Technics SU-Z55 transformer

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It's a thermal fuse. I can tell you that of the several dozens or so of th ese fuses I've seen fail over the years, not one of them was because the XF R ran too hot, nor did they fail because of any kind of short or inherent X FR problem. They just go for no reason.

As to whether or not to jump it, that's another issue. I have an Onkyo rec eiver that I use for my HT, and it's got a jumped out thermal. If you want to be safe, I guess you can epoxy a thermal fuse to the core or even stuff a small one between the windings and wire it in the primary:

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** AC supply transformers do not have regular fuses buried inside them. If they have a thermal fuse, then it is invariably wired in series with the primary and fitted in close contact with the winding. ** Seems you have access to the terminals on this "fuse" - which may be a temperature sensor or the like.

Do some more testing and see what you find, cos at the moment your story is full on contradictions.

.... Phil

** I wonder how you know that?

A thermal fuse in the primary is specified to open BEFORE damage happens to the windings of the tranny. In case of a "double insulated" device like th e Technics, it is the only user protection against electric shock hazard re sulting from a damaged insulation between primary and secondary of the supp ly transformer.

** An internal short or overload on the secondary would blow the AC supply fuse.

** There is always a reason.

eceiver that I use for my HT, and it's got a jumped out thermal. If you wan t to be safe, I guess you can epoxy a thermal fuse to the core or even stuf f a small one between the windings and wire it in the primary:

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** The device in the link is a self resetting type with a rather low operat ing temp of 75C - most thermal fuses used for this job one time only and rated at 115C to 130C.

.... Phil

Because bypassing them at the primary junction board allowed full and norma l operation. Every single I ever did this to never had any issue whatsoeve r, nor did they show any signs of overheating visually. They all ran cool a nd at full power after bypassing. If the fuses were rated to open that clo se to the normal operating temp of the XFRs, then they were either badly ma de and failed over too little hours, or spec'd incorrectly IMO. As to whet her other techs experienced situations where the AC XFR was indeed overheat ing by external loads or shorted turns causing the fuse to open, I certainl y couldn't say, nor did I. I only know that none of the ones I encountered were nothing more than nuisance failures.

Strangely, the transformers that I have seen overheat and burn in older 60s and 70s vintage electronics (both TV and audio) were *not* equipped with t hermal fuses. They just sat there and puked tar.

ating temp of 75C - most thermal fuses used for this job one time only an d rated at 115C to 130C.

Perhaps, and while other higher values can be selected, a XFR running at 11

5C would be hotter than I've ever seen one of these run at. I suppose one could try it and if it kept tripping, another value could be tried.

Update:

Of course the thermal fuse was buried, but terminals are accessible, otherwise I couldn't tell it was open. It's unlikely that a secondary dead short would take out the thermal fuse before the main AC fuse, so I just bridged the thermal fuse and the amplifier works perfectly. Can someone suggest a failure mode where the thermal fuse would blow before the main AC fuse? This amplifier has a double insulation AC input, so no earth wire. Its chassis has a ground post anyway, so it could be connected to some external equipment that is earthed. Thanks Frank

I would suspect that like other things (circuit breakers for example), over time with repeated heating and cooling, these either become more sensitive or don't respond at all to conditions. I have replace a few thermal fuses in transformers in the past as well. Replace it with another thermal fuse and move on. Thermal fuses are not expensive. From a liability standpoi nt, I always replace the fuse rather than just bypassing it.

Dan

I'm not familiar with thermal fuses used in transformers, but I have worked on a few toaster ovens and coffee makers that have thermal fuses. These fuses are typically attached to the hot plate or heating box. I believe, internally, they are made with a spring and a wax bead. When the wax melts, the spring moves and opens the circuit.

Given enough time (not sure if it is operating time or merely age), these fuses trip on their own. I have replaced them with new thermal fuses with the same temperature threshold and the unit works fine for many years (until the fuse gets old again).

--
G. Paul Ziemba 
FreeBSD unix: 
 7:46AM  up 108 days, 11:25, 11 users, load averages: 0.48, 0.62, 0.66

Yes, there is _always_ a reason. But any device of this nature is a wearing part, if only over a very long time. Essentially, they get tired and simply fail - especially if run close to the margins. So the reason may be as simple as old age.

This is not to suggest that simply replacing the fuse and call it done is the correct approach even though that may work for some while. Any blown fuse not for obvious and/or immediately apparent reasons requires additional diagnostics.

Peter Wieck Melrose Park, PA

G. Paul Ziemba prodded the keyboard with:

Some that I have seen are what looks like a blob of solder hung between two wires inside the plastic case. I once tried to repair one by using a soldering iron. It just dissolved into a blob on the iron tip.

--
Best Regards: 
                      Baron.

snipped-for-privacy@aol.com prodded the keyboard with:

Based on my limited experience with these embedded thermal fuses, I would say that the failure mechanism would be like that of a dry solder joint.

--
Best Regards: 
                      Baron.

** Which is as expected, given that the fuse is DESIGNED to open before any harm occurs to the insulation. It is never OK for one to open afterwards.

You are completely missing the point.

** What does it say on typical thermal fuses ? 120C or the like ?

That is way above the normal operating temp of most transformers.

** You cannot know that, unless you were able to check the transformer's internal temp at the time of the fuse going open. ** Not strange at all, those items used a safety ground conductor and so did not need a thermal fuse for safety.

The AC transformers fitted to "double insulated" items are designed to be very safe unless they overheat - the thermal fuse prevents that one remaining scenario.

Replacing thermal fuses is a dodgy practice and bridging them out is highly dangerous.

.... Phil

** Don't leave it like that - you have removed the most important safety feature in the whole amplifier. ** A sustained overload, high ambient temp or both will cause that.

** One way to make the amp safe is to fit a 3 core AC lead to it and remove the double square symbol.

.... Phil

Isn't the overload likely to blow the main AC fuse?

I'll do this. Thanks

Frank

cted

move

The only reason that fuse opened was nuisance fatigue due to age and therma l cycling. Trust me on this. Of all the ones I've ever seen fail, every o ne was on an older piece. Never saw this fuse fail on warranty jobs. The f ew XFRs I've seen that actually did overheat showed signs of discoloration or burning on the winding wrap or darkening of the clear coat. I'll bet yo ur transformer is as clean and bright as the day it went in.

I always add a C14 socket to my older two wire equipment even though I've h eard that it's not technically allowed to make such mods (something to do w ith UL certification, even if it's an improvement). It's a bit of work and assumes you have the room to hog out the rear panel. I have a box of C14 ty pe sockets with the built in noise filters that I scavenged from scrapped p lasma TVs that I've used space permitting, but I like the new smaller three pin setup LG uses. If you don't bury a thermal fuse in the windings, then I would add a three wire setup for the one in a bazillion chance that there 's a short or AC over-voltage that manages to get the windings hot enough t o compromise the isolation of the windings without managing to blow any fus es..

** Supply fuses in most audio amplifiers are normally sized to allow the po wer transformer to operate well beyond its long term capacity - so it can d o so in the short term. As a consequence, they do not protect the transform er from overheating. That is what a fitting thermal device is for.

The AC fuse will open under various fault conditions that cause enough curr ent.

.... Phil