Argon SA1 audio amplifier power-supply - fuse blown after fixing the short?

[Babbar]

I found a project in the electronics trash yesterday. A nice Argon SA1 audio amp with some electrical issue. Took it apart and saw that the 4A250V fuse was blown so started lifting components to find the short. Eventually got to two MOSFETs (model FTA14N50C) that are shorted and when i took them out the short was gone.

Now i didn't have any replacements of the same model of course but i found a couple with pretty similar spec i thought (K10A60D & K12A50D) and put them in. Also replaced the blown fuse with a 3.5A250V one. Checked another time for short circuit and plugged it in.

The new fuse blew immediately and also the 10A fuse in the wall.

So my questions are:
1. Any ideas what i should look for as the reason for the issue?
2. Is this completely expected as i replaced the MOSFETs with similar but not same specs or that the two replacements are a little different in spec?
3. Could the fuse be the reason somehow?

When connecting the power supply it was not connected to any other part of the amplifier, just the power supply by itself.

[kasfamily]

Hi, need more photos, check the diode bridge

[CapLeaker]

Ya…No! You can't pair 2 different Part number mosfets together and call it good. There is so much more than just match the power rating. The mosfet also has to turn on an off in a timely critical manner. So if an overlap occurred, no wonder it went bang!
And:
and you didn't check the gate drive circuit and it may be stuck on.

[Babbar]

Hi, need more photos, check the diode bridge

Good suggestion! I checked the rectifier and it is indeed borked as well. Measures open on both +/~ and -/~. Need to source a replacement now, model GBU606 (6 A 600 V).

[Babbar]

Ya…No! You can't pair 2 different Part number mosfets together and call it good. There is so much more than just match the power rating. The mosfet also has to turn on an off in a timely critical manner. So if an overlap occurred, no wonder it went bang!
And:
and you didn't check the gate drive circuit and it may be stuck on.

You live you learn =P. Thanks for the clear info on do's and don'ts of replacing MOSFETs! Would it have been a better idea or maybe even good idea if i had replaced the two broken ones with two matched K10A60D or two K12A50D? Or should you always aim for an exact replacement with original?

How do you check the gate drive circuit state? Will it always blow if it is stuck in the on state?

[CapLeaker]

Usually the bridge rectifier fails short. I don't believe 4 diodes in there are open. Look at the gate of the mosfet on the PCB and follow it back. Make sure the components of the gate drive circuit and the gate driver is good.
if the mosfet shorted on all three legs, the gate drive circuit got full power on to it and it doesn't like that. If there was a 14N50 in there, best is to put a 14N50 back. The 10A60:isn't a good fit at all. You can put something like 14N65 in there. A 12N60 is a bit low but should work in a bind.

[Babbar]

Usually the bridge rectifier fails short. I don't believe 4 diodes in there are open. Look at the gate of the mosfet on the PCB and follow it back. Make sure the components of the gate drive circuit and the gate driver is good.
if the mosfet shorted on all three legs, the gate drive circuit got full power on to it and it doesn't like that. If there was a 14N50 in there, best is to put a 14N50 back. The 10A60:isn't a good fit at all. You can put something like 14N65 in there. A 12N60 is a bit low but should work in a bind.

Followed the gate of the MOSFETs back through some resistors and diodes to an ST L6599A IC which is a high-voltage resonant controller according to the spec i found. Don't know how to check it fully for faults but at least didn't find any shorts on the way to it or on the connecting pin (nr 15 HVG). Planning to order replacements for the MOSFETs and bridge rectifier this evening and hope to get the amp running again without more bangs =).

[CapLeaker]

I don't think the bridge rectifier is bad, you can check that by leaving the Mosfets out and plug the PSU in. Then measure the DC voltage on the main filter capacitors. Other wise your main fuse would have been blown on the fuse panel and the rectifier would look a bit blown up.

[Babbar]

I don't think the bridge rectifier is bad, you can check that by leaving the Mosfets out and plug the PSU in. Then measure the DC voltage on the main filter capacitors. Other wise your main fuse would have been blown on the fuse panel and the rectifier would look a bit blown up.

The rectifier doesn't look blown in any way that i can see but when i use my multimeter it measures dead short on both +\~ and -/~ so i'm thinking it is gone for sure. My main fuse did blow when i plugged in the amplifier, wrote that in my initial message that both the device fuse and wall fuse blew.

I appreciate all your help on this little project which hopefully leads somewhere, if nothing else some new knowledge!

[CapLeaker]

Yeah, if the main fuse blew, could be that the rectifier is forked. Make sure you got still continuity from the plug line and neutral to the AC inputs of the rectifier. Not that something is open now.

[Babbar]

Yeah, if the main fuse blew, could be that the rectifier is forked. Make sure you got still continuity from the plug line and neutral to the AC inputs of the rectifier. Not that something is open now.

Ok, just checked and AC line and neutral still have good connections to the AC inputs of the rectifier.

[CapLeaker]

Then you are good there. It does happen that an inductive filter coil opens up. Usually the wire blows off the leg. If that happens just undo one turn and solder it back to the leg.

[Babbar]

Then you are good there. It does happen that an inductive filter coil opens up. Usually the wire blows off the leg. If that happens just undo one turn and solder it back to the leg.

Ok cool, will remember that if/when i come across it! Going to order the parts now, should show up in a couple of days so then the fun continues =).

[Babbar]

Parts arrived today, put in the new rectifier and power MOSFETs and nothing blew up, the lights come on and i can connect my phone via Bluetooth. So far, so great =). But... there is no sound. I've tried two different BT sources and connected via line-in. Also tried two different speakers. All seems to work without issue but without any sound coming to the speakers. Have no idea how to continue at the moment, any ideas welcome! Also, what do you think, should i focus my energy on the PSU or the motherboard at this time? From what i can tell using my IR camera there is nothing getting hot on the PSU meaning not much power is being drawn by anything. Attaching some pics of the whole setup assembled.

[Babbar]

Taken the amp apart again and looking at the power supply board but not getting anywhere =). At the moment feeling like i'm out of my depth with this repair as i don't know what to expect. It's confusing that the device seems to work fully; powering on and allowing bluetooth connection etc but at the same time i get the feeling that the PSU has some major issues.

Having the PSU plugged in but disconnected from the main board, what should i expect from the various connectors on the power board? If it says on the MB +12V and G on a connector which leads from the PSU, should i expect 12V to always be available on that connector on the PSU regardless of if it is connected or not to the MB? Silly questions perhaps but as i said, i'm in a confused state.

Attaching a picture of the back of the board with rectifier highlighted yellow and the replaced mosfets highlighted red. With the PSU having power i measure ~320V output from the rectifier and reaching the left mosfet drain but that is as far as the power goes from what i can tell...

[CapLeaker]

Turn the sucker on and measure the DC voltages on the secondary of the power supply going to the main board. If all these voltages are there, try different audio inputs. Look at the Class D brick. Get the spec sheet and poke around at it with an oscilloscope. Check if that class d brink is getting audio, etc.

[markeye]

Hello, I don't know how the repair went in the end, but I bought this amplifier today.
1. I would try a firmware upgrade, a usbA - usbA cable is enough for that.
2. I saw somewhere on the web that you can buy replacement boards from the amplifier for normal prices, or maybe the manufacturer will know about it.
3. It's a shame that service manuals are currently unavailable, the EU could make it mandatory to publish them.