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Old 21st February 2008, 06:29 PM   #11
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thanks for the trolling

of course - if you want me to repair the amp (for a fee) i will do that, but i didn't think this forum was about touting for business (hence the name 'DIY')

jonathan
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Old 21st February 2008, 06:35 PM   #12
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..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
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Old 21st February 2008, 06:56 PM   #13
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Jonathan,
There are more diplomatic ways to refuse an offer.

Lee,
Others might have given wrong info, the intention was to help. A few hints from an experienced tech will certainly be appreciated.

/Hugo
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Old 21st February 2008, 07:56 PM   #14
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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 )

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

thanks again,
jonathan
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Old 21st February 2008, 11:11 PM   #15
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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
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Old 22nd February 2008, 12:28 AM   #16
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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 )

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
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Old 22nd February 2008, 09:52 AM   #17
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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
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Old 22nd February 2008, 10:05 AM   #18
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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
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Old 28th February 2008, 11:44 AM   #19
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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.
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Old 28th February 2008, 03:34 PM   #20
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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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