| Johnnymac426 |
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 |
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| AudioSystemsEng |
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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| Johnnymac426 |
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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| Johnnymac426 |
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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| AudioSystemsEng |
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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| Johnnymac426 |
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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| AudioSystemsEng |
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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| AudioSystemsEng |
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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| Johnnymac426 |
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 :bawling:
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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| lee.basham |
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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| AudioSystemsEng |
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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| AudioSystemsEng |
..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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| Netlist |
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 :cop: |
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| AudioSystemsEng |
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 |
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| lee.basham |
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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| AudioSystemsEng |
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 |
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| lee.basham |
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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| lee.basham |
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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| stevothepsycho |
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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| lee.basham |
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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| cleverzippy |
Lee, I have just bought an RA3001. It was sold as not working and as I suspected the 47R soft-start resistor has gone o/c along with the 2A fuse. I had one of these amps from new 1o years ago and it did the same thing then. I phoned C-Audio at the time and they sent me a new resistor and told me there was a mod I could do which involved pulling a diode or resistor on the board - I can't remeber which. Can you help.
Thanks
Adrian. |
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| lee.basham |
Hi Adrian,
1. you have an RA3001, therefore this already has the mod tracked in
2. the soft start fuse IS NOT 2Amp, it MUST be F1.6A
3. the soft start resistor must be 47R
that said, it is almost certain that you have a further fault on the unit.
My suggestion is to start by ensuring that the above are rectified and then prepare a mains lead with a light bulb in series with the live wire and armed with this give me a call on 07799386256 or 07768551482 and i should be able to talk you through a diagnosis.
Lee |
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| AudioSystemsEng |
Hi Lee,
I've got an RA3000 with an interesting fault. I've had this one since new, but its been in storage for about 5 years.
When i took it out recently and checked it over it seems to have turned into a low-pass filter.
putting a 1kHz sine into it (with no load) gives me a sine out at low levels... as i increase the level the sine turns into a pretty perfect triangle wave.
At 1 kHz it is not possible to get full amplitude out of the amp.
Both channels behave identically. The 15V, 90V and 100V rails all seem fine, I've also injected the signal after the Input op-amps, with the same result.
Running a frequency sweep shows that the amp is working well at, say, 100Hz, but it's behaving as a LP filter as i increase the freq.
Any ideas?
jonathan |
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| lee.basham |
Sounds like the line fuses are missing or o/c the fuses are alongside the heatsink either side of the TX.
with these out (off load) you will get a sine wave at low levels but when you turn the level up it will look triangular. if you turn the amp off at this point and drop the line fuses down you should see a decaying sinewave with both fuses in, if when you drop a fuse the output disapears then you have a fault on the output stage.
one thing this amp suffers from is loose fuses in the fuse holders. sometimes on load you can hear them signing along to the audio. remove the fuse, and with an insulated pair of pliers squeeze the fuse lugs together, when refitting the fuse drop it down so it is central on the fuse lugs and then press down in the middle to ensure both ends pop in at the same time, if the fuse topples and only goes in one side then start again as you will have bent the lugs and be back where you started.
Alternately remove the fuses and holders and fit 10A fuse wire across the points and solder into position, I find these rarely come loose.
Let me know what you find.
Lee |
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| AudioSystemsEng |
hmm - i guess its possible that the fuses are o/c.
i'm nowhere near the amp or cct layout at the moment as i'm not due in my workshop for another 3 weeks.
are you suggesting that the LT (75V) rails can charge up from the HT rails ?
i was getting 98V HT and about 90V LT (which i thought was a little high).
would this explain the low-pass behavior of the amp with no load ?
i was getting 6dB/oct rolling off at about 200Hz.
i'll post what i find next time i'm in the workshop, although its at he back of the q now - i've got 14 crowns to do first for a paying client :-)
thanks again,
jonathan |
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| cleverzippy |
Hi Lee, thanks for your help on the phone. I have found the defective 'fet, a P channel BUZ905s, and have removed it. The bulb now dims and everything seems to work OK. I have ordered the part - a 905 - no 's' designation for Monday. Can you remind me what I need to do now with regard to checking the other 'fet's for possible wear and tear.
Thanks
Adi. |
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| lee.basham |
switch the amp on and with no signal and no load (Quiescent state) using a DMM set to mV range measure across each gate resistor (Not with respect to ground but across the resistor legs) you should have ZERO volts drop, any voltage here even 5mV will indicate a soft FET and will need to be replaced. failure to replace them will lead to their demise and the probable weakening of other and it then become a viscous circle.
on the plus side you may have none or just one other which is dodgy. it is incredibly rare to have a whole lot of FETs fail so rare i would not suspect it unless there were some obvious outside cause.
the S suffix indicate a single die device the D suffix indicates a double die device. you only require the single die device in an RA.
thanks
Lee |
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| AGS |
Hi
A friend of mine have tried to repair this beast.
5 MOSFET's were gone to the eternal transistor heaven, all in the same channel. ( 4x 2SJ162 + one 2SK1058)
He purchased new ones, and replaced the faulty ones.
And he replaced the fuse in series with the softstart resistor with a new one, only problem was that this was a T10A fuse...
He switched on the amp, and at least 4 of the new MOSFET's went the same way as the old ones.
He then sendt the amp to me....
I followed Lee's advice (60W light bulb trick), and found that the softstart resistor was defect.
In lack of a replacement resistor, I replaced it with this light bulb, and disconnected power wires from PSU to the MOSFET's, and switched on the amp.
The light bulb glows brightly and dim down slowly, so PSU and brigdes seems OK.
Meassures +/- 68V and + 6V from PSU - I think this is normal, but can anyone confirm this?
I would love to have diagrams for this amp...
The new MOSFET's did not have a branding, so I can't tell if this is a Hitachi/Renesas one, but I guess they were not good enough for this amp.
Any sugestions on what went wrong?
And what to look for, before I replace the MOSFET's?
Best regards,
Alf Gunnar Sørensen |
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| AudioSystemsEng |
I'm not familiar with the RA4000, i had a feeling the 2SJ162 + 2SK1058's were obsolete ?
I have to say that simply replacing dead output devices and switching it back on is NOT the way to repair an amp (ok.. it works with old peaveys).
you really need to go through a complete procedure of testing with a low voltage current limited power supply to establish that the amp is behaving correctly before you hook it up to the mains.
lee here is very good with the C-audio's so hopefully he'll help you if he's not too busy.
jonathan |
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| Kevin.kinchin |
Hi Lee, cant e-mail you yet until i'm an 'upgraded' member !
Just like to thank you for your advice with my SRX3801. Two soft output devices replaced and the amp has stopped overheating.
With only about 5mV across the gate resistors this was not enough to bring the protect in, hence throwinig me a little.
In the past with the many XR and RA amps we have used for PA work the 2SJ and 2SK devices seem to fail and bring in the protect circuit immediately and then the light bulb trick sorts the down time out pretty quickly.
Hope the new modular amps and powered speakers are going well for you and keep up the posts on here as your advice is of great help to a many of us. :smash: |
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