Arcam Alpha 9 problem

[Colin151]

Hi was wondering if anyone can help.

I have an Arcam Alpha 9 amp which is blowing fuses. I rang Arcam and this apparently is likely to be a faulty output transistor.
I don't know a vast amount about DIY or electronics but really want to try to fix it myself as they charge £150 for it. I found a source for the output transistor, a IRFP240 MOSFET which are a couple of pounds each.

I have located the 4 MOSFETS in the amp (on the heatsink) and they seem easy to remove and replace. Was just wondering how I check the present ones to see is any any faulty or shorting. Can you test them without the amp running (as obviously it won't run as the fuse blows)? And should I check other parts like rectifiers, bias transistors, drivers, diodes resistors etc for shorts?

Any basic help (and I mean basic!) would be much appreciated.

Cheers,
Colin

[h_a]

Well, fixing things without having some experience is not the best idea, honestly.

First, I don't know why an output transistor should be the fault. Sure nothing lasts forever, but given sufficient cooling these devices last forever. Unless you shorted them. Did you?

It could be anything from bad isolation between fet and heatsink, to bad regulators.

Further, the 2x IRFP240 per channel are likely to be matched (if they're paralleled), that is that both devices are specifically selected to work together. I don't know wether the Arcam is expensive enough so that these are matched or not. But likely they are. So don't just replace _one_ - otherwise distortion will increase.

If they aren't paralleled, it probably doesn't matter. But you don't know and that's not good.

To check the output transistors you would need to desolder them and measure wether they are conducting without gate-voltage to get a first impression.

Just changing parts without searching for the problem is a very time consuming way to fix an amp.

All the best, Hannes

[AndrewT]

Hi,
the surrounding components will confuse your measurements but it's worth putting a multimeter across the FET pins to identify which device, if any, have a dead short between pins.
Set the meter to low ohms or even milliohms.

If there is a low voltage switch for the ohms scale then try measuring with the low voltage switch in both positions.

edit,
I think the 9 has just 1pair on each channel output i.e. 4 in total. If this is the case then matching is not an issue.

[Colin151]

Hi Andrew and Hannes,

Many thanks for that.

Hannes said I should desolder the IRFP240 MOSFETS to check for shorts. But you (Andrew)seem to suggest I can check them for shorts with a multimeter whilst still attached to the board. IS this right?

The IRFP240's obvsiouly have 3 pins. should I check 2 certain pins in particular?, or all combinations? I heard that gate to drain shorts are common (the left and middle pin).

And if it doesn't seem to be the MOSFETS how do I identify and check the regulators?!

Cheers and sorry I'm not very experienced!

Colin

[AndrewT]

Originally posted by Colin151
seem to suggest I can check them for shorts with a multimeter whilst still attached to the board. IS this right?

The IRFP240's obvsiouly have 3 pins. should I check 2 certain pins in particular?, or all combinations? I heard that gate to drain shorts are common (the left and middle pin).

yes do first check in situ.
Check all combinations in both directions and on both standard and low voltage settings (48readings).
record each reading for each device.
The shorted device should stand out from it's good partners.
But if there are two shorted devices, it's more difficult to identify the damaged one.

[Colin151]

Originally posted by AndrewT
yes do first check in situ.
Check all combinations in both directions and on both standard and low voltage settings (48readings).
record each reading for each device.
The shorted device should stand out from it's good partners.
But if there are two shorted devices, it's more difficult to identify the damaged one.

"Check all combinations in both directions and on both standard and low voltage settings (48readings)."?

Sorry Andrew I am a bit of a Muppet what do you mean by this? When you say "Standard and low voltage" settings what does this mean?, - do I need to send a voltage through the MOSFET first with a multimeter? (I read something about doing this).

Please note I have another working Alpha 9 amp with good MOSFETS so I suppose I could check the MOSFETS in that amp for comparison.

Cheers
Colin

[Colin151]

Here is a pic of one pair of the Mosfets in my non-working Alpha 9

[AndrewT]

Hi,
that looks like one output pair per channel.
This avoids the need to match parameters.
Just buy replacements and fit. But you do need to check the drivers and possibly further upstream as well.

Most multimeters apply a voltage to the device under test (DUT) and measure the current. The electronics convert this to ohms and this appears on the display (analogue or digital). The voltage sent to the device is usually over 1Vdc and passes through all normal diodes, in one direction, but not the other way.
Some Digital Multimeters (DMM) have a switch to reduce the applied DUT voltage to something less than the 600mV so not turning on any semiconductor junctions, i.e. diodes and transistors.

For a two pin device you need to check first one way with red to pin1 and black to pin2, then swap the leads around black to pin1 and red to pin2. record the readings.

For a three pin device there are six combinations. Many of the readings will the the same or similar, record them all. Test 4DUTs using both the low voltage option and the standard voltage and you have 48 readings to record. If one channel is undamaged then it should be easier to identify the damaged device. But due to your test voltage leaking through adjacent components, it can often be confusing. Forming a table (like a spreadsheet) helps identify the readings that are the same and hopefully show those that are odd (zero ohms).

[h_a]

that looks like one output pair per channel.
This avoids the need to match parameters.

Could you please shortly explain that? This is not a complementary pair thus it would in general need matching. Or do you simply regard the increased distortion for a single pair not being significant?

Thank you, Hanne

[Colin151]

Thanks very much Andrew.

I tested the Mosfets last night with a multimeter on ohms and one set of pairs was definitely different to the other. One set had 0 Ohms readings on the pins and the other pair inifnity. On my working identical Alpha 9 amp (whose Mosfets I tested as well) they were all infinity at the pins. So the 0 ohms reading mosfets look like the dodgy ones.
So I can order some more and try to replace.

You mention I must also check the drivers and upstream. Hmm, sorry again, - what are the drivers?, what do they look like and how do I check them?!! and the stuff upstream?!!!!

I am learning slowly. Wish I had done some more electronics at school!!

Thanks again for you help and patience!

Cheers,
Colin