I have an Amp which I am trying to re[air, I need to change the MOSFET, the P-type had 2 x J-351.
These are hard to come by unlike the J-352. Would it be possible to use the J-352 in place of the J-351, I have been led to believe that the only difference being that the J-352 is up to 200V drain source compared to the 180v max.
https://www.web-bcs.com/transistor/tmfet/j0/J351.php
https://www.web-bcs.com/transistor/tmfet/j0/J352.php
Is this safe, recommended?
Thanks in advance.
These are hard to come by unlike the J-352. Would it be possible to use the J-352 in place of the J-351, I have been led to believe that the only difference being that the J-352 is up to 200V drain source compared to the 180v max.
https://www.web-bcs.com/transistor/tmfet/j0/J351.php
https://www.web-bcs.com/transistor/tmfet/j0/J352.php
Is this safe, recommended?
Thanks in advance.
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Perfectly OK to use the 200 volt part in place of the 180 volt.
https://pdf.datasheetcatalog.com/datasheet/HitachiSemiconductor/mXuruvr.pdf
Make sure you buy only from genuine suppliers as there are many fakes around.
https://pdf.datasheetcatalog.com/datasheet/HitachiSemiconductor/mXuruvr.pdf
Make sure you buy only from genuine suppliers as there are many fakes around.
Thanks - my Amp was blowing the fuse for P-Mosfet when I powered it up after a moment of madness when I switched the positive and negative feed to the board.
I ordered the J-352 - installed them but now when i power the amp up both fuses are blowing so it seems I made matters worse. The component came from China so a fake I am guessing, however would that cause out0right failure or just not last as long?
I ordered the J-352 - installed them but now when i power the amp up both fuses are blowing so it seems I made matters worse. The component came from China so a fake I am guessing, however would that cause out0right failure or just not last as long?
The suspect could literally be 'anything', not even a FET.
These parts look like Lateral FET's to me (going off the data sheet characteristics) and the only reputable source of those are new devices from Exicon.
https://www.exicon.info/products.php
If you have even momentarily reversed supply polarity there could be other damage... not possible to second guess that without testing.
You need to work methodically, use a DBT (dim bulb tester) to limit current that can be drawn and fault find in the normal way. The DBT helps prevent damage and blowing of output transistors. I would definitely suspect your Chinese replacements as being fake though.
These parts look like Lateral FET's to me (going off the data sheet characteristics) and the only reputable source of those are new devices from Exicon.
https://www.exicon.info/products.php
If you have even momentarily reversed supply polarity there could be other damage... not possible to second guess that without testing.
You need to work methodically, use a DBT (dim bulb tester) to limit current that can be drawn and fault find in the normal way. The DBT helps prevent damage and blowing of output transistors. I would definitely suspect your Chinese replacements as being fake though.
They are in fact listed as replacements. The 2SK2221 is the N channel version (which your amp may or may not use)
There aren't a great deal of components on the board, 7 transistors (incluing the 4 MOSFETS) and a bunch of resistors and a couple of caps. I have a good working board on the other channel and using a multimeter I comparing each component and the only difference i can find is the MOSFET - the 2 x N channel are on order but I fear will be fakes also.
There is one other small transistor that is giving me a different reasding, it is a BC558 with a negative base pin. On the good board I get a reading of 843 on the emitter and 808 on the collector. On the channel i am fixing readings are 774 and 748 respectively - not sure if that is significant?
There is one other small transistor that is giving me a different reasding, it is a BC558 with a negative base pin. On the good board I get a reading of 843 on the emitter and 808 on the collector. On the channel i am fixing readings are 774 and 748 respectively - not sure if that is significant?
Thanks, unfortunately I don't have one. I've search for the Amp on Google T400 MOSFET reveals not very much. Only other info is "Class Ltd. London" on the amp. Has "Tower Amp" in yellow letter but otherwise i'm in the dark.
You're right--- a relatively simple circuit. It may be worth the effort to trace out the circuit.
Again, good luck!
Again, good luck!
The BC558 is a common device if it needed them. I'm not quite following what you mean by a 'negative base pin' 🙂 Transistors really have to be checked out of circuit to stop interaction with other parts. The BC558 is a PNP device and the base should read to both collector and emitter provided the black meter lead is on the base and you use the 'diode check' range on your meter. There should be no reading between collector and emitter with the red lead on the emitter. I wouldn't suspect the transistor at this point though.
Looking at the board and it appears the FET's are in parallel pairs which suggests for testing you could use just one complementary pair (N and P channel). I would strongly recommend using a bulb tester though.
This looks like a PA/stage amp... lets move the thread and perhaps ask.
Looking at the board and it appears the FET's are in parallel pairs which suggests for testing you could use just one complementary pair (N and P channel). I would strongly recommend using a bulb tester though.
This looks like a PA/stage amp... lets move the thread and perhaps ask.
To avoid damage due to applying reverse power supply voltages, place a diode from the positive to ground and the negative to ground. Of course not in a conducting fashion. In the event as described in a previous post, only the fuse will blow and probably the diode.
I managed to pick up some replacement FETs - fitted and bolted down and sound quality is great.
However, after the amp has been powered down over night, when I first power it up I get quite bad humming sound, Today when this happened I turned it off then back on and the noise began to rise, I then adjusted the volume and a flapping sound began to rise. Powered off and on the third attempt there was just a humming, like a bad earth when a turntable hasn't had the ground connected - but it was stable and didn't rise.
When I add a sound source and play something the humming/feedback/flapping stops and playback is fine. Would it be a bad idea to add some caps across D and S on the FETs?
However, after the amp has been powered down over night, when I first power it up I get quite bad humming sound, Today when this happened I turned it off then back on and the noise began to rise, I then adjusted the volume and a flapping sound began to rise. Powered off and on the third attempt there was just a humming, like a bad earth when a turntable hasn't had the ground connected - but it was stable and didn't rise.
When I add a sound source and play something the humming/feedback/flapping stops and playback is fine. Would it be a bad idea to add some caps across D and S on the FETs?
Absolutely disastrous 😉 don't even think of doing that.
So it works 👍 Maybe there is a problem with the input sockets if it consistently behaves when you connect something to it and play something. Are they 1/4 inch input jacks or phono sockets?
1/4" input jacks yes. Just tried again I didn't get any of the flapping, just a bit of a hum when I switch on, the hum is there without the input source plugged in.
Seems that this channel is getting a little warmer than the other - as mentioned FETs are now screwed down. It could be that a previous underlying issue meant this channel ran hotter than the other before but I wasn't really paying too much attention.
Maybe I just need to let it play for a period of time and monitor?
Regarding the cap mod, apologies should have been between G and D, not D and S!
Seems that this channel is getting a little warmer than the other - as mentioned FETs are now screwed down. It could be that a previous underlying issue meant this channel ran hotter than the other before but I wasn't really paying too much attention.
Maybe I just need to let it play for a period of time and monitor?
Regarding the cap mod, apologies should have been between G and D, not D and S!
It might be worth cleaning the input jacks and particularly if they have contacts that lift up and open as you plug anything in. In any case some clean paper towel soaked in iso or switch cleaner and twirled in the socket is no bad idea to clean them.
If there are no bias presets on the board then it will run at fixed bias and differences in FET characteristics would change the current (and hence the overall heat). Without examining the circuit it is impossible to tell you how to alter that.
You could measure the DC voltage across those four large resistors that go to the FET's. Are they orange (0.33 ohm) or red (0.22 ohm) bands? The one next to the top one looks discoloured or burned. Worth checking it is OK.
Typical current per FET would be 100ma so you might expect to see about 33 millivolts or 22 millivolts across each resistor depending on value.
If there are no bias presets on the board then it will run at fixed bias and differences in FET characteristics would change the current (and hence the overall heat). Without examining the circuit it is impossible to tell you how to alter that.
You could measure the DC voltage across those four large resistors that go to the FET's. Are they orange (0.33 ohm) or red (0.22 ohm) bands? The one next to the top one looks discoloured or burned. Worth checking it is OK.
Typical current per FET would be 100ma so you might expect to see about 33 millivolts or 22 millivolts across each resistor depending on value.
Checked the resistors, on the 20v setting my DMM is reading 0.02 - I checked on that top one which looks dis-coloured in the photo and the next one down. I also checked the phono lead before it plugs into the speaking black on the centre red on the outer, voltage seems fluctuate from 0.15 up to 0.5 - every time I try a reading it seems to be different.
Sound was very good this morning, no feedback or weird noises - almost feels like it is settling down/in - if that is such a thing?
I thought about maybe adding a fan to the setup, one of those approx. 10x10cm computer rack fans that run of 12v - any reason why I shouldn't do this?
Sound was very good this morning, no feedback or weird noises - almost feels like it is settling down/in - if that is such a thing?
I thought about maybe adding a fan to the setup, one of those approx. 10x10cm computer rack fans that run of 12v - any reason why I shouldn't do this?
0.02 or 20mv sounds fine. It sounds like it is all OK and you have to remember its not a complex circuit like a HiFi amp.
A fan can't do any harm if you feel it needs it... and I'm not sure it does really... and you would have to rig up a suitable power supply.
A fan can't do any harm if you feel it needs it... and I'm not sure it does really... and you would have to rig up a suitable power supply.
Hmmm. I wouldn't go within 100 miles of that 😀 Note that it specifically says 220v volt on the unit (UK mains can be as high as 253 volts) and also there are even spelling mistakes on the label. Iuput for input. Ask yourself what the quality is likely to be for that price, how long will it last, is it safe and how electrically noisy is it (radiated noise and interference).
As I said earlier, I'm not even sure a fan is needed here, it was obviously designed from the start not to need one.
As I said earlier, I'm not even sure a fan is needed here, it was obviously designed from the start not to need one.