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Old 21st February 2012, 12:09 AM   #1
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Default THERMALLY FUSED TRANSFORMER

My harman Kardon HK990 amplifier suddenly dead. I suspect one of the transformers -the one which power preamp section- has failed because thermal FUSE is open. The transformer has a thermal FUSE that opens at 125ºC
I want to know if is possible to replace the fuse. I have read that sometimes fuse is inside the transformer and it is very difficult to change it. I have read too that I MUST DISCAR the transformer because it may not BE SECURE...
What to do? Discard the transformer? Open it and try to chage the fuse????
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Old 21st February 2012, 01:31 AM   #2
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The 'fuse' is generally buried in the primary winding of the transformer. This would require the rewinding of the transformer to ensure there is no internal short that caused it to fail in the first place.

I have seen many units that have taps with and without the fuse in the circuit, though I would not recommend using the unfused tap, especially if the fuse has opened already.

I'd suggest replacing the transformer if the internal fuse has failed.
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Old 23rd February 2012, 01:52 AM   #3
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Those fuses can fail from repeated thermal cycling rather than a fault condition, though as jk already mentioned you need to determine if there is a fault such as a winding or downstream component short causing excessive current.

If you can determine there is no fault, or you want to monitor the circuit when you next power it up to check operation, you can replace the thermal fuse and hope for the best that it's the only problem.

It is usually not difficult at all to change one, you simply take the plastic wrap off the primary winding, the fuse is usually right there at the top because it was machine wound all the way then the fuse added after that. Solder a replacement in, wrap a few layers of tape around it or put insulating material behind it, then put the outer tape back around the primary windings.

Of course replacing the transformer is the more common way and if the transformer is PCB mounted it could be difficult to access the fuse if other components are blocking your tools, but if the primary reads open on a multimeter you might as well try, OR substitute a different transformer mounted elsewhere and just jumper wire it into the circuit (also cut the secondary winding or trace from it on the dead transformer so your new transformer isn't loaded down by it.
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Old 28th February 2012, 04:06 AM   #4
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thermal fuses can be shunted by twisting its leads (if you can pry them) then insulating the leads from the transformer body. then, tracing the primary voltage and inserting the replacement fuse on one of the power lines-don't try to change the burnt fuse - simply short it out and insulate then fuse 'out'. be careful since you are working on the power supply.
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Old 1st March 2012, 02:42 PM   #5
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The pre-amp transformer must be quite small in comparison to the main transformer.
Replace it, it should be quite cheap.

If the internal temperature fuse has opened, then ask yourself, "why?".
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Old 23rd March 2012, 07:46 PM   #6
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I was able to change thermal fuse and put a new thermal fuse. Idid not see anything elese wrong. HK990 is working OK since them...

Harman KArdon repair service offered me the original NEW transformer. Total price is about 60 euros. Question is:

If everything is WORKING OK ¿What I have to do? Change the entire transformer or leave the actual, that is working OK?

There are two oppisite ideas:

a) If thermal fuse OPENS, transforme MUST be changed because transformers is not safe

b) Thermal fuse is here for protecting transformer, so, if fuse changed and everything OK, no need to change the entire transformer.

I thing this was a simple overheat, and notunig else. I did not see anything burned....

WHAT TO DO??????????
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Old 23rd March 2012, 08:03 PM   #7
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Quote:
Originally Posted by tiomiguel View Post
WHAT TO DO??????????
This is Diyaudio. I think everyone will agree with me when I say, toss it.
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Old 23rd March 2012, 08:41 PM   #8
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Assuming you inspected the windings and did not see any damage to the insulating coating nor heat damage elsewhere, AND that the replacement thermal fuse was rated for an appropriate trip temperature, I would not buy a new transformer.

Unless the last user of the amp was operating it in an abnormal environment with high ambient air temperature, very high load, or a shorted output (or very low impedance speaker load), odds are the thermal fuse blew merely because it had been subject to too many thermal cycles. Thermal fuses of this type will fail eventually even if the temperature never exceeded their trip point temperature rating.

So in summary, I would use the amp as is, keeping an eye on it and not leaving it unattended for a little while to be sure there is no undetected fault still present that is causing excessive power loss and strain on the transformer, AND I would measure the output voltages (again?) to make sure the voltage looks correct, to rule out the possibility of there being a shorted winding. However, thermal expansion can change the mechanical aspect of contact conduction, I would measure voltage both when it is cold, then wait for it to heat up then measure again.
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Old 23rd March 2012, 11:38 PM   #9
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I would spend 20 Euros on an IR thermometer! That way you can check the transformer properly. You would also have a useful tool for everything from checking your home for heat leaks to cooking the perfect dish.
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Old 24th March 2012, 12:19 AM   #10
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^ Ok, but what does it solve to know the temperature alone? If the current transformer is viable and is replaced with a new version of the same thing, it should reach the same operating temp under the same conditions.
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