| Arcam 9P blowing fuses, please help. - Click HERE for Original Thread |
| 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 |
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| 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 |
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| 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 |
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| 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 |
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| 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 |
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| 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 |
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| 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 |
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| 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. |
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| 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 |
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| jaycee |
| The outputs are N-channel MOSFETS. If you ask Arcam for the service manual they should supply it. |
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| anatech |
Hi Ryan,
$200 USD to fix it? I'm in the wrong country!!
The two diodes are flyback diodes. They are installed reverse biased across the output transistors. If the output is shorted, the diodes will test short unless you unscrew the collector bolts from the transistors. The diodes are not part of the Zobel network, that's an R-C network, remember?
You should give repairing this amp a shot since you can access the equipment.
Replace the driver transistors even if they test okay. All tests must be done out of circuit to be accurate. See if you can get a hold of a variac to use when powering this amp up after repair. I do not have the schematic, so I can only guide you in general terms.
Start by taking digital pictures as you go. That will show you how the parts were oriented. I do this - because I'm lazy for one.
-Chris |
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| anatech |
Hi jaycee,
You posted as I was composing. Thanks for the info. You wouldn't have a schematic would you? You could guide Ryan on his repair.
Ryan,
Make sure the gate resistors are okay, if they are open, you will blow the new outputs as you power up.
-Chris |
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| wintermute |
ok looks like you have cooked one channel then :( I'd still check the speakers out before you reconnect them, a short on output could cause the transistors to fail, I've been lucky in that respect with my amp as I have had a couple of shorts (even a stray strand of speaker wire will do it) but It's allways blown the rail fuses rather than the output devices.
provided the replacement parts aren't too expensive, then yeah I'd say go for it :) makes things a lot easier too if you have one good channel for reference!
Tony. |
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| iolaus |
Thanks for all the tips and encouragement everyone.
Chris, I guess I wasn't thinking when I wrote about the Zobel network... should have given that some more thought before I posted. :)
Interesting about the flyback diodes, I guess I'll have to get the mosfets out before I can really test those. And while we're on the topic of testing, do I simply need to test all the resistors, diodes, and caps in the output stage of the bad channel (assuming I replace all the mosfets) before proceeding with repair? From the testing I've done so far (preliminary on resistors and diodes) I haven't run into any issues... other than the flyback diodes.
Could it really be a simple task of replacing the mosfets to bring my Arcam back to life? That seems almost too good to be true *knock on wood*
Thanks again everyone!
P.S. I'll contact Arcam about getting a service manual too. |
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| anatech |
Hi Ryan,
That's a tough question for me to answer. I don't have the unit in front of me and I don't have a schematic for it.
You can measure the resistors in circuit unless you get a reading that does not agree with the value marked on the part. Caps need to be tested out of circuit for a true answer. I would tend the replace the drivers and bias transistor anywhy since the outputs shorted to gate as well. I would be wary of the gate resistors to. Use metal oxide resistors for replacement if you can get them.
Don't power the unit up before posting again. I'd like to give you some pointers that will be fresh in your head when you reach that step.
-Chris |
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| burnedfingers |
If I may throw 2 cents in here also.
I would highly suggest replacing the driver transistors as well as the bias transistor. This would be better than chancing a transistor that has been stressed. The cost will be small. |
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| pinkmouse |
| Arcam are very good with schematics, whenever I have needed one they have usually sent it back on the same day I emailed them. Unfortunately I have never asked for an Alpha 9, or I'd mail it to you direct. ;) |
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| iolaus |
Hey guys,
Arcam provided me with the 9P service manual which I've put up here for viewing. Can anyone give me a list of components I should pay particular attention to (and maybe ones that should be replaced for sure)?
Thanks,
Ryan
P.S. If there are forum rules against posting the schematic please let me know and I'll remove it. I'm assuming since Arcam provided the schematic so freely they won't be too concerned with me sharing it (at least temporarily). |
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| jaycee |
I doubt Arcam are too bothered now that the Alpha series is discontinued.
It's a more complex amp than I remembered, most of it would be the auto biasing circuit. Obviously check the output mosfets, they have most likely shorted drain to source. You'll also want to check the resistors labled R1/RA1 and R101/RA101, the 0.22R 3W ones to see if they have failed. You're going to want to check the gate zeners D3/D4 and D103/104 respectively too. I would also check Q3, Q4 and Q5 (and the equivalents in the other channel) to make sure they havent toasted. Any resistor marked "FU" (ie Fusible) needs checking too as it may have gone open circuit.
You might be lucky and only the outputs have toasted. Someone on this board posted with a damaged Alpha 7 that is very similar circuitry (minus the auto bias basically) and all that went were the two mosfets.
You may also want to check the speaker relay as they are not very high current and tend to weld when the outputs fail. |
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| anatech |
Hi Ryan,
Thanks for the post.
Jaycee has given great advise. One thing he was mentioning is that some resistors can look perfectly fine and yet be blown. Do not trust your eyes, measure them and write down what is okay and not. This will save you some time.
-Chris |
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| iolaus |
| Thanks for the tips guys. I'll start taking some measurements after work today and document/post any trouble spots I find. |
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| iolaus |
Hey all,
Sorry for the delay in my response but things were a little hectic the last few days. I just got around to doing some measurements on the amp tonight and here are my preliminary results.
Q1: Shorted
Q2: Shorted
R1: OK
Ra1: OK
D3: Shorted*
D4: Shorted*
Q3: OK
Q4: OK
Q5: OK
* if I understood correctly, Chris made it sound as if these diodes will read as shorts because of the damaged mosfets. I'm planning on ordering replacements just in case and re-measuring once the mosfets (Q1 & Q2) have been replaced.
Everything on the other channel checks out so far. Haven't checked the speaker relays yet because I haven't taken the amp apart far enough to get at the under side.
Any suggestions on a next step? Do I need to do more detailed analysis or do I know enough now to begin some repairs? Either way, any suggestions for replacement parts and suppliers would be greatly appreciated.
Thanks again for all your help everyone,
Ryan |
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| clem_o |
Ryan,
Suggest putting an incandescent bulb in series with the power line to limit the current in case something else is busted, this may save you having to fork out more money to replace costly output transistors (Q1 and Q2).
BTW, you may want to re-test your KEFs on "normal" music and make sure that you haven't blown anything - 5KHz at "moderate" volume puts a lot of stress on the tweeter unit of that Uni-Q, and they really are NOT units that will stand up to abuse. The tweeters are ferro-fluid cooled, but the magnet stucture is small (as they are inside the midrange driver) and, since there's no such thing as a new 105/3, its also possible that the fluid's already dried up.
Replacement units for the tweeter (they are replaced by pair, to keep the performance of the loudspeakers balanced) are available from KEF UK, though these will need a slight mod to mount properly (and may require a little tweak to the crossover network). If you need details (though I certainly hope your pair is still OK) on this just send me an e-mail...
Cheers!
Clem |
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| anatech |
Hi Ryan,
Clem is right. High power 5 KHz will take the tweeters out right away. Used to do KEF warranty. Use the KEF tweeters and proceed carefully.
Undo the screws holding the TO-3 outputs down and you should be able to measure the diodes.
-Chris |
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