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Old 18th December 2010, 04:43 AM   #11
anatech is offline anatech  Canada
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Hi Trevor,
Shorted outputs will not blow that thermal fuse. That kind of damage will only take out the soft start resistor and possibly the thermal fuse on it. The only possible explanation for the thermal opening in that amp is a high core temperature. That means silly high dissipation for hours until the core managed to heat up to that point.

One of the tests for that amplifier involves running it just before clipping into a 2 ohm load. The only thing I've ever seen is the breaker opening on that dedicated circuit to my bench. No fuses ever went, nothing. Therefore, to succeed in opening the thermal, I can only imagine that the amp was run hot at a party (probably) for a few hours without proper ventilation. The interior chassis temperature would have been very close to the heat sink temperature with restricted air flow and the transformer would not have been able to lose the heat into ambient. Then the thermal fuse opens without any other catastrophic failures. The reliability of the amp may have suffered simply due to M.T.T.F. (mean time to failure) being accelerated due to high heat. Once the fuse opened, no other damage is possible if the amp is connected as it would be for it's intended use. Watch this, someone was using it to run a shaker table.

Even if the AC mains fuse was blown and someone tried the cigarette paper trick to short the mains fuse, the soft start resistor and thermal would have both died next. The only other way to damage anything electrically would be to introduce power through the speaker terminals or input jacks. I doubt that happened.

Test the output transistors for shorts, speaker output line to each power supply rail. You should read either open or a diode drop (then reverse the leads).

To open the thermal fuse in anything, you must get the power transformer core very hot, and that normally will take an hour to three hours of abuse. There is no excuse for this fault, and it took hours to fail. This shows intent or stupidity, take your pick. We used to see many New Year's Eve party victims with open power transformers. No smell. The way most of these people accomplished this was to direct air flow over the output heat sink(s). The thermal cutout normally does not operate because the heat sink temperature never rises to that critical level. Unfortunately, the power transformer has no additional cooling and it's internal temperature reaches the open temperature of the thermal fuse. Then, it's game over.

So you see, if the outputs or rectifiers failed, the primary fuse would open. Failing that, the soft start circuit would fail. After this happens, no other damage can occur until those circuits are repaired. I guess someone could have shorted that soft start circuit, but that implies they had access to the service manual. That means they know better. I know you can open these thermal fuses with greatly excessive current flow, but that takes some time and those other parts will fail first. The soft start resistor is the thing that normally opens in that case.

As for the power transformer, you're venturing in a direction that only has negative outcomes. There is no rush, it's only an amplifier. The time you spend would have been better served looking for a replacement, consider Hammond along with Plitron to see if they have any options for you. Check the cost for new while you're at it. You could have been calling around to PA companies, music stores or even service shops. A close replacement is very likely out there somewhere for you.

Stop messing about with that thing and use your time productively. You never know how close a dead 250 wpc amplifier may be. Sorry if I sound a bit harsh, I don't intend that. However, the work you are doing now is not useful in my view. I've seen too many amps (all brand names) come in for service where the owner has had the same idea as you do. More than once they had taken a repairable situation and made it into a replace only job. You are very close to that now.

Good luck with this, I really do mean that! I am wishing you success.

-Chris
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Old 18th December 2010, 03:24 PM   #12
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Thanks for the detailed and informative post Chris. If I don't have any success bypassing what I think is the thermofuse on the two white cloth covered leads, I'll definitely be looking at replacement transformer options next.
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Old 18th December 2010, 11:52 PM   #13
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Well sometimes ignorance and persistence pay off

I connected the two exposed leads which were covered by soft white cloth insulation, since I was pretty sure they were what put the thermofuse in the main winding, and quick test with a multimeter suggested I was right. Next I installed the transformer in the amp and tested the output windings with AC connected, and all seemed good. Then I hooked up all the connections, powered up, and after a few seconds the protection circuit clicked off, confirming operation.

I've now had tunes playing for about an hour, and all seems fine. The heat sinks are warm at about 38 degrees C, and it's sounding great

When time permits I'll re-cap, upgrade the adjustment pots, reset bias & DC offset, and do some critical listening.

Last edited by electrafixion71; 19th December 2010 at 12:10 AM.
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Old 19th December 2010, 12:47 AM   #14
6L6 is offline 6L6  United States
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Ohh! It worked! That's great!!

I have been following this thread from the beginning and am very happy that you have found success!

Please post more photos as your project progresses!
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Old 19th December 2010, 01:16 AM   #15
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Originally Posted by 6L6 View Post
Ohh! It worked! That's great!!

I have been following this thread from the beginning and am very happy that you have found success!

Please post more photos as your project progresses!
Thanks. Attached is how the transformer looked right before I connected the two wires to bypass the thermofuse. I basically connected points A & B by joining the wires in the white cloth insulation.
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File Type: jpg Transformer Thermofuse 2.jpg (437.9 KB, 121 views)
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Old 19th December 2010, 01:46 AM   #16
6L6 is offline 6L6  United States
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Are you going to re-pot the case now or just leave it as-is?
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Old 19th December 2010, 01:56 AM   #17
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Originally Posted by 6L6 View Post
Are you going to re-pot the case now or just leave it as-is?
I'm going to see if I can buy a small amount of potting compound just to "finish it" but I'm not sure if it's really necessary or not.

Attached is the picture of the amp and matching components running in my basement right now.
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File Type: jpg Nak System 7s.jpg (363.3 KB, 115 views)
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Old 19th December 2010, 02:15 AM   #18
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Air flow for the amp fins???
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Old 19th December 2010, 02:37 AM   #19
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Originally Posted by p. jordan View Post
Air flow for the amp fins???
It's been running for hours like this now, and the heatsinks are still at 38 degrees C.

If at a later date I hook up my power hungry / low impedance Thiel CS2.3 speakers, I'll make the amp has lots of breathing room, and watch it really closely.
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Old 19th December 2010, 02:50 AM   #20
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That's great! I have a Nakamichi PA-7, and a Bose 1801.
Both of these units get warm, and would like to know what
you use to measure the fin temperature.

Thanks,
Phil
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