Help to fix C-Audio RA3000 Amplifier

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Hi everyone,
I’m new to this, so I hope I’ve done this right.
One of our band’s amps, tan RA3000 has died on us. The symptoms are similar to another member’s, 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 channel’s -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 – the leds don’t 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
 
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
 
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
 
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
 
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
 
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
 
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
 
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
 
..also, while i think about it, the soft start resistor isn't a safety component and the way c-audio used it is boody dangerous - no wonder they went bust.
shame - those RA's are one of the only mosfet's with a proper driver stage (as published in elektor magazine in the 80's).

also - what's the deal with the GB series being the same product as those crowns ?
j
 
Thanks hugo,
i do try to keep my posts helpful and avoid getting personal.
i try to help people that i think have a limited understanding of electronics (and probably only own a multimeter) - the main message i was trying to give jonnymac was 'please be careful and don't get in too deep if you don'y really know what's going on'
i don't tend to give out circuit diags because if you are a novice, you will often mis-understand them.
i believe that the professionals amongst us don't need circuit diags (they're all the same anyway :D )

although i do occasionally have to draw them out by hand for future reference.

thanks again,
jonathan
 
Hi All,
I wasn't suggesting he send it to me to fix, I was suggesting he call me and get advice on how to determine the fault with the minimum of effort. I have repaired enough of these units to, in most cases, diagnose them over the phone. my method of diagnosis is based on the 'do no damage' approach and certainly shorting out the soft start resistor could easily lead to further damage. The use of the soft start resistor in this circuit is perfectly safe and provided the soft start fuse is the correct type and rating and considering the number of years that these units have been in service it obviously works.

When I suggested it should only take an hour this is the usual time to fix one of these amps given the correct advice and service approach.

I also like to speak to people before I send circuits to determine their level of competence as I too do not wish to have a death or injury on my conscience.

for the record, C Audio didn't go bust, Harman bought and gave the products to Crown, hence the GB thing.

Lee
 
Thank you for giving a more rational response, it doesn't really do to insult other members.

i stand by my remarks about the version with the aluminium-clad soft start resistor (the version you made with the long ceramic resitor is much better).
if i remember correctly, the soft start resistor was protected by a T1.6A fuse ? either way, we had a number of units in with the 'damp soft start circuitry' where the resistor had expanded. i really dont think it would meet the 3mm creepage requirement for a CE mark today.

As i said before - the RA series are an excellent, reliable amp. (much better than those awful TR850 ones you asked me to send back because they had so many missing mods they shouldn't have got out of the factory :dead: )

if you have it, you should offer jonnymac a copy of the soft-start damp-proofing mod sheet - very useful

thanks for the info on the GB series - it did have us a bit baffled.
jonathan
 
Hi All

Please Note (Excluding the TR850 which has a T500mA)) Where fitted, the soft start fuse in ALL C Audio amplifiers MUST BE A F1.6A
IMPORTANT: A T1.6 or anything higher is effectively a nail and will not fail under fault conditions and Will lead to further damage, starting with the soft start resistor and moving on through the amp if the resistor is bypassed.

If you have any doubt about an amplifier (and you should if its in for repair) i would suggest you assemble a light bulb fixture, this comprises a standard 60W light bulb in a holder in series with the live wire of a power lead. you can do this elegantly with a switch and sockets etc or simply by splicing the light bulb socket into a mains lead. PLEASE ensure that you use an insulated light bulb holder for this purpose.

Typical results using the light bulb.

Amp has a blown/Short circuit output device. light bulb glows brightly (No damage to amp as bulb takes all the current)

Amp has a short circuit bridge rectifier/other similar failure in PSU.
light bulb glows brightly (No damage to amp as bulb takes all the current)

Amp has driver stage failure pulling excess current. light bulb glows brightly (No damage to amp as bulb takes all the current).

in the event of an amp with a shorted output device but a blown line fuse, or an amp which is not fatally faulty, the common result is a light bulb which glows brightly to begin with but will rapidly dim down.

Once you are happy that the amp is not going to burst into flames or blow all your trips you can swap over to full mains (VIA A MAINS ISOLATION TRANSFORMER).

I use this technique with all repairs and it works, in cases where the transformer itself has melted giving a short across the mains this method has given a clear and bright indication of a fault without recourse to resetting trips /Clocks etc or further damage to to product.

I hope that this will be of some help to other engineers out there and remember you are responsible for your own safety and that of others around you and the eventual user of the product after servicing it. You should at all times ensure that when working with live product you do so in a safe manner and that the product is safe for use. Electricity can be a dangerous friend, treat it with respect.

Lee
 
Quick reply to JohnnyMac,

If you use the light bulb method to power up the transformer on its own you will be able to determine if the transformer is faulty. (Make sure all the secondaries are seperated. the DC resistance of this transformer primary is very low, so a 1 Ohm reading is normal, it is after all just a big piece of wire.

the very early RA units were supplied with a transformer tray with a small dogleg in the metalwork for strength however this was quickly changed when it was found the transformer rubbed. there should have been a strip of neoprene rubber along that edge to prevent rubbing but these have sometimes disappeared over the years.


Having read the initial fault description it sounds like the unit originally worked except for being stuck in protect. if you do find that the transformer is OK (you might well be lucky) i would suggest you put the amplifier back together as it was when you started, and try the light bulb again on the amp as a whole. then give me a call and we can try to sort it out for you.

Lee
 
I've got a very similar problem with a RA3001. The protect just flicks on and off on both channels repeatedly.

There's no visible damage on the boards, all the fuses are fine etc.

Just wondered if I should be following the advice given above for this problem or if there is a known fault with these symptoms.

Cheers,

Steve.
 
Hi Steve,

Do you have a signal going into the amp when it cycles protect, if yes try it without the signal applied does it still do it.

If it does, is it clicking around once every second?

If it doesn't check the fuses, they may be intact but the fuse holders themselves may be splayed apart and not connecting, unplug the amp, then remove the four line fuses and with a pair of pliers gently squeeze the fuse tags together. when fitting the fuses place them on top of the fuse holder and when they are centred press down firmly till they click.

If this doesn't solve the problem, call me on 07768551482 to discuss further.
 
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