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Old 9th June 2012, 04:35 AM   #1
bushy is offline bushy  Australia
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Default Blown power filter cap

Hi all,
I've got a Mackie SRM450 active PA speaker which about 2 years ago blew it's fuse. I narrowed the problem down to being the main power transformer, by the fact that with the secondary side disconnected from the board, it still blew the fuse, but with the primary side disconnected, it didn't. I enquired about a new transformer, got told a price, picked myself up off the floor, and forgot about it for the next two years

Last week I decided that it was taking up too much space in my garage to justify not fixing it, so I ordered the transformer, put it in, and hey presto, it lived! I went into the next room to find somebody to share in my excitement, when I heard BOOM, and returned to find a room full of smoke and a smell not dissimilar to an extended fireworks display.

Turns out one of the main power filter capacitors had blown (and taken out the fuse once more, albeit not quite fast enough). So my next move is to replace all four filter caps, but I thought I should see if I can't get some answers to some niggling questions first...

-Is it likely that the capacitor just blew from having no use for couple of years and then being dragged out of retirement?
-Is it likely (or possible) that the faulty capacitor took out the original transformer?
-Is there anything else I should be replacing (or testing) while I'm at it? Anything that may have been damaged in the process, or something that may actually be the root cause of both the transformer AND the capacitor? I'll attach the relevant part of the schematic below.

I wasn't running it hard when it blew - it was in my lounge room, so it was actually at a very low volume, especially considering the use my SRM450's usually get. The cap that let the smoke out was C170.

Thanks guys, I appreciate any insights
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Old 9th June 2012, 06:25 AM   #2
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That's why clever guys invented DMMs. Before that people just plugged their gear in mains to check what would happen. I hope your new transformer survived.

I don't know the age of the amp but I would first check all parts in the power supply if they're OK, replace what's faulty and then replace all electrolytic caps anyway. Old news butt the caps are the most likely source for errors as they dry out and/or crappy quality is used etc. but repeating well known stuff is not necessary.

10.000 uF 50V won't hurt if they fit.
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Last edited by jean-paul; 9th June 2012 at 06:27 AM.
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Old 9th June 2012, 06:33 AM   #3
Mooly is offline Mooly  United Kingdom
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Very possible defective caps could damage (overheat) the transformer if they were drawing excess leakage current. You would know by the caps getting hot.

If there is any doubt then replace them. Transformers stand a lot of abuse. If they are overloaded it is the heat that kills them by melting insulation on the windings.
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Old 9th June 2012, 10:31 PM   #4
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50 V caps having to withstand 47.5 V idle... that's kinda tight in the long run. (Even if ratings for electrolytics tend to be "WV" = working voltage, not abs max.) It's not unusual for electrolytics to develop spots in the dielectric that cannot withstand full rated voltage in old age. This can lead to significant current flowing, and it's quite possible that this eventually resulted in the demise of the original xmfr.

I'd recommend 6800 F 63 V types.

Maybe fashion a bulb tester, too.
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Old 9th June 2012, 10:36 PM   #5
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10.000 uF 63V then.
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Old 10th June 2012, 04:46 PM   #6
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The synchronicity of this is startling. I began troubleshooting one of my pair of 450s Friday night after it had developed a hum problem. When I pulled the amp plate out, C170 & C171 (the 45V rail caps--the middle pair of the four-cap column) were slightly, but obviously, bulging on top.

The major limitation here are the cap diameters, and the clearance height between the top of the board and the cabinet recess the amp module fits into. There's 47mm clearance between the surface of the board and the cabinet wall. The stock 6800uF 50V 85-degree caps measure 25x42mm.

I put in some time yesterday running down replacement candidates. I started out wanting to install 10,000uF 63V 105-degree devices, but kept coming up against the very real physical limitations of this application.

What I finally decided on are some Cornell Dubilier 10,000uF 50V 85-degree snap-ins. 10-week lead time from Mouser, but I'm willing to wait. These are 25x40mm in CDE's datasheet (not "25.4mm" as Mouser declares on their product page).

If you want something right now, Mouser has stock on the 6,800uF 50V versions of the above. At 25x30mm, they'll fit easily.
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Old 12th June 2012, 09:56 AM   #7
bushy is offline bushy  Australia
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If it's any help to you, mine still has the sticker on top with the mackie part number...they'll probably sting you for them but they'll fit

I'm thinking a light bulb tester is a good idea too, but unfortunately for me, a genius politician who obviously has supreme knowledge on things like this outlawed incandescent light globes around two years ago on the grounds that they're not eco-friendly And believe me, I tried for days on end to find a way to get some for another project, and it was dead ends all round. Does anyone know if any other type of globe will work the same? I know compact fluro's/LED won't work, but what about the halogen style ones?
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Old 12th June 2012, 10:50 AM   #8
bushy is offline bushy  Australia
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...I mean, incandescent globes got outlawed in Australia, not the world in general
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Old 12th June 2012, 11:22 AM   #9
AndrewT is online now AndrewT  Scotland
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The halogen replacements for the incandescent should work.
They are both tungsten filament and both have the desired PTC characteristic.
I have not bought any of these halogen replacements. They are charging ~2.50 each, whereas the old style were ~0.50 each.
Do you have any working "old" incandescent bulbs around the house/in the garage/in the loft?
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Old 24th November 2012, 08:41 PM   #10
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Default So did the hum stop?

Quote:
Originally Posted by robovox View Post
The synchronicity of this is startling. I began troubleshooting one of my pair of 450s Friday night after it had developed a hum problem. When I pulled the amp plate out, C170 & C171 (the 45V rail caps--the middle pair of the four-cap column) were slightly, but obviously, bulging on top.

The major limitation here are the cap diameters, and the clearance height between the top of the board and the cabinet recess the amp module fits into. There's 47mm clearance between the surface of the board and the cabinet wall. The stock 6800uF 50V 85-degree caps measure 25x42mm.

I put in some time yesterday running down replacement candidates. I started out wanting to install 10,000uF 63V 105-degree devices, but kept coming up against the very real physical limitations of this application.

What I finally decided on are some Cornell Dubilier 10,000uF 50V 85-degree snap-ins. 10-week lead time from Mouser, but I'm willing to wait. These are 25x40mm in CDE's datasheet (not "25.4mm" as Mouser declares on their product page).

If you want something right now, Mouser has stock on the 6,800uF 50V versions of the above. At 25x30mm, they'll fit easily.
Just pulled apart my 1st gen srm 450 due to humming when using microphone, Noticed that those caps are slightly bulged on mine as well. Wondering how the replacements worked out?
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