Pundit
Post by paulsaints on Mar 11, 2017 9:42:40 GMT 12
How do I use multi-meter to measure capacitors?
I have picked up a Cambridge Audio R50 and some spare drivers but even when the T27's measure well they are not playing so I need to look at the crossovers and measure the capacitors.
Thanks.
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Pundit
Post by Gryffles on Mar 11, 2017 10:04:14 GMT 12
May just be easier to replace as they will need to be removed from the circuit to measure anyway.
Electrolytic or film Paul?
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Pundit
Post by paulsaints on Mar 11, 2017 10:49:43 GMT 12
I am not sure - here is a photo. And are you saying you can't check them as installed - they need to come out to check? Basically the coles super tweeter and T27 measure ok but are not playing so I am looking at the crossovers health. They share the black negative cable.   pictures of image
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Post by Owen Y on Mar 11, 2017 10:53:01 GMT 12
paulsaints - those are fine spkrs, big cabinets. cool '70s grille face design. Gryffles is pretty much right, you will need to at least lift one leg of ea cap in order to isolate & measure. But what to measure? (i) capacitance - may have varied over time, not necessarily silence a tweeter output (although this could kill the tweet). (ii) ESR - trickier to measure, but again, shouldn't prevent a working tweeter making sound. Caps can die of course (short, overheat) but also maybe unlikely with lspkr voltages. Old age? Yes, possible deterioration or failure. If you don't have Capacitance on your DMM, then you can use Ohms & the normal behaviour of a cap when connected, is to show a short, followed by ohms ramping up as the cap 'charges'. Suspect the 'blocking' caps attached to the T27 +ve leg/s - 2 x 4.5uF I believe (xover online). Check also the shunt coil attached to the T27 -ve terminal (lift one leg again) - it should measure a fraction of an ohm I'm guessing, not be either a short (0 ohms) or open cct. The T27s themselves should measure 6~8 ohms DCR, I'd think. Keep us posted.
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Post by Owen Y on Mar 11, 2017 10:59:10 GMT 12
PS. disconnect one leg of the tweeters too, when measuring - careful not to apply heat to tweeter terminals for too long (a couple of secs max should do it).
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Post by Owen Y on Mar 11, 2017 11:08:56 GMT 12
First make sure that you have good tweeters - lift & measure ohms as above. Then, depending on how much effort you want to put in, I would replace all caps of that age (measure first will give you an idea of variation from specif). You don't need boutique audio caps, but modern quality caps are much better now. The black Elcaps are electrolytics (non polar I believe) & the Wima would be a metallised filmcap (this latter could be still OK). This is what I found, seems to match: 
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Pundit
Post by Gryffles on Mar 13, 2017 15:49:01 GMT 12
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Pundit
Post by paulsaints on Mar 17, 2017 20:50:17 GMT 12
How did I get on? good question. When I realised I had to remove the crossovers and remove the capacitors to measure them I realised I was into a longer project. One speaker has the coles and the T27 not working, one has the coles working but the T27 not. Happily I have 2 spare T27's that measure well. The problem is that swapping in a T27 that measure well on the bench still doesn't create sound. (The T27's I have taken off also measure well on the bench). I have since realised A) that I need to practice my wire to wire soldering before having a second go, B) if this is a crossover/capacitor issue then I have a bigger project than just swapping drivers. I am quite happy about that but probably won't be making fast progress. When I do I'll report back. Meanwhile away assisting with family care after arrival of new grandchild. But ... no-one has actually explained how to use the meter. I have figured out that it needs to be on the Ohm setting, but I have little ohms, big Ohms and Kohms to choose from - what's the right setting?
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Post by Graham on Mar 18, 2017 8:36:32 GMT 12
Hi Paul
I have never had conclusive results when trying to measure capacitors. The readings never seem to be the same.
Being as how you have to remove them from the circuit to measure them anyway as mentioned, and they are usually readily available and cheap, I figure you might as well just replace them.
Cheers
Graham
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Post by Owen Y on Mar 18, 2017 14:01:48 GMT 12
But ... no-one has actually explained how to use the meter. I have figured out that it needs to be on the Ohm setting, but I have little ohms, big Ohms and Kohms to choose from - what's the right setting? Hi paulsaints - as mentioned above, you really need a Capacitance meter. If you don't have this on your DMM, then you can only gain an indication of the cap's behaviour using the Ohms meter - (i) this is easier with an analogue meter & (ii) you need some experience to interprete behaviour. Since the lspkrs are 40yrs old & they are good lspkrs (IMO) & the drivers appear OK, then yes, I would completely check over the xovers - the drivers are mostly disconnected anyway. And, as mentioned previously, you do not have to remove the caps to measure, you only have to desolder/just lift one leg to isolate it. (This also applies to the Rs & chokes.) PS. The problem is not necessary w the cap/s. PPS. Congrats on your new grandchild!
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Pundit
Post by paulsaints on Mar 20, 2017 14:06:56 GMT 12
Thanks for that. I see that the same setting on the DMM says "CAP". What value is that measuring in? farads??
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Post by Owen Y on Mar 20, 2017 14:53:39 GMT 12
Thanks for that. I see that the same setting on the DMM says "CAP". What value is that measuring in? farads?? The meter will usually show the units - eg. 'uF' (microfarads).
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Post by Owen Y on Mar 20, 2017 18:55:42 GMT 12
Take one component at a time, methodically. Happy to help, just ask. Study the above crossover diagram - focus initially on caps C5, C6, C7 because these caps are there to protect the tweeters from being cooked by the LF signal 
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Post by Owen Y on May 27, 2017 12:09:54 GMT 12
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Post by Owen Y on Jun 29, 2017 22:39:04 GMT 12
paulsaints - did you manage to get those R50s up & running. Beautiful vintage mid-70s(?) lspkrs: You can still get crossover upgrade parts for them. Big, at 1.080m high x 0.3 wide x 0.45m deep. I esp like the ultra cool looking grille design:  I know of the existence of another pair in Akld.
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Pundit
Post by paulsaints on Jul 7, 2017 14:27:09 GMT 12
Hi Owen - thanks for your continuing interest. They are still in the project phase. Life got very busy. I realised that the fitted drivers and my spare drivers where all ok, but that the crossover must be the problem area. I also had to face the fact that my soldering skills are cr...p. So I've decided that I am really committed to this as a project, enough to wait and find the time to up-skill on soldering before I get back into it. On the other hand if the pair in Auckland are perfect I could be persuaded to swap ... But really I want the project - no hurry - just need to learn to solder ten times better.
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Post by Owen Y on Jul 7, 2017 15:00:48 GMT 12
H paulsaints - by way of further encouragement here's a reasonably good guide to electronics soldering: Feel free to ask. Did you say that there is a 2nd pr of R50s avail to you?
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Pundit
Post by paulsaints on Jul 7, 2017 17:38:32 GMT 12
Thanks for the video - I need a hunk of hands-on time. I've got a pile of boards out of old VCR's to practice on.
I don't have a second pair - you mentioned a set in Auckland. -Paul
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