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Old 20th November 2007, 11:02 PM   #1
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Join Date: Nov 2007
Default Help to fix C-Audio RA3000 Amplifier

Hi everyone,
Iím new to this, so I hope Iíve done this right.
One of our bandís amps, tan RA3000 has died on us. The symptoms are similar to another memberís, who I believe had help from Lee. The power light comes on, the fan whirrs and the lowest set of level leds (-40) come on, plus the r/h channelís -20 led, but the protect circuit stays on, and there is nothing at the outputs.
I have tried a signal at the inputs and varied the input level controls to no effect Ė the leds donít alter. Looking inside shows no obvious damage, and the fuses are all okay.
I would try to sort it out myself if I had a diagram, but any help that might get it going quickly would be appreciated. I tried to email Lee but as I have just joined was not able to.
Hope someone can help.
Thanks
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Old 12th December 2007, 05:08 PM   #2
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Join Date: Dec 2007
Grrr - i typed a long explanation then lost it !

here it is bullet-pointed:

C-audio RA series amps have a protection circuit that is very sensitive to damp (including damp dust on the transistor legs).

1. Establish that the actual amp channeles are fine.

a quick but brutal way to do this is:
a) replace the 80V channel fuses with quick blow glass 2A fuses.
(do a channel at a time, remember to discharge the HT caps trough a 47R/25W (say), resistor - or you will get a surprise.

b) locate the soft start resistor (depending on the board revision, this is either a 3inch long ceramic at the front, or a aluminium cased 47R/50W mounted to the heatsink, and bypass it with a wire link.
(remember THERE IS MAINS GOING THROUGH THIS RESISTOR!!).

c) turn the amp on for a second, then off again.
if any fuses blew, then you have a channel down.

d) if its ok so far, turn it on again. carefully test for +/- 75 to 85 V at both end of each fuse.
(use either of the black binding posts as 0V ref.)

e) if the power rails seem ok, test the voltage on the cases of the mosfets. on each channel, the cases are all linked and go through the speaker relay to the output.
the max voltage on these should be +/- 350mV.

f) a dc offset greater than 350mV should trigger the protect cct, you may need to fix the offset with the preset, or fix the channel.

g) if there is none, or a small dc offset, try a little sine wave into the amp with no speaker connected. - at should appear at the output devices cases.

h) you've established the amp channel is 'ok' (i.e. not the cause of the protect fault).


2. If the PSU and amp channels are 'ok', then the fault must be in the protect circuit.

discharge all caps with a resistor.
clean with a toothbrush and alchohol.
make sure all the black dust is out from between the small transistor legs.
from memory, i think they were BF422/BF423's about 3 or 4 of them.

when the board is fully dry and clean... see if its better.

after that, remove those small transistors, any 1N4148 dides around them.
use a tester to test the components around the ptotect circuit (if i remember correctly, this is at the back next to the fan so it can get nice and damp!.
replace the BF's with new ones (cost £0.80 for the lot !).
C-audio said once its all going, encase the legs of those transistors in silicone sealant.
I preferred to spray the area liberally with Electrolube DCA conformal coating (tropical! ).

hope this helps

j
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Old 12th December 2007, 06:03 PM   #3
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Hi J,
I appreciate the very detailed reply - I shall have a good look at all those points in your post this weekend.
Thanks for your advice.
Johnnymac426
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Old 15th December 2007, 09:37 PM   #4
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Default C-Audio RA3000

Hi
Thanks to J I tried the tests - all okay until I turned on the power with a link across the soft start resistor. The psu fuse blew on turn-on, though the channel protection fuses were all okay (I left the 2A ones in). Even with the channel fuses out of circuit the psu blew on turn-on, except when the link was removed. I could think of possible causes, but if anyone has any clues or a circuit diagram, I would be grateful.
Thanks, in hope.
Johnnymac
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Old 15th December 2007, 11:49 PM   #5
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Join Date: Dec 2007
hi - i do have circuit diags somewhere - A3 i think !

it sounds like you might have a problem with the power supply
try this (i have to say, i personally use more scientific methods), remove all 4 amp channel fuses.
replace the mains fuse with something like a T1.6A fuse. put the link back across the resistor.
if the fuse blows, its most likely you have a shorted (dead) rectifier.
if the fuse doesn't blow, you should be able to measure about +/- 75V on the supply side of each fuse holder.
be careful - the caps hold a lot of charge !

even if the fuse blows, you may be able to detect a voltage stored in the caps of one channel - this will give you an insight into which rectifier to suspect.

dead or dried out caps usually go hi-impedance rather than shorted, so its less likely to be these blowing the fuse.

option 3 is that the transformer has shorted primary - but i see this VERY rarely.

ways to progress:
you need to isolate the transformer secondaries from the rectifiers.
personally, i would just concentrate on the high current secondaries..
The RA-3000 actually has 3 supplies... +/- 15V for the op-amps, +/-75V for the output stage. and +/- 80V for the drivers.
i can't remember if the 80V supply uses 10V floating supplies in series with the High Current one, or whether it uses seperate 80V windings, either way, its not important at the moment.

As the secondaries are soldered directly to the board, you might as well remove the rectifiers. there are several benefits:
1) this will isolate the secondaries.
2) it will also isolate the caps
3) as we suspect the rectifiers, they're probably going to have to come out anyway.

with the 2 big rectifiers out, a T1.6A mains fuse, and the soft-start resistor linked.. the mains fuse should stay in tact.
if not, look for faults between the mains input connector and the transformer primary.
suspect the primary last.

if the fuse stays ok.. use a diode tester to check the rectifiers (4 diodes in there, google it
the diodes should all have the same forward voltage - usually 0.48-0.54V (less than for a signal diode).

finally, test the caps.
discharge them (again!) with a 47R/25W resistor (sometimes they charge up a bit even though you've left them disconnected!).
test them for short-circuit.
compare thier capacitances - if you haven't got a capacitance meter, you can use an old-school multimeter.
in the 10 Ohms range, the meter will show 0 ohms, which increases exponentially with time.
each of the caps should behave in the same way.

hope this is enough to go on.

do be careful - 250V in these things y'know
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Old 17th January 2008, 07:36 PM   #6
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Default C-Audio Amplifier

Hi,
Back again! Sorry not to have given you the courtesy of a reply to your helpful post until now, but have been away.
I have checked the amp today and I think your first thoughts were right - a power supply fault. The main fuse was blowing every time I switched on with the other supply fuses out of circuit and with the link across the soft start resistor. So I traced from the mains input to the fuse - no shorts. I desoldered the bridge rectifiers and the toriod outputs and still the main fuse blew.
Having freed the toroid from its bolt I found that it had been touching against the back edge of its mounting plate (bolted too closely) and an area of the tape had been chafed - it also looked slightly brittle as if it had heated up, though apart from discoloring of the lacquer where the windings had been against the plate none of it appeared to have burned through.
I think any damage has been done inside - a quick resistance check of the primary showed 1ohm. I know primaries have a low resistance but surely not that low?
So, unless I can find a cheap replacement transformer, that's it for this amp!
Anyway, thanks for all your help.
Johnnymac
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Old 17th January 2008, 08:50 PM   #7
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Join Date: Dec 2007
cool - be careful with torriods in general...
the secondary windings should be the outermost (you can verify this with a meter, seeing as the copper is already exposed).
make sure there is no possibility of adjacent turns shorting to each other.
a shorted turn can will have a very high current running through it and can damage itself, the primary, and / or the soft start circuit.
This is one of the reasons there is thick insulation on the underside of the amp lid directly above the transformer's mounting bolt (bolt + case = shorted turn! ).

if you are convinced that the transformer is safe to use, then go ahead.
you should also check the condition of the soft start resistor..... if you have the model of the RA3000 with the long ceamic resistor then it'll be fine.
If your amp has the golden coloured aluminium resistor, them you should inspect this carefully.
The problem is that this resistor doesn't really like being in an RA3000, if the amp stays in 'protect' for any period of time (as little as 30 seconds), the windings and ceramics inside the alu tube expand - usually loosening one of the terminals. This means that the winding can come in contact with the alu case.
The winding is at 240V mains, the case is connected through the heatsink to earth.
the fuse or RCD should cut the power IF THE AMP IS PROPERLY EARTHED ,
but it should be remembered that even with good eathing in the power cable, the chasis will momentarily be at a potential of 120V while the fault exsists.
if in doubt, replace the soft-start resistor.

Good luck with the amp :-)
J
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Old 17th January 2008, 09:55 PM   #8
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Join Date: Dec 2007
Hi,
just read your post again... 1 Ohm is too low for that primary.
I guess it must've got damaged by the shorted turn.

as an experiment, you can drive the secondary from about 12V AC through a current limiting resistor - 47R / 5W say.

you should see about 25-35V coming out of the primary.

Getting a replacement transformer will not be easy :-(
this particular design has extra secondaries - you wont find them in most other amps.
there are 75-0-75 for the output stage as usual for a 1200W Amp.
but there are some other windings.
there may be a 15-0-15 for the input circuitry (op-amps) - but i don't think there is on these (i havent got the cicuit diags to hand).
but there is definately a +/- 80V supply in this amp the power the driver stage.
again, without the diags, i cant remember whether there is an 80-0-80 low current set of windings, or i have a suspicion that they use 2 x 10V windings added to the 75-0-75 to up it (the 80V supplies are low current compared to the 75V ones).
The only other amp i know that uses this design is the Studiomaster Mosfet 1000 (1980's), but the transformer is too high to fit in an RA3000.
Another option is to get the transformer from an old BK electronics MXF900 (AKA sound management DNA900) and use that for the 75-0-75. (BK will sell this transformer as a spare).
Then get a small torroidal 10-0-10 1Amp transformer from RS.
you're gonna need to work out how to fit them in, and how to wire them correctly - a bit of a hassle really.

option3 - put the amp on ebay as 'faulty, spares or repair' - be honest about whats wrong with it and you'll still get about £100 for it.

hope thats of some use,
J
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Old 18th January 2008, 09:11 PM   #9
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Hi J
Thanks for your replies. I'll have another look at that toroid, just to make sure, but I think that's the problem
The band has got another amp in the meantime, so I can take my time on this.

Thanks for all your help so far.
John
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Old 21st February 2008, 03:50 PM   #10
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Join Date: May 2006
Location: Cambridgeshire
Hi J,
If you want proper help to fix this unit which doesn't involve shorting out safety components and dismantling the amplifier give me a call on 07768551482. it's only an RA and shouldn't more than 1 hour to fix (unless you have trashed something doing some of the silly suggestions I seen on this thread).

I can also supply circuit diagrams for these units in electronic format.

As the Service and product manager of C Audio (Until Harman killed it off) I know a little bit about the amps.

Lee Basham
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