Arcam 9P blowing fuses, please help.

[iolaus]

Hey all,

Last night I ran into some trouble with my Arcam Alpha 9P. I just got a pair of KEF 105/3s which are rated at 4 Ohms (and I have since checked out to be 4.1 Ohm and 4.3 Ohm respectively). I was doing some high-frequency output testing on the KEFs with a 5KHz sine wave (at medium volume) when my 9P turned off.

I checked out the 9P to find the fuse was blown so I let the amp cool, disconnected the load, and replaced the blown fuse with the provided replacement. Upon turning the amp on, sure enough, the replacement blew too.

Now... I'm in busted electronics depression mode.

I'm a computer engineer (which around here means about 75% EE and 25% Csci) so I'm very familiar with electronics but haven't worked much with audio amplifiers. Can anyone give me some tips on where I should start in diagnosing and trouble-shooting the problem? If I'm not able to correct the problem does anyone know what it might cost me to have it repaired?

Thank you,
Ryan

[anatech]

Hi Ryan,
I would test the outputs for a short. Rectifiers too.

Audio amps normally have an RC network on the output called a "zobel network". Amps do not like to be run at high frequencies, nor do your speakers. Don't do this again.

-Chris

[iolaus]

Chris,

Thank you for the information, I have certainly learned my lesson! What is the issue with the zobel network and high frequencies? Is it most likely the sustained high frequency that gave my Arcam a heart-attack rather than the 4 Ohm load?

I'll test the output transistors for shorts as soon as I get home from work today (hopefully I can do it without having to do too much deconstruction). As for the rectifiers, is there an easy way to identify them? Are they going to be close to the transformer?

If it turns out some of my output transitors have shorted, are there any other components I should also check for damage?

Thanks again for taking the time to help me out,
Ryan

[anatech]

Hi Ryan,
There is no shortage of help on this board. Many good hearted souls here.

The Zobel network is a cap in series with a resistor. The resistor varies between 2.2 ohms and 10 ohms. As the frequency goes up, the impedance of the cap goes down. This puts the resistor in parallel with your load resistance. This may generate a current high enough to open the capacitor or resistor. The amplifier may then become unstable and oscillate. This will overheat the output stage and cause it to fail. Various causes for this, doesn't matter because the end result is blown outputs and or drivers.

So check the outputs , drivers, bias transistor and back until you start seeing good parts. I would suggest a trained audio technician work on your amplifier. Someone who actually does this for a living and is good at it. It's not the big things that will get you normally, it's the small details.

-Chris

[iolaus]

Chris,

I'm wondering if I have any chance of getting a technician to repair the amp for something less than its worth (around $200-$300). If not, I think I'll probably use it as a learning experience and take a stab at it myself. (gotta put all those EE classes to practical use some day) I'm familiar with the output transistors but I guess I'm not familiar with what you mean by the 'drivers' and would I be correct in thinking that the 'bias transistor' will be a transistor mounted on the same heatsink as the output transistor in order to bias the output transistors in response to thermal changes? Also, as far as the Zobel network being a possible source of failure, will I need to check all caps/resistors going back from the output for shorts to verify if one has failed?

Thanks again for your help,
Ryan

[anatech]

Hi Ryan,
The amp should be repairable for less than the value. If it's a write off (unlikely) then use it to build another amp project listed on this board.

The drivers are the transistors that "drive" the outputs. You need to check for shorts, DC beta and leakage currents when you test transistors. Do not ever use ECG/NTE branded parts. If you do, just fill the unit with gas, light it and walk away. (I feel strongly about this).

You need to check the capacitors for value and quality. The resistors for tolerance, burn marks exclude them from further use.

First, get the schematic at least. The service manual is the best. Use good service equipment. HP or Fluke bench meters, or a Fluke hand held (87 series is the best). A proper LCR meter will make your life easier (bridge is okay if you are patient). A temperature controlled soldering station is a must. Solder sucker and braid. An oscilloscope is invaluable. An audio oscillator is needed, a distortion analyser is very handy but not required. A good transistor tester makes your life easier. Heathkit IT-18 or IT-121 are good. I use the IT-18 the most on the bench. If you have access to these you have a good chance of repairing the unit to it's original specification.

-Chris

[iolaus]

Chris,

I do have (or have access to) most of the equipment mentioned but I still might call around a bit and see if I could get the amped repaired for a reasonable price. I did some testing just now (as soon as I got home) and found that your prediction seems to be dead on. The output transistors on one channel are shot (both showing very low impedances between leads). And also, as you predicted, on the bad channel I found 2 shorted diodes shortly before the output (I'm guessing part of the Zobel network?). However, all resistors and diodes before the 2 shorted diodes appear to check out. I haven't been able to check the bias transistors yet as they are hard to get at because of their heatsinks. I'll get those removed and test them as well though. Given this new info, does it seem likely that there may be more damage than I'm finding or did I possibly luck out and only blow up the very end of my output stage on one channel?

Exremely impressed by your predictions,
Ryan

[wintermute]

Hi Ryan,

You didn't specify whether it was rail fuses or the main power fuse that went. In any case have you disconnected your speakers from the amp?

If not disconnect them and try again, you may have a short within the speaker or speaker wires.

if it is not the rail fuses but the main fuse, then you may have a short in the PS. perhaps a diode has gone short circuit....

Tony.

[iolaus]

Tony,

It is, infact, the main fuse and it blows with no load attached to the amp as soon as the amp is powered on. I've tested as Chris suggested and it does appear that the output transistors on one channel are blown as well as two didoes right before the output transistors. However, I have found no other signs of damage thus far and no burnt-electronics smell or scorch marks.

I've called around and it seems it will cost me $150-$200 to have the amp repaired (which is very close to its value) so I think I may just take a stab at repairing it myself.

If anyone has suggestions on additional testing I should do before attempting repair it would be appreciated. Also, once I disassemble the amp further I'm sure I'll have to ask here what parts will be suitable replacements for the bad transistors, diodes, and any other dead components I've yet to find.

Thanks,
Ryan

[jaycee]

The outputs are N-channel MOSFETS. If you ask Arcam for the service manual they should supply it.