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Old 10th September 2008, 04:20 PM   #21
diyAudio Member
Join Date: Sep 2008
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.
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Old 11th September 2008, 06:11 AM   #22
diyAudio Member
Join Date: May 2006
Location: Cambridgeshire
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.

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Old 11th September 2008, 10:14 AM   #23
diyAudio Member
Join Date: Dec 2007
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?
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Old 11th September 2008, 08:21 PM   #24
diyAudio Member
Join Date: May 2006
Location: Cambridgeshire
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.

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Old 12th September 2008, 12:06 PM   #25
diyAudio Member
Join Date: Dec 2007
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,
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Old 12th September 2008, 08:05 PM   #26
diyAudio Member
Join Date: Sep 2008
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.

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Old 15th September 2008, 06:54 PM   #27
diyAudio Member
Join Date: May 2006
Location: Cambridgeshire
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.

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Old 29th September 2008, 03:45 PM   #28
AGS is offline AGS  Norway
diyAudio Member
Join Date: Sep 2008
Default RA4001


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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Old 29th September 2008, 04:11 PM   #29
diyAudio Member
Join Date: Dec 2007
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.

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Old 16th October 2008, 02:21 PM   #30
diyAudio Member
Join Date: Oct 2008
Thumbs up SRX3801

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.
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