Replaced blown Mosfets on amp, now they are overheating. (Behringer NX3000D) - Page 1
Forum section: electronics / beginners

author: Thridel
author_rank: Newbie
timestamp: January 21, 2023, 03:41:32 pm
content: Hi everyone!I'll start with pointing out that I'm a electronics novice and English is not my first language so bear with meI am working with a Diy bass project and I've built two sealed 10" subs. I bought a used Behringer NX3000D to drive the subs and on my first test play i blew one of the output transistors on channel B due to a very stupid mistake, i had accidently made a short circuit in the Speakon cable.The amp gave away a loud pop and the magic smoke could be seen and the amp went in protection mode. When i opened the case i saw that one of the four output transistors was blown.I found a schematic for the NX3000 model and it's the same layout as my amplifier but not the exact numbers on components. I guess it's because the added DSP in my version. The model is IRFB4227 and i couldn't find any replacements at Mouser or Farnell so i ordered some new from Ebay. I replaced both the B-channels mosfets ( T-49 and T-48) and checked surrounding components to the best of my knowledge for faults but i couldn't find any so i put everything together again, plugged it in and started it but the protection leds still flashed.Took everything out again and investigated further and i found out that there was no connection between the drain on T-49 and the source on T-48. I noticed that some of the contact surface for the drain pin on T-49 was gone so i guess that's the problem?! I studied the schematics and used channel A for reference measuring when comparing to channel B and came to the conclusion that i just need to solder a jump cable from the drain pin on T-49 to the connection on Coil L5. Said and done, now channel B measures as channel A and i once again put everything together and started it up.Success!! Now the protection leds were off and everything seemed fine! Connected the subs and they played nice and everything seems like its working but the i noticed the fan ramped up quite fast and it didn't sound and felt right so i checked with an IR thermometer and as I suspected, the heatsink gets pretty warm in just a few minutes. This is with the subs disconnected and with no input signal, everything in idle. I measured the output voltage and it shows 0. Gate voltage measures the same on both channels with a offset about 6 V pair to pair.Now I'm out of ideas, is it something with the gate drivers? Or is it just due to bad replacements? Or did i make a huge assumption when i added the jumper cable?I don't have any oscilloscope and couldn't read one even if i had it so I'm stuck!Any help is helpful and guess you guys can spot the mistake in an instant.Cheers!

author: wasedadoc
author_rank: Super Contributor
timestamp: January 21, 2023, 04:04:29 pm
content: Insulating mica between MOSFET and heatsink?

author: Thridel
author_rank: Newbie
timestamp: January 21, 2023, 04:35:14 pm
content: I reused the ones that was already there. I added some computer thermal paste between the mosfet and mica. I don’t have any continuity between heat sink and mosfet.

author: Terry Bites
author_rank: Super Contributor
timestamp: January 21, 2023, 04:41:19 pm
content: Check that complementary pair driver transistors are ok.You really need to scope the driver waveforms.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: January 21, 2023, 05:48:08 pm
content: You could take your mosfets out and check themCheck your bias on the mosfet gate again. It should not turn on with a gate volt below 3-4 voltsI think you said bias was 6 V. I think this is too high according to the specs that would turn it on.I do not think the bias should be more than 3 volts. 4 volts will start turning it ON.

author: wraper
author_rank: Unknown
timestamp: January 21, 2023, 05:55:59 pm
content: Since it's a class D amp, very likely transistors are counterfeits with much higher RDS(on).

author: wraper
author_rank: Unknown
timestamp: January 21, 2023, 06:05:25 pm
content: How much did you pay for them? If it's <$5 a piece, there is nearly zero chance they are genuine

author: Thridel
author_rank: Newbie
timestamp: January 21, 2023, 07:12:14 pm
content: Quote from: Terry Bites on January 21, 2023, 04:41:19 pmIf i measure them on board they seem to be ok, do i need to desolder them to know for sure?I'm just curious , I like to learn, but assume that nothing is wrong with any other component, shouldn't it be possible to just swap out the mosfets (if they are genuine and the same type) and get it running without the need for a scope?Quote from: Wallace Gasiewicz on January 21, 2023, 05:48:08 pmIf I remember correctly T-29 and T-48 was around 10 volts and T-47 and T-49 were around 3,5 volts. I'm assuming that's ok since they measured the same on both channels?Quote from: wraper on January 21, 2023, 06:05:25 pmI payed around 4 Euros a piece, i didn't have any faith them being genuine but i thought it didn't matterthatmuch but it may very well be the culprit of my problem. Thing is, it's the same price on Mouser e.g. and i guess they don't sell counterfeit stuff?Does anyone know where i can source genuine parts in Europe so i can at least test with "the real deal" before going further down the road? I live in a pretty remotely place and I don't know anyone with a scope, and have a long long way to the nearest town and if I need to send the unit away to a pro i would most likely cost as same or more then it's worth

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: January 21, 2023, 07:24:24 pm
content: If the bias on T-29 and T-48 is ten volts and we are talking about the same mosfet IRFB4227That is definitely way ON. Constantly ON.The other mosfets bias may be OK

author: Thridel
author_rank: Newbie
timestamp: January 21, 2023, 07:40:36 pm
content: Is there a possibility that both gate drivers are at fault then or can i backtrack further to see if i need to replace anything before it?

author: Bud
author_rank: Super Contributor
timestamp: January 21, 2023, 08:08:43 pm
content: Think about it logically for a sec. Overheating means the parts dissipate more power then they are supposed to. With everything else being equal to the original ones (gate drive, Vcc, etc) means they likely have higher Rds. I am with Wrapper on this one that you may have bought counterfeit parts.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: January 21, 2023, 08:29:21 pm
content: I said;If the bias on T-29 and T-48 is ten volts and we are talking about the same mosfet IRFB4227That is definitely way ON. Constantly ON.The other mosfets bias may be OKMay I addMaybe just check the bias resistors in circuit without removing the mosfets first. You have good ones and questionable ones to compare.You might find shorted resistors.Then:The two mosfets with gate voltages of ten volts may be very faulty.I would remove and check the mosfets with 10 volt biasI would check the gate voltages of these mosfets without them in the board.If there is really 10 volts bias, yes look to the drivers and bias circuits. including any resistors providing bias.And yes, the mosfets you bought may be faulty from the start.Google fake IRFB4227

author: Thridel
author_rank: Newbie
timestamp: January 22, 2023, 08:58:44 pm
content: Yeah sorry, we are talking about the same IRFB4227.Thanks for all the answers, I'll try to find some genuine Mosfets to try with first, it feels like it can be as suggested that the Rds is way off from spec so I'll try that first.