Technics SU-Z55 transformer

[frank]

Hi all,

I have on the bench a Technics SU-Z55 with a faulty transformer.
First of all, does anyone have a copy of the service manual? I've
been able to only find the schematic.
The transformer appears to have a fuse or a thermal fuse buried
between the primary and the secondary windings. The primary itself is
not interrupted, but the buried fuse is interrupted.
I plan to disconnect all the connection to the secondaries and try
to power the transformer through a light bulb, that should tell me
if the transformer primary is shorted or something else happened.
A regular fuse just after the power switch is still ok, so I really wonder
what caused the transformer internal fuse to burn.
Ideas? Should the transformer have the correct voltages, what would you do?
Just bypass the open thermal fuse?
Thanks
Frank IZ8DWF

[ohg...@gmail.com]

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

As to whether or not to jump it, that's another issue. I have an Onkyo receiver 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:

http://www.allelectronics.com/item/thsw-75/thermal-circuit-protector-75-c/1.html

[Phil Allison]

frank wrote:

> Hi all,
>
> I have on the bench a Technics SU-Z55 with a faulty transformer.
> First of all, does anyone have a copy of the service manual? I've
> been able to only find the schematic.
> The transformer appears to have a fuse or a thermal fuse buried
> between the primary and the secondary windings. The primary itself is
> not interrupted, but the buried fuse is interrupted.
>

** 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.

> I plan to disconnect all the connection to the secondaries and try
> to power the transformer through a light bulb, that should tell me
> if the transformer primary is shorted or something else happened.
> A regular fuse just after the power switch is still ok, so I really wonder
> what caused the transformer internal fuse to burn.
> Ideas? Should the transformer have the correct voltages, what would you do?
> Just bypass the open thermal fuse?
>

** 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

[Phil Allison]

ohg...@gmail.com wrote:

>
>
> It's a thermal fuse. I can tell you that of the several dozens or
> so of these fuses I've seen fail over the years, not one of them was
> because the XFR ran too hot,
>

** 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 the Technics, it is the only user protection against electric shock hazard resulting from a damaged insulation between primary and secondary of the supply transformer.

> nor did they fail because of any kind of short or inherent XFR problem.

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

> They just go for no reason.
>

** There is always a reason.

> As to whether or not to jump it, that's another issue. I have an Onkyo receiver 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:
>
> http://www.allelectronics.com/item/thsw-75/thermal-circuit-protector-75-c/1.html
>

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

.... Phil

[ohg...@gmail.com]

On Tuesday, December 6, 2016 at 8:49:28 PM UTC-5, Phil Allison wrote:
> ohg...@gmail.com wrote:
>
> >
> >
> > It's a thermal fuse. I can tell you that of the several dozens or
> > so of these fuses I've seen fail over the years, not one of them was
> > because the XFR ran too hot,
> >
>
> ** I wonder how you know that?

Because bypassing them at the primary junction board allowed full and normal operation. Every single I ever did this to never had any issue whatsoever, nor did they show any signs of overheating visually. They all ran cool and at full power after bypassing. If the fuses were rated to open that close to the normal operating temp of the XFRs, then they were either badly made and failed over too little hours, or spec'd incorrectly IMO. As to whether other techs experienced situations where the AC XFR was indeed overheating by external loads or shorted turns causing the fuse to open, I certainly 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 thermal fuses. They just sat there and puked tar.

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

Perhaps, and while other higher values can be selected, a XFR running at 115C 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.

[frank]

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

[dansabr...@yahoo.com]

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 standpoint, I always replace the fuse rather than just bypassing it.

Dan

[G. Paul Ziemba]

frank <fr...@invalid.net> writes:

>Can someone suggest a failure mode where the thermal fuse would blow before
>the main AC fuse?

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

[pf...@aol.com]

On Tuesday, December 6, 2016 at 8:49:28 PM UTC-5, Phil Allison wrote:

>

> ** There is always a reason.

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

[Baron]

G. Paul Ziemba prodded the keyboard with:

> frank <fr...@invalid.net> writes:
>
>>Can someone suggest a failure mode where the thermal fuse would blow
>>before the main AC fuse?
>
> 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).

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.

[Baron]

pf...@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.

[Phil Allison]

ohg...@gmail.com wrote:

>
> > >
> > > It's a thermal fuse. I can tell you that of the several dozens or
> > > so of these fuses I've seen fail over the years, not one of them was
> > > because the XFR ran too hot,
> > >
> >
> > ** I wonder how you know that?
>
> Because bypassing them at the primary junction board allowed full
> and normal operation.
>

** 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.

>> If the fuses were rated to open that close to the normal operating
>> temp of the XFRs,

** What does it say on typical thermal fuses ?

120C or the like ?

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

> As to whether other techs experienced situations where the AC XFR
> was indeed overheating by external loads or shorted turns causing
> the fuse to open, I certainly couldn't say, nor did I. I only know
> that none of the ones I encountered were nothing more than nuisance
> failures.
>

** You cannot know that, unless you were able to check the transformer's internal temp at the time of the fuse going open.

> 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 thermal fuses. They just sat there and puked tar.

** 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

[Phil Allison]

frank wrote:

> 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.
>

** Don't leave it like that - you have removed the most important safety feature in the whole amplifier.

> Can someone suggest a failure mode where the thermal fuse would blow before
> the main AC fuse?
>

** A sustained overload, high ambient temp or both will cause that.

> 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.
>

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


.... Phil

[frank]

Phil Allison <palli...@gmail.com> wrote:
>
> ** A sustained overload, high ambient temp or both will cause that.

Isn't the overload likely to blow 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.
>>
>
> ** One way to make the amp safe is to fit a 3 core AC lead to it and remove
> the double square symbol.

I'll do this.
Thanks

Frank

[ohg...@gmail.com]

The only reason that fuse opened was nuisance fatigue due to age and thermal cycling. Trust me on this. Of all the ones I've ever seen fail, every one was on an older piece. Never saw this fuse fail on warranty jobs. The few 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 your 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 heard that it's not technically allowed to make such mods (something to do with 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 type sockets with the built in noise filters that I scavenged from scrapped plasma 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 to compromise the isolation of the windings without managing to blow any fuses..

[Phil Allison]

frank wrote:
>
> Phil Allison

> >
> > ** A sustained overload, high ambient temp or both will cause that.
>
> Isn't the overload likely to blow the main AC fuse?
>

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

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


.... Phil