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Old 6th April 2011, 08:43 PM   #1
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Default Acoustat 2-MH blowing fuses

Hi-
I've owned my Acoustats for almost 29 years, and still love listening to them. Recently, one began blowing the audio fuse- only twice so far since January, but this has never happened before, so I am concerned.

I did find a (hand-drawn) schematic online and am planning to open the box and look for any obvious signs of stress, but just wanted to check if anybody may have advice?

I did see one thread where "bear" said that the audio fuse can be bypassed..any thoughts on that? (My feeling is that given 27+ years of no-blow, something is amiss..)

(btw, I did try swapping L/R speakers to eliminate the amp/preamp as a cause.)

These were upgraded with Medallion transformers. Driven by an Amber Series 70 power amp.

thanks!
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Old 6th April 2011, 10:07 PM   #2
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Do you have the 3amp or the 5amp fuse? After lots of nuisance fuse blowing, Acoustat changed the fuses to 5 amp. Most people use the 5 amp (slo blo).

However, 5amp doesn't protect the speaker very well from playing loudly. You can still melt the transformer by playing loudly enough, just under the 5 amp limit, for long enough time. I myself have done this. It's even possible (though it wouldn't be easy) to blow the audio transformer with the 3 amp fuse also (though you'd possibly replace a lot of fuses first).

The fuse protects from extreme accidents (unplugging cables, for example) or amplifier failure. It can also serve as a warning that you are playing too loudly. But it does not perfectly protect from playing too loud for too long. The transformers heat up over time while you are playing loudly, and ultimately the insulation breaks down. In my opinion, there should also have been a thermal sensor, but possibly it wouldn't be easy to get it to work right.

Anyway, some people do go fuseless. You can once you know where the limits are and if you stick to them perfectly. But if, all of a sudden, you are blowing fuses, not particularly by playing much more loudly, you ought investigate a little further...

If the HV supply is weak, you will need much more amplifier power to get the same level of loudness. Perhaps that is the issue. The HV supplies do get weak over age.

Another problem might be subsonics and/or supersonics from your amplifier.

Finally, the worst case may be if you have already melted one of the audio transformers. It may work up to a point, and then short, causing fuse to blow. Then you'd have to get a new transformer. Replacements are being made.

On this same website there is a blog by The Acoustat AnswerMan or something like that, Andy Szabo, who worked at Acoustat during the Hafler era.

Last edited by charlesp210; 6th April 2011 at 10:12 PM.
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Old 6th April 2011, 10:35 PM   #3
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The power supply fuse?
The speaker line fuse?

Big difference.

I bypassed the speaker line fuse - but I also stated that this means that you knowingly assume some risk in favor of slightly better sound.

Check the DCR (with a DVM) by measuring across the two speaker in connections - no amp or cable on it. The two boxes should be rather close. If there is a LOWER DCR (ohms) on the one that blows fuses, then you may have a smoked xfmr... let's see what you come back with...

_-_-bear
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Last edited by bear; 6th April 2011 at 10:37 PM.
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Old 7th April 2011, 04:09 PM   #4
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I'm assuming it is the speaker line fuse- it was a 3A slo-blo. This is the only fuse accessible from the back panel, correct?

I only had a 5A Slo-blo to replace it, so that's what is in now. I'll check the DCR and report back.

thanks for the replies! I did see the AnswerMan blog, lots of good info there, too.

cheers!
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Old 7th April 2011, 07:24 PM   #5
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Quote:
Originally Posted by marks2l View Post
Hi-
I've owned my Acoustats for almost 29 years, and still love listening to them. Recently, one began blowing the audio fuse- only twice so far since January, but this has never happened before, so I am concerned.

I did find a (hand-drawn) schematic online and am planning to open the box and look for any obvious signs of stress, but just wanted to check if anybody may have advice?

I did see one thread where "bear" said that the audio fuse can be bypassed..any thoughts on that? (My feeling is that given 27+ years of no-blow, something is amiss..)

(btw, I did try swapping L/R speakers to eliminate the amp/preamp as a cause.)

These were upgraded with Medallion transformers. Driven by an Amber Series 70 power amp.

thanks!
If you have been using the same speakers and amplifier for many years, and presuming that your listening habits have not changed, then sudden fuse blowing should be a subject of concern. More than likely, one of your transformers (most likely the LF transformer) has developed a breakdown of the insulation, and is arcing on musical peaks. If this is the case, you will not be able to measure any difference with an ohmmeter, since the problem occurs only when the speaker is subjected to high voltage. If one or more turns of the transformer are permanently shorted, you might be able to measure a difference with an ohmmeter, but the difference might be very small and within the usual variance between units.

A suggested, you could have a problem with the bias supply producing a low voltage, but you would probably notice a diminished volume from that speaker at all times (you didn't mention that problem).

It is true that Acoustat increased the fuse value from 3A to 5A (both values are slow-blow type) due to ocassional 'nuisance' blowing of the 3A fuses. Although I usually recommend that most users can safely use the 5-amp fuse, I don't think increasing the fuse value is the best long-term solution in your case. If you have a failing transformer, increasing the fuse value will only hasten its eventual and complete failure!

By the way, I DO NOT recommend bypassing or removing the fuse. Whatever small improvement there may be in the sound, it is not worth subjecting both your speakers and amplifier to potential catastrophic failure. A fuse will not protect under all circumstances, but it will significantly improve your chances of avoiding damage to your equipment.

You can try a visual inspection of the interface guts, but you probably won't see anything. With speakers as old as yours, you will probably have a lot of soot inside (black dust attracted by the high voltage), which makes looking for burn marks difficult. But do look, and you might get lucky.

Transformers do occassionaly appear on eBay from seller 'Soundvalves', or complete MK-121 interfaces that you could use as a source for parts.

Good luck, and write again if you need more help!
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Old 7th April 2011, 07:38 PM   #6
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Default more suggestions

Since you have the Model 2MH, there is also a remote possibility there could be a problem with the woofer crossover or the woofer itself. If I remember correctly, the crossover components are accessible through the interface mounting hole.

Also, in my last post I mentioned the LF transformer and MK-121 interfaces. My bad. The 2MH was mated with an MK-131 interface,which used only one audio transformer per interface. Unfortunately, finding a relacement transformer for the MK-131 will probably be more difficult, as there were not that many of them made, and the MH series was discontinued long before the more common MK-121 went out of production.
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Old 8th April 2011, 08:07 PM   #7
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Thanks- this is great info.

Actually, I did notice the output was down on the one side, but was attributing that to my cranky preamp and rearranged speaker/furniture placement..long story, but I had to move this speaker away from the side wall, and my listening chair is off to one side. I'll listen for this when I get back home this weekend.

Ahh, my whole system is old and creaky (like me ), just hope it isn't finally giving out on me. These speakers have made me smile so many times...

Thanks again!!
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Old 13th April 2011, 09:19 PM   #8
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Well, I did take a serious listen this weekend- and the speaker is definitely down a bit in volume and clarity..it seems to be more muted, with much more HF rolloff than the other.

We are in the process of moving, once I am settled in the new place I'll dig into the box and see what's what.

Thanks again for the help.
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Old 2nd July 2011, 01:06 PM   #9
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Quote:
Originally Posted by bear View Post
The power supply fuse?
The speaker line fuse?

Big difference.

I bypassed the speaker line fuse - but I also stated that this means that you knowingly assume some risk in favor of slightly better sound.

Check the DCR (with a DVM) by measuring across the two speaker in connections - no amp or cable on it. The two boxes should be rather close. If there is a LOWER DCR (ohms) on the one that blows fuses, then you may have a smoked xfmr... let's see what you come back with...

_-_-bear
Yes sir, the best fuse for Acoustat's are NO fuse 27 years without no problems BUT use your brains.

PS: See the making of all steel Acoustat's 1+1s modified by Jocelyn Jeanson Quebec Canada in my Flickr galery.

http://www.flickr.com/photos/mracoustat/

Click the image to open in full size.
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Last edited by MrAcoustat; 2nd July 2011 at 01:10 PM.
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Old 30th July 2011, 10:14 PM   #10
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Dear Mark,

I had the same issue as the one you meet, some time ago, together with a loss in sound level on one of the speaker.
Since then, I changed the high tension caps (0,01uF - 6KV). The problem disapeared, and now, when I blow the 3A fuses, it is only when playing very very loud (rock' or women's lyrics at high levels).

Another possibility can be that your amp is also old, and might have some distortion issue. Acoustat are not very 'kind' for the amps. But this is only a guess.

Good luck,
Mathieu
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