Nakamichi PA5 Thermal Fuse

Buried DEEP in the transformer and appears open.

No continuity from the AC plug to the big R401 2.2 ohm power resistor.



Guidance?



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You can have it rebuilt, if you have no choice, and meanwhile try to take the bobbin off the core.
It has to sense either the winding temperature, or the core temperature.
Just short it out. And check if the winding is okay.
You checked the relay and so on, and the transformer out of circuit? That means directly, after removing all connections to the rest of the circuit on the primary side?
Failures are rare...


If needed, send it out for rewinding, it won't be that expensive.
 
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NTE 8167, $ 3 or so...


Limitor Thermal Fuse +121°C 15 A, 250V ac

RS Stock No. 797-6042
Brand Limitor
Manufacturers Part No. SF 119 R
HSN Code 85361050


Put the temperature and current rating, along with the voltage, search for thermal fuse.


The transformer windings are intact, right?
 
NTE 8167, $ 3 or so...


Limitor Thermal Fuse +121°C 15 A, 250V ac

RS Stock No. 797-6042
Brand Limitor
Manufacturers Part No. SF 119 R
HSN Code 85361050


Put the temperature and current rating, along with the voltage, search for thermal fuse.


The transformer windings are intact, right?

Thank you.
Yes, transformer is good.
Fuse on order, once I get that in I get to see what's on the other side that made it blow.
 
I get to see what's on the other side that made it blow.
Easy, and MUST be solved or you´ll make big damage, possibly catch fire and if unattended might burn your home down.

The 20W 2.2 ohm resistor is a soft start current limiter.

When amp is turned ON it limits charge current BUT dissipates a lot of heat (in fact way above the 20W rating) but no big deal if very short time.

There is an auxiliary circuit which after a couple seconds (no more) shorts that resistor with relay contacts.

IF that circuit fails (bad relay or relay trigger) 20W resistor overheats and will catch fire if left ON long enough, so the thermal fuse under it saves your home.

So "just" replacing it is not enough, personally I would replace the relay (peace of mind) and check trigger works.
Should activate relay coil a few seconds (no more than 5 seconds) at turn-on.
 
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Easy, and MUST be solved or you´ll make big damage, possibly catch fire and if unattended might burn your home down.

The 20W 2.2 ohm resistor is a soft start current limiter.

When amp is turned ON it limits charge current BUT dissipates a lot of heat (in fact way above the 20W rating) but no big deal if very short time.

There is an auxiliary circuit which after a couple seconds (no more) shorts that resistor with relay contacts.

IF that circuit fails (bad relay or relay trigger) 20W resistor overheats and will catch fire if left ON long enough, so the thermal fuse under it saves your home.

So "just" replacing it is not enough, personally I would replace the relay (peace of mind) and check trigger works.
Should activate relay coil a few seconds (no more than 5 seconds) at turn-on.


Thank you for explaining the circuit, I'll see if I can find a relay.



Any limitations/issues with using a Dim Bulb Tester on initial startup?
 
The circuit relay turns on for a few seconds, then off?
Can a NTC be used to get a similar action in place of the resistor?
J M Fahey is far more experienced than I am, please heed his advice.


Dim Bulb Tester should be standard procedure after PSU repair.
 
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FIXED! Thank for the guidance.
Installed the new thermal fuse and checked every output for shorts, all good.
Put a fire extinguisher on the bench and punched the button. MUSIC!

Checking bias, someone had it over 100mv (spec is 40mv) and it was HOT. Probably the reason for the open fuse.