you have been running them very loud you might have burnt out the speaker coils.
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Old 2nd Jun 2020, 12:13 pm   #9
Tom mish 720
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Default Re: Mission 720 Xover help.

Problem solved thanks chaps: Joints looked ok but started resoldering a couple this side of the Xover and the - to the fuse carrier was the problem. Up and running now. Lesson learnt, will get a multi meter and test continuity first. All speakers behaving now so assume caps functioning to some degree however point taken about age so i will see what I can get as replacements. Any tips on uk suppliers? Falcon seem to come up on Google...
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Old 3rd Jun 2020, 6:50 am   #10
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Default Re: Mission 720 Xover help.

That's good, glad you sussed it. Falcon and Hifi Collective deal in audio caps & posh caps as well other stuff. The lads at Hifi Collective are helpful too, but prices tend to be on the high side as they're only little firms. The big 3 will be cheapest, Mouser, Digikey & Farnell.

Problem is your looking for some odd value's and if you replace like for like then you'll be after non polarised electrolytic's, not easy to get and the big 3 don't really carry em as they're speciallist. If you use other cap types like plastic film that makes things easier. As regards value you don't need to use the value Mission did as they're paralleled, so as long as you use either one cap of the correct value or two caps to get the right value it doesn't matter, as long as ESR is low that is.

Now here comes the big problem, you really want to be using 1% caps if poss or if not you need to be selecting them by measuring em, why? cos they're part of a filter and there are two sets. Not 100% essential but whilst your at it you may as well do a good job.

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Old 3rd Jun 2020, 9:14 am   #11
Tom mish 720
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Default Re: Mission 720 Xover help.

Cheers Andy. A quick look at hi fi collective comes up with: (existing and Mundorf like for like replacements)

6uF 50v (Mundorf E Cap Plain 5.6 uF)

16 uF /50v + 25 uF /50v in parallel (E Cap raw 33 uF +E Cap Plain 8.2 uF in parallel) 4.5

60 uF /50v (Ecap 56 uF raw +3.9 uF Plain in parallel)

16 uF /50v (Ecap Raw 15 uF)

10 uF /50v (Ecap Raw 10 uF)

I make that about 50 quid for both cabs (assuming I can fit them on the board)

There are also
Jantzen Bi-polar Electrolytic – Bit cheaper

And non electrolytic:
Jantzen Poly Cross Cap
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Old 3rd Jun 2020, 11:36 am   #12
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Default Re: Mission 720 Xover help.

It's your money, and you're free to do as you wish, but I would gently advise you to not be tempted by "posh" capacitors - the standard "Elcap" types will be absolutely fine - https://www.falconacoustics.co.uk/al...itors-50v.html

For non-electrolytics, industry-standard polypropylene types work perfectly. For example, https://uk.rs-online.com/web/p/polyp...itors/7270041/

Resist the temptation to replace electrolytics with non-electrolytics because it could change the behaviour of the crossover. Designers would have taken the higher ESR of electrolytics into account. Unfortunately, "internet wisdom" doesn't always realise that crossovers can be a delicate balancing act.

Ultimately, posh capacitors are fitted for marketing reasons, not for real performance gains. Most "posh" caps sell to DIYers or "expert" tweakers - many of whom, shall we say, "have a little knowledge", or are concerned with their standing in the hi-fi lunatic fringe...

All that said, I would want to test the original capacitors first. Yes, they are old enough (and of a type) to be treated with suspicion, but if they have degraded, your speakers will sound different afterwards - and I'd like to understand the effect of each. Cheap Chinese component testers (around a tenner) will report on value and ESR, and are well worth investing in.
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Old 3rd Jun 2020, 11:51 am   #13
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Default Re: Mission 720 Xover help.

Quote:
Originally Posted by mhennessy View Post
Ultimately, posh capacitors are fitted for marketing reasons, not for real performance gains. Most "posh" caps sell to DIYers or "expert" tweakers - many of whom, shall we say, "have a little knowledge", or are concerned with their standing in the hi-fi lunatic fringe...
They sell mainly into a market of people not excessively burdened with engineering mathematics, or people who wish to show off what expensive things they can afford, while implying that their level of perception is exalted far above the norm.

Some boutique capacitors are perfectly fine and of good quality. I've used them when other parts of the values and ratings I wanted weren't available, or not available in small enough quantities.

I'm not so worried about the effects of replacing electrolytics with non-polarised parts. Sure, the ESR will be somewhat less, but this can be corrected by adding a small value but robust resistor in series. The advantage is that these parts should last indefinitely. I hate having to repeat a repair job (Quoth he waiting for someone to deliver a new tailgate switch for the range rover... its third replacement!)

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Old 3rd Jun 2020, 12:48 pm   #14
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Default Re: Mission 720 Xover help.

Yes, they're mostly fine in all areas apart from price and marketing I've seen quite a few that are re-labelled standard parts.

In many cases, the change in ESR will be irrelevant. But occasionally it will be. If I ever get the time, I'll repeat something I did in the 1990s to a Rogers LS7 - now that I have a measurement mic, it'll be interesting to see if I can measure what I'm convinced I heard at the time (after a couple of weeks the polypropylene caps came out and the original Elcaps went back in!).

But without the means to measure, how do you determine the effect of the new caps, and how can you be sure you've restored the speaker to its original specification? It's a minefield
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Old 3rd Jun 2020, 1:04 pm   #15
Tom mish 720
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Default Re: Mission 720 Xover help.

Advice noted thanks, I've got no problem with spending as little as possible! Falcon Alcaps seem to fit the bill- a combo of the various electrolytics should do it as follows:
6uF = alcap 6uF 50v
41uF = alcap 2x20uF 50v
60uF = alcap 60uF 50v
10uF = alcap 10uF 100v
16uF = alcap low loss 16uF 50v
All comes in about 15 quid worst case for 2 cabs - more like it.
Yes, cheapo meter on its way so I would test any caps before I changed. Question is if its not way out of spec do I change based purely on age and assumption that it can't last as long as a new one...
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Old 3rd Jun 2020, 2:22 pm   #16
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Default Re: Mission 720 Xover help.

"Question is if its not way out of spec do I change based purely on age and assumption that it can't last as long as a new one... " Complicated, a tricky one, and down to the application and owner/tech/customer and quite a few other variables.

If we have a coupling cap in a vintage valve amp that tests fine but if it it fails could cause damage, it's probably best to replace. The amps on the bench anyway, if not replaced now it may never get done before failing and causing damage to expensive components. So an easy one yes?,Replace it - however the owner wants to keep the amp original, so back to square one.

In your case a cap is unlikely to cause mega damage if it fails but you've got the speakers apart, ten caps are about the same price as 20 after postage, why not change em, then you know they'll be good for a few years more, they'll probably see you out.

BTW, how did the wire on the fuse fail? Snapped due to vibration or bad solder joint?

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Old 3rd Jun 2020, 4:12 pm   #17
Tom mish 720
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Default Re: Mission 720 Xover help.

Must have been a bad solder Andy. Looked dull ish but i was still surprised. Having said that none of the solders look to have been wiped at manufacture and they all look like theve been done by me...i.e not very well. Every one has Flux residue and it's crap they way they then just hot glued the xover to the back cab wall. Ive drilled and screwed the xover back on with a bit of foam behind to damp any cab vibrations in future. I love these speakers but they were really banged together quickly and they look shoddy inside. Guess they were built at a cost and for a purpose.
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Old 3rd Jun 2020, 7:42 pm   #18
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Default Re: Mission 720 Xover help.

When the Cambridge R50 was first reviewed, Ralph West's impression of the first pair he was sent was so bad that he called the manufacturer straight away - the top end was positively shrieking. Tests on this sample showed a treble peak in the order of 12dB, and the cause turned out to be capacitors of the right value but of different type fitted in the crossover because of supply problems. When the specified components were fitted, the unit behaved as expected. I can't remember the exact details without looking it up, but I think the design used non-polarised electrolytics and the substitutes were polystyrene. Anyway, it shows that secondary characeristics can sometimes cause problems in crosovers.
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Old 3rd Jun 2020, 10:56 pm   #19
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Default Re: Mission 720 Xover help.

They did make wound polystyrene capacitors up to some large sizes. I used to have one in a junk drawer I'd inherited. 0.22uF I think it was, but with only twin lead-out wires from each electrode, it had horrible resonances.

I suspect it may have been resonance rather then just low ESR, Ted. 12dB is a lot more than I'd expect ESR to have created and the peaky nature would fit.

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Old 3rd Jun 2020, 11:36 pm   #20
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Default Re: Mission 720 Xover help.

Makes sense. It does seem a bit lively just for ESR. I'll look up the review and see if there's any more detail.

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