I'm repairing an amplifier - there is a Resistor with a thermal fuse in the power supply, and the thermal fuse has blown. This same component has been replaced at least twice before. It seems to last a couple of years and then go. We use the amp for a couple of hours each week on Sunday, and we aren't overloading the amp.
I'm considering replacing it with a non-fused resistor of the same value. Is this dumb? 🙂
The resistor+fuse is in the main power supply line. It is already in series with a thermistor, so there is double-protection. I also have the option of adding in a normal replaceable fuse. The particular resistor+fuse is also a custom part for Yamaha, so it's also hard to get, costs about $20 and takes me a couple of hours to bring it home and fix it. It would be a couple of minutes and very cheap to replace a normal fuse.
Any thoughts or suggestions?
I'm considering replacing it with a non-fused resistor of the same value. Is this dumb? 🙂
The resistor+fuse is in the main power supply line. It is already in series with a thermistor, so there is double-protection. I also have the option of adding in a normal replaceable fuse. The particular resistor+fuse is also a custom part for Yamaha, so it's also hard to get, costs about $20 and takes me a couple of hours to bring it home and fix it. It would be a couple of minutes and very cheap to replace a normal fuse.
Any thoughts or suggestions?
What model is the amplifier? If there is a design fault, Yamaha would have issued a technical service note. Otherwise if the fuse keeps blowing there is a reason why. Disregard the failing protection at your own peril.
You can put a fusible resistor?
Just a thought...
Those are cheap, a few cents each, can you post a photo of the blown part?
Just a thought...
Those are cheap, a few cents each, can you post a photo of the blown part?
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Yes, it's R116. The part is about $13, which is not cheap, but not so bad, and I've found some local stock. It would just be nice to avoid having to do this again in the future, as it's gone three times already (a couple of years between failures).
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R116 is shown as 6.8R 5W and is in series with TH101 (8R2 cold NTC). Both those parts are shorted by relay RY101 contact once the 27V rail on the smps is up.
Assuming the amp is working ok (ie. the rail voltages and reg output levels and idle currents are ok) then R116 (and fuse) could plausibly be stressed by a quick repeat turn-on, whereby the mains caps are discharged but TH101 is still hot. I'd recommend fixing a note to the power on button to not turn the amp on again until waiting say at least 2 minutes.
If you had the part number of R116, and a datasheet could be found, then replacement resistor/fuse parts could possibly be selected with good confidence. I wouldn't replace the part without a suitable fuse, as collateral damage could occur if there was some form of failure. Also the resistor preferably has a specified surge current capability.
The fusing characteristic is in http://www.micron-e.co.jp/english/www/common/pdf/CEG-O-100809-02-T.pdf
Assuming the amp is working ok (ie. the rail voltages and reg output levels and idle currents are ok) then R116 (and fuse) could plausibly be stressed by a quick repeat turn-on, whereby the mains caps are discharged but TH101 is still hot. I'd recommend fixing a note to the power on button to not turn the amp on again until waiting say at least 2 minutes.
If you had the part number of R116, and a datasheet could be found, then replacement resistor/fuse parts could possibly be selected with good confidence. I wouldn't replace the part without a suitable fuse, as collateral damage could occur if there was some form of failure. Also the resistor preferably has a specified surge current capability.
The fusing characteristic is in http://www.micron-e.co.jp/english/www/common/pdf/CEG-O-100809-02-T.pdf
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There's nothing special about that resistor apart from that it is a vertical format ceramic. It's quite possible TH101 is faulty and that's stressing the resistor, so try replacing that at the same time. Anyway I would have no qualms whatever about doubling it's power rating and/or using a more robust design. Something like this: https://au.mouser.com/ProductDetail/Ohmite/TWW10J6R8E?qs=sGAEpiMZZMtlubZbdhIBIJ9l1veHkqGcN5c8WBz9gIk%3D
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If I can get the thermistor part easily, I'll replace that too. The one on the board at the moment has obviously been replaced previously (and it's badly soldered onto a lifted track), and is measuring around 3.7R, which is way off. It's glued in place, and I can't read the value, so it might even be the wrong part.
Another interesting detail is that we never turn off the amp by the power switch. The switch is left on, and the amp comes on when the power comes on through a remote-controlled power socket. In this scenario, C155 is always shorted, and R116 and TH101 are only in the circuit for a very brief moment at power on, and possibly power off. As far as I can see, these resistors are there purely to handle transient spikes at startup of the SMPS, and if the device is turned on with the switch, the C155 could also supply some startup current, but in our use case, that can't happen, so maybe that helps to stress the resistor? I have an electrical engineering degree, but my knowledge of AC circuits is very rusty.
I'm going to repair with the proper parts, and will check the static voltages to make sure they are in spec, but I'm just trying to figure it out to give me confidence that replacing R116 and TH101 will stop this happening again in another year, or whether I should also be looking for some other cause.
Another interesting detail is that we never turn off the amp by the power switch. The switch is left on, and the amp comes on when the power comes on through a remote-controlled power socket. In this scenario, C155 is always shorted, and R116 and TH101 are only in the circuit for a very brief moment at power on, and possibly power off. As far as I can see, these resistors are there purely to handle transient spikes at startup of the SMPS, and if the device is turned on with the switch, the C155 could also supply some startup current, but in our use case, that can't happen, so maybe that helps to stress the resistor? I have an electrical engineering degree, but my knowledge of AC circuits is very rusty.
I'm going to repair with the proper parts, and will check the static voltages to make sure they are in spec, but I'm just trying to figure it out to give me confidence that replacing R116 and TH101 will stop this happening again in another year, or whether I should also be looking for some other cause.
The spec for the thermistor says a max 470µF cap, but the amp has 1000µF. I think the 6.8Ω resistor is there to reduce current through the thermister, but I doubt that it is a high enough value to keep the thermistor in its safe operating area. I'd probably put in a higher rated current one like this https://www.tme.eu/Document/7bb2b00fc5c8a46d5d1a413afe9d8aed/MF72-3-8R2M.pdf
surfdabbler, I'm not sure that C155 has an influence ?? Even if the amp was remotely energised, wrt R116-TH101, the start-up process is the same as using the amp's switch. You haven't indicated how the remote controlled power socket is controlled, so human error may still be possible (for close off-to-on events) and possibly even mains related glitches or repeated auto-reclose actions.
You have indicated R116 has failed only a couple of times, but it could be that one or some of those events could have been accelerated by a degraded TH101. I would recommend you replace both parts with correct spares, and assess the remote control aspect. Using different parts for R116 and TH101 may be fine for your diy use, but if this amp is in some level of public use, then that doesn't seem wise as the collateral damage has not been adequately assessed here imho.
You have indicated R116 has failed only a couple of times, but it could be that one or some of those events could have been accelerated by a degraded TH101. I would recommend you replace both parts with correct spares, and assess the remote control aspect. Using different parts for R116 and TH101 may be fine for your diy use, but if this amp is in some level of public use, then that doesn't seem wise as the collateral damage has not been adequately assessed here imho.