Hi,
My sweet Mcintosh MC225 just blew something out inside upon powering it up. The amp fuse was intact, but the fuse in the fuse box of the house was blown out. I opened the amp up to see what happened. Turned out to be the electronic part that is connected in line between the mains input wire and the power transformer. I have never seen such a part before, so I decided to ask people around here for help. Even though I am not an electronic scientist, I am pretty handy with soldering iron, so I can do basic things myself.
The part itself looks is a black (carbon?) disc about 2 cm in diameter and about 2 mm thick, with two wires soldered to each side of it. What happened was – one of the wires that are soldered to that black disk has come loose. There was faint smell of burned rubber inside. I looked in the schematics of the amp, it seems to be some special resistor, as it is marked by the symbol of resistor ("saw teeth") in a circle, with letter T within the circle under the resistor symbol. It is marked as R 45 in the schematics (which again indicates it as a resistor) and „>25 Ω cold” (at least it seems like Ω, as the schematic is a bit unclear). I guess it’s a thermistor, but I’ll better ask than be sorry, because as far as I know thermistors are marked differently - symbol of resistor crossed by a slant L shape line, no circles around.
What does it mean „>25 Ω cold”? That it should be of over 25 ohms while cold, and as it heats up it drops the resistance down?
Probably the part is of 1960’s and not available in original version. Any specific considerations when picking a current alternative?
Any advice appreciated very much.
Thanks!
My sweet Mcintosh MC225 just blew something out inside upon powering it up. The amp fuse was intact, but the fuse in the fuse box of the house was blown out. I opened the amp up to see what happened. Turned out to be the electronic part that is connected in line between the mains input wire and the power transformer. I have never seen such a part before, so I decided to ask people around here for help. Even though I am not an electronic scientist, I am pretty handy with soldering iron, so I can do basic things myself.
The part itself looks is a black (carbon?) disc about 2 cm in diameter and about 2 mm thick, with two wires soldered to each side of it. What happened was – one of the wires that are soldered to that black disk has come loose. There was faint smell of burned rubber inside. I looked in the schematics of the amp, it seems to be some special resistor, as it is marked by the symbol of resistor ("saw teeth") in a circle, with letter T within the circle under the resistor symbol. It is marked as R 45 in the schematics (which again indicates it as a resistor) and „>25 Ω cold” (at least it seems like Ω, as the schematic is a bit unclear). I guess it’s a thermistor, but I’ll better ask than be sorry, because as far as I know thermistors are marked differently - symbol of resistor crossed by a slant L shape line, no circles around.
What does it mean „>25 Ω cold”? That it should be of over 25 ohms while cold, and as it heats up it drops the resistance down?
Probably the part is of 1960’s and not available in original version. Any specific considerations when picking a current alternative?
Any advice appreciated very much.
Thanks!
From your description I would think it is an NTC thermistor, commonly used as surge protection for transformers. Large power amplifiers have an NTC and a relay contact on a delay.
It is 25R when cold and probably 1 or 2R when hot.
Something like this; PTGL12AR270M9C01B0 | Murata 27Ω 2300mW PTC Thermistor, 14 Dia. x 6.5mm | Murata
It is 25R when cold and probably 1 or 2R when hot.
Something like this; PTGL12AR270M9C01B0 | Murata 27Ω 2300mW PTC Thermistor, 14 Dia. x 6.5mm | Murata
Thanks!
One more question:
The Mcintosh amp is 110 V, and I feed it through a step down transformer 220-110. Are thermistors voltage specific? In descriptions that I found on internet shops where they sell thermistors, no info on working voltage is mentioned. Is this because it does not matter or they just neglect to show it?
One more question:
The Mcintosh amp is 110 V, and I feed it through a step down transformer 220-110. Are thermistors voltage specific? In descriptions that I found on internet shops where they sell thermistors, no info on working voltage is mentioned. Is this because it does not matter or they just neglect to show it?
A thermistor is a varying resistor so no working voltage is relevant, having said that, if there are say 10volts across the thermistor, it will dissipate 4watts, (25R cold) quickly warm up and the resistance will reduce, as will the voltage until the dissipation equals the mean power of the varying resistor.
I hope my explanation helps. There is a part number on the RS site page that should help you trace a replacement.
I hope my explanation helps. There is a part number on the RS site page that should help you trace a replacement.
Excuse me Jon, that's the wrong part, it's a PTC.Goes in high resistance if to hot, protection for chokes, motors ...
The TS needs a NTC ! If in Europ, he can find it here,
Conrad
art.code 468053-62
type nr. B57237-S220-M
NTC 22Ω - 2,8A
It's not 27Ω but close, shoud be no problem.
Mona
The TS needs a NTC ! If in Europ, he can find it here,
Conrad
art.code 468053-62
type nr. B57237-S220-M
NTC 22Ω - 2,8A
It's not 27Ω but close, shoud be no problem.
Mona
Hi,
That makes no sense. If it was a PTC it would be buried in the transformer windings,
and > 25 ohm cold would be a pretty meaningless specification for such a part.
rgds, sreten.
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yes. it is an NTC thermistor.
you need to consider the current capability of the NTC. resistance need to be in the ballpark. 20-30ohm will do the job.
you need to consider the current capability of the NTC. resistance need to be in the ballpark. 20-30ohm will do the job.
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