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repairing c-audio st600

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Courtec View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Courtec Quote  Post ReplyReply Direct Link To This Post Posted: 27 November 2015 at 10:53am
Dear all,

I recently received a ST 600i C-audio amp with a dead channel. The mosfets are oke, there is someting else going on. Does anybody have a service manual for these amps?

Many thanks in advance. I'll keep you posted about the solution.

regards,
Koert.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote snowflake Quote  Post ReplyReply Direct Link To This Post Posted: 26 March 2017 at 7:00pm
amp in original post is down again. clip light stays on and relay on that channel doesn't click on. fuse FS2 is blown and transistors TR6 and TR7 are visibly blown - all the others look okay but not tested.

haven't been inside an amp for a few years other than to blow the dust out. planning to replace the fuse and then start comparing the two channels. try and find the problem that made it blow before replacing TR6 and TR7. is this the right approach or should I replace TR6 and TR7 and then try to fault find?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kedwardsleisure Quote  Post ReplyReply Direct Link To This Post Posted: 27 March 2017 at 3:01pm
you need to make sure all the fets are ok, with renesas fets the recommended way is to replace all of them in the affected channel. It only takes one lazy fet to spring the amp into oscillation then it will fall over. And re-cap the amp if you havent already.
Kevin

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Post Options Post Options   Thanks (0) Thanks(0)   Quote snowflake Quote  Post ReplyReply Direct Link To This Post Posted: 27 March 2017 at 4:50pm
Originally posted by kedwardsleisure kedwardsleisure wrote:

you need to make sure all the fets are ok, with renesas fets the recommended way is to replace all of them in the affected channel. It only takes one lazy fet to spring the amp into oscillation then it will fall over. And re-cap the amp if you havent already.


I was afraid you might say something like that.

so four pairs of these:

http://www.ebay.co.uk/itm/Matched-2SJ162-2SK1058-J162-K1058-HITACHI-RENESAS-High-Power-Audio-MOSFET-/191590215049?hash=item2c9baaad89:g:ASYAAOSwBLlVZ5od

and replace the four 10pF caps across the fets.

borderline whether it is worth doing this repair but will be my bit for the environment.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote snowflake Quote  Post ReplyReply Direct Link To This Post Posted: 27 March 2017 at 4:55pm
or do you mean the power supply caps?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kedwardsleisure Quote  Post ReplyReply Direct Link To This Post Posted: 27 March 2017 at 7:46pm
the LV rail caps etc especially if it's lived a hard life.
Ebay fets always a bit dodgy.
Kevin

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Post Options Post Options   Thanks (0) Thanks(0)   Quote snowflake Quote  Post ReplyReply Direct Link To This Post Posted: 27 March 2017 at 11:12pm
Originally posted by kedwardsleisure kedwardsleisure wrote:

the LV rail caps etc especially if it's lived a hard life.
Ebay fets always a bit dodgy.


one on each channel, c15 and c38 4700uF
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Post Options Post Options   Thanks (0) Thanks(0)   Quote digitallake Quote  Post ReplyReply Direct Link To This Post Posted: 28 March 2017 at 12:53am
Hi,
This popped up on my email notifications from a few years ago.
I would agree with Mr Edwards advice that the amp is getting to the age now where there will be many areas of attention needed in addition to repairing the specific faults - replacing capacitors, etc. The cause of the instability and output stage failure could be a number of reasons or combination, so it's best to make sure all the basics are in place first and also make sure all the manufacturer's recommended modifications are done (there's quite a lot, the mod sheet is here http://tophat.myzen.co.uk/service_data/c_audio_ST400-600.pdf )

I'd agree that it's sensible to replace all the output devices due to type, unless you have a way to make absolutely sure an individual transistor is OK before re-using it. I would normally remove all the output devices from the affected channel, and test them individually but a simple low-current test may not indicate a reliable component.

If only one channel has failed, then I would proceed by firstly doing all the basics to both channels, such as the re-capping and the recommended mods, and anything else obvious like dry solder joints or corroded PCB traces (common in these amps) I would then remove all the output devices from the *working* channel too, and proceed to compare the remainder of the amp channels with each other, node by node, with a curve tracer. If the channels pass that test (are identical signatures throughout) with no output devices fitted, then proceed to re-fit known-good output devices to both channels and re-test by curve tracing on the output stages to make sure before applying power.

When power is eventually applied after all the cold / low-current testing has passed then it should be done in a safe way, by limiting the current to avoid any damage from fault areas that may still exist.

Because it is a fairly old amp, the procedures above or similar, depending on how pedantic you want to be, are probably necessary to get a reliable amp again, and it will probably be a fair amount of work to cover all possible areas of potential causes of the repeated failure of the output or other stages.

Cheers Jim
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