Arcam Delta 290 Fuse keeps blowing

[skenwrick]

I have an Arcam Delat 290 that has been working fine for many years. Recently I was careless when moving speakers and the red and black speaker cables touched while the AMP was switched on causing it to cut out.

Now I can't get the amp to power up - it just blows the fuse each time (the T1A6 surge protect fuse inside the amp).

Can anyone suggest what might be the cause of this and offer some suggestions for diagnosing or fixing the problem.

I am not an electronics expert - but would be happy to attempt diy repairs if necessary. I have a volt/amp/resistance meter..

Thanks in advance for any advice...

Sean Kenwrick

[Tekko]

Hello! I did the samke thing with a diy amp based on a stk module. You most likely blew the output transistors.

[JonHarrison]

Sean,

Replace the blown fuse with another 1.6Amp anti-surge. Turn the volume down to minimum and switch both speaker outputs off. Then, with all inputs and outputs disconnected from the amp (i.e. input phonos and speaker output cables removed) power on.

If I remember correctly the power led starts off orange and then after a short while the output relays click and the led changes to green. At what point does the fuse blow ?

Jon

[skenwrick]

I don't have any input or output connectors plugged in and I have the volume turned down to minimum - the fuse just blows as soon as I switch on (with a blue flash).

[JonHarrison]

Sean,

I believe the amp has an IC that protects the output against overload, it is possible though that the output transistors or something else in the output stage has failed short circuit following your accident.

The amp runs on +/-44V rails. With the amp off measure the resistance between +44 and 0V and -44V and 0V. There are three wires running from the toroid to the pcb, you can measure these. If you measure 0 ohms between either pair then you have found the short and the reason the fuse keeps blowing. Are you able to remove the output transistors from the pcb (there should be four bolted to a large heatsink), then try measuring the resistance again ? I'm afraid I don't know which of the transistors are left channel / right channel or positive / negative but perhaps you can work this out for yourself. It may be that one or all have failed.

Do you know which version of the amp you have? What is the serial number? Arcam used different transistors during the lifecycle.

Jon

[skenwrick]

The serial number is D29013849 - I will try and test the resistance like you suggested and I will post the results..

many thanks
Sean

[JonHarrison]

Hi Sean,

13849 looks like it's one of the later amps. I guess it's got a processor/normal switch on the back ? These used IRFP240 mosfets which are still readily available from RS/Farnell etc. for about £4-5 each.

Jon

[skenwrick]

There are three cables coming from the toroid (blue,green,blue) and I measured the resistance between the blue and green cables (for both blue cables).

I set it to the lowest setting (200 Ohms) and it is showing 01.6 for both cables. I guess this means a resistance of 1.6 ohms between the blue and green cables.

Do you think that this indicates a short? It is not zero but I get 00.8 from just touching the probes together.

I have unbolted the mosfet transistors from the heat sink but they don't seem to want to come out. Are they soldered into the PCB? If so I guess I will have to get the PCB out which looks like quite a tricky job since all the output connectors and switches woould have to be removed as far as I can tell.

If you think that they should just pop out then I might try applying a bit more force...

Let me know what you think..

thanks
Sean

[JonHarrison]

Sean,

>0.8ohms. Do you think that this indicates a short?

Pretty close but I probably should have actually asked you to measure it on the pcb having disconnected the blue/green wires first sorry. What you have measured is everything across the supply in parallel with the secondary of the transformer which itself will have a low resistance. If you can disconnect the wires and measure again please do. Depending upon which way round you have the probes you'll probably see the resistance changing as you charge up the caps.

>I have unbolted the mosfet transistors from the heat sink but they don't seem to want to come out. Are they soldered into the PCB?

Yes! Don't tug on them, they will need to be desoldered.

>I will have to get the PCB out which looks like quite a tricky job since all the output connectors and switches woould have to be removed as far as I can tell.

You shouldn't have to take any knobs off. I think you can unscrew the front facia from behind or the side and pull this forward. The phonos are screwed to the rear panel by a few screws and the pcb is screwed to the chassis by a few more screws. Once you have removed all of the screws the pcb should just pull forward and up away from the chassis. The toroid is heavy!

Jon

[JonHarrison]

Sean,

Rather than measuring at the input you could approach the problem from the other end. If you can get to them you could actually measure the output mosfets. Measure the resistance between the three legs of each device (left-right,left-middle,right-middle). I they are working correctly then I don't think any measurement should be less than a few hundred ohms, say about 500-600 or so.

Jon