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Author Topic: NSM 35M Amp  (Read 693 times)

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DJO

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NSM 35M Amp
« on: November 20, 2017, 11:02:06 am »
Hi guys, I'm new here so hopfully I'm in the right place. I have a 1972 NSM 160Bb which has suddenly developed a fault on the amplifier. At some point in its life the amp has been changed from a 70s to a 35m. I dont have a problem with that as its been plenty loud enough. The problem is the usual one of no sound. Whilst checking through the components I found one of two legs broken from a small stud like device bolted through the case. The legs come through a hole on the centre of the PCB and connect to Two tags. Is this a thermal fuse or thermister of some kind? its not shown on the schematic. Any lead would be welcome. Thanks in advance

lilshawn

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Re: NSM 35M Amp
« Reply #1 on: November 21, 2017, 12:54:30 pm »
I'd think it's simply an over temp switch. basically cuts the 50 volts going to the driver transistors if they get shorted or driven super hot.

you have a little nubbin of wire there where it broke off, measure across it and the remaining wire  with a ohmmeter and see if it's dead short (switch) or has resistance (thermistor)

if it's just a temp switch you can probably just jump the switch points with a wire to get going again until you can source another.

if indeed that's what it is,  something like a "normally closed" 90c would be decent enough for this application. Just find a way to clamp it down.

if it is some kind of compensation thermistor, you will have to find a part number of it and find it on the schematic to find out what it's specs are.

DJO

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Re: NSM 35M Amp
« Reply #2 on: November 21, 2017, 01:30:20 pm »
Thanks for the quick response. It certainly seem to be a thermister. I managed to tag a small piece of wire onto the nubbin as you call it and measured the resistance, 6.2k. I then held it on the soldering iron and the resistance started to tumble rapidly. I think I have located it on the schematic marked "HL1". I think the first value is 6k but could be a 6R. K makes more sense because of my initial reading. Could bridge it with a resistor for test purposes.
« Last Edit: November 21, 2017, 01:44:44 pm by DJO »

lilshawn

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Re: NSM 35M Amp
« Reply #3 on: November 21, 2017, 05:05:15 pm »
A thermistor typically has it's rating given at 25c room temperature. IE: if it's 25 degrees celsius, it's value should be 6K depending if it's cooler or warmer the value will increase or decrease from that point.

for instance a 10k NTC (Negative temperature coefficient) thermistor (what you typically find these days) will have a value of around 18k at 10 celsius, 10k at 25 celsius, and about 4 k at 50 celsius.

a PTC (Positive temperature coefficient) thermistor will behave in the opposite manner (gaining more resistance as it warms  and less as it cools)

This sounds indeed like a thermistor.

DJO

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Re: NSM 35M Amp
« Reply #4 on: November 23, 2017, 06:45:12 am »
Thanks for the info. It must be a NTC thermistor. All I have to do now is find one.

lilshawn

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Re: NSM 35M Amp
« Reply #5 on: November 24, 2017, 03:48:34 pm »
It might be a little tough to find one EXACTLY like that one, but there are plenty of 6k thermistors on Ebay. what they have likely done is drilled a hole in the top of that bolt and epoxied a "bead" type thermistor into the top. the 6k is the important bit. the temperature response curves are all going to be about the same.

dig out the old one... epoxy in the new one.

IF you happen to have 2 of those in your amp, do them both....that way it will stay balanced.

DJO

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Re: NSM 35M Amp
« Reply #6 on: November 25, 2017, 11:57:39 am »
Never thought about trying that, a good idea but I dont know which verion of eBay you've seen the 6K beads on but they're certainly not on my version.

lilshawn

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Re: NSM 35M Amp
« Reply #7 on: November 25, 2017, 04:18:31 pm »
im in canada soy results are a little different than us or eu. but in anycase, just plotz up your ebay and search "6k NTC thermistor" pick one of the epoxy bead types (not the thin metal film or big metal container types)  youll see what i mean when you search and see them.


ed12

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Re: NSM 35M Amp
« Reply #8 on: November 25, 2017, 04:48:00 pm »
http://parts.amientertainment.net/Products/NSM/
they carry it
expensive
part # for nsm
222-485
1 per channel
if this is all that is wrong with it ? replace the pair "pair"
if it blew the mains to amp 60volt rail fuse ?
chk and replace needed futher parts
p.s johns jukes also carry them
they are required stock for any nsm repair shop


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Shipping something from the U.S. to Canada for repair/exchange?  Please use USPS to avoid (additional?/excessive?) shipping charges.  PM me if you have any questions.

lilshawn

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