Arcam Alpha 9p blowing fuse

[topperdoggle]

Hi all,

I have a 9 / 9p bi-amp set up with some Monitor Audio floorstanders. All has been well until suddenly last night the 9p went dead. I opened it up and the internal 230v mains fuse in the lower right corner had blown. There was a spare on the board, so I replaced it and it blew immediately again.

Can somebody give me some tips on where to go from here? I'm not an electronics guru, but I know how to solder and use a multimeter.

Furthermore, the "T1A6L" fuse as marked on the PCB, doesn't show up in Google. Does anybody know the spec for this fuse (it's a different socket for 230v and 115v - I need for 230v)? I think I'll have to buy a few replacements.

Thanks in advance.

[KatieandDad]

T1A is just a 1A fuse.

These are notorious for blowing output transistors if the speaker leads are inadvertently shorted together.

Unfortunately output device failure also often causes the drivers to fail.

BUT, before we go down that route, can you see any bulging or leaking of the large PSU capacitors ?

[KatieandDad]

To help all the helpful guys here is the schematic of the Arcam 9P.

I would start by checking the large reservoir caps. Then check the four Output Mos-Fets

[topperdoggle]

Hi, thanks for responding. I've ordered some 1A 230V slow-blow fuses from Ebay.

As for the rest of your post, I don't see why the speaker leads would suddenly short together, the amp hadn't moved. And I don't know what you mean by output device failure - do you mean an amp failure might have blown my speakers?

Second post - the two massive caps aren't bulging or leaking. I'm not sure how else to "check" them with a standard multimeter. What part numbers on the schematic are the output mosfets, and how do I check them?

I get the feeling that I need a bit of extra hand-holding here, hope I'm not being too thick...

[topperdoggle]

If anyone's still reading, I wanted to add that I took the spare fuse out of the integrated amp (1.6A by the way not 1A), disconnected the power amp from everything except the mains and... pop. So I know it's not the speakers (currently bi-wired to the integrated amp, all working okay).

So as I said above, I 'm not sure exactly *how* to test the caps and the four output mosfets with my standard multimeter. I hope I can do them in-circuit. On the bright side, I have locate the latter, they're the big ones attached to the heatsinks, right?

I don't mind buying some better diagnostic tools than my standard voltmeter if they aren't too expensive.

I seem to remember something from the past about disconnecting some of the transformer coils then switching the amp on to isolate the problem, any mileage in doing that here?

[tauro0221]

Hi,
Normally when the main fuse blow most of the time there is a shorted in the rectifiers or in the main filter caps section. Check the caps + pos side to ground or the - neg caps to ground. Remembered that there are caps for the neg voltage and the pos it is grounded. If you have a low resistance reading means that you have a shorted. If you find that there is a shorted then by isolating each components one at a time you maybe able to isolate which one it is causing the short.The schematic does not shows the power supply components so I can not advices which components to check.

[jaycee]

In this case it is likely that a MOSFET has failed short. Desolder the IRFP240 devices and test them. I guarantee you'll find one failed source to drain. Fortunately they are cheap.

[KatieandDad]

JAYCEE - Looks like this guy needs a bit more guidance than just a description.

TOPPERDOGGLE - I'm agreeing with JAYCEE.

What you need to do is to remove Q1, Q2, Q101 and Q102. They are MOS-FET devices so are slightly sensitive to static discharge. Just make sure that everything is connected to EARTH, including YOU and the soldering iron.

If you place the IRFP240 onto its back, the left hand leg is the GATE, the centre leg is the DRAIN and the right hand leg is the SOURCE.

With a multimeter you should get very high reistance between the DRAIN and the SOURCE in both directions and you should get INFINITY OHMs between the GATE and both the DRAIN and the SOURCE.


If either reading is LOW then the device is SHOT.


Arcam don't usually use the TO3 version IRF240. I would have expected the TO247 Plastic version IRFP240.


IRFP240s are cheap. Try Cricklewood Electronic, Farnell or even RS. Cricklewood have them for £2.50 each.


JUST BEWARE. If you do find that one of the output devices have a short circuit on its GATE then it will probably have taken out is driver transistor in the process.

These MOS-FET amps rely upon the GATE insulation resistance to keep the GATE current low so-as to reduce the dissipation in the drive circuitry.

[topperdoggle]

Thanks people. I'll have a crack at testing these guys over the weekend. I'm assuming I have to desolder them i.e. I can't test them in-circuit?

As for the possibility of the driver transistor being shot, I'll test these four first then come back on that if necessary.

[jaycee]

Yes you must desolder them to be sure. Before you test, short the Gate pin to Source - MOSFETs are fairly easily turned on (ooer) and its possible to get a false reading. If you get zero resistance both ways between source and drain, or a reading between Gate and Source or Drain, it's shot.