C-Audio RA3000 Amplifier in protect mode

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**FIXED** C-Audio RA3000 Amplifier in protect mode **FIXED**

Hi all,

I've had this amplifier a few years now and it's had an easy life in my hands.

The other day the protect light came on and I can't see why.

The symptoms are the lower set of level LED's (-40, -20) come on, plus the protect circuit LED stays on and there is nothing at the outputs.

Looking inside shows no obvious damage and the fuses are all ok.

I've found the circuit diagrams but my skills are a little rusty in diag and aren't too strong.

I would love some help in trying to fix the amplifier, if you guys can help that would be great.

I've checked the voltages from the transformer and they seem to be ok. I've checked the voltages from the cases of the mosfets to the ground, I'll post these values up shortly as I'm not sure if the reads are ok.

I'll also post some photos up when I get home.

Any help would be great.

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The amp is now coming out of protect mode.

The fault was all down to R69, a 24k resistor going open circuit.

I'd like to thank ChrisTech and amptech for there help. I've learnt so much from this issue and I'm looking forward to my test project.
 
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The guts.
p33EgEMh.jpg


The voltages I got when doing some quick measurements.
tPO3jWEh.jpg


Voltages between the mosfet cases and negative terminal of the speaker output post. This are all in mV.

Voltages on the output of the transformer are in volts expect the zero output is at 1.3mV.

Some tracks don't look to good and there had been one repair done on the one.
okaQU9oh.jpg


I've checked a couple of the others that don't look to good but they seem to conduct ok.
I've soldered a cable in to the worst looking track in the hope that would sort it but no joy.
 
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Last night I was reading about DC offset and this morning I measured both channels and found large offset on both channels.

Channel A 7.11v
Channel B 7.28v

So, this is the reason for the protect mode being active.

Now, I need to find out where this offset is coming from and how to trace it back.

A small explanation on how to do this would be great.
 
Start at the beginningg and work your way back.
Check the input stage voltages.
As they both have offset it is most likely to be something both amplifier channels share in common. As it also registers on the input meters start with the input stage.
I believe there is an opamp on the input is it receiving the correct power supply voltages? Is there DC on the output of the opamp. I believe there are more circuit diagrams. But you need to look for things in common to both channels. Is it in bridge mode?
I will scum the circuits when I'm not walking to work.
 
Over my lunch break I checked for DC offset at the end of the input stage, at the two chokes and I got

Choke 1, 4.7mV
Choke 2, 35.8mV

What do you guys think of those values?

I see that those values are similar to the values I am going on the cases of the mosfet's.

I also checked to two caps C25 and C56 on the board with my multimeter.

C56 seemed to charge up of via the Ohm setting on the DMM and the I turned the DMM on to the volt setting and it had slightly charged and was loosing that charge. Guess that ones ok.

C25 seems dead. It shown nothing on Ohm mode and no voltage in that mode.

I'm going to remove them later and double check them out the board to make sure.

Great work Mr Snell in pointing me in the right direction.

If that C25 is definitely faulty I'll replace the other to as the cost of shipping of the component will be more than the item.

Am I thinking correct that once that faulty cap is replaced that the amp should come out of protect or is it likely that more components could have no been taken out?
 
Well IC1 is not happy to start with 1vdc offset on its output is not great!
How about pin 7 too?
I would suggest replacing C5, 16 (input stage) and check the IC voltages again.
I would also suggest C54, 60, 24, 29 also (the voltage across Z3 is a tad low. It could be the Zener or the capacitor.
Use Panasonic FR series capacitors.
 
RS is excellent for components and customer service. They stock all the capacitors and the chip.

As the fault is the same (apparent fault) on both channels the only other thing I could suggest is that it is actually the protection circuit messing things up. Try lifting one end of both R63 and R64 at the same time and measure the output at the inductor again.
 
Hi, I worked on these amps many many years ago and found there tracks did show signs of damage.

What ever you do, don't touch the gate pin with the meter lead with the amp powered on!.

Dc faults in amplifiers start from around the input tail stages ie the small transistors.. transistors such as mpsa92 and mpsa42 can be used.

Also carry out cold checks on resistors as there may be o/c one..

These amplifiers can do 600w per chan and 1200 high mode.
 
Chris, I think you are right about the protection circuit being at fault. I read that if I connect an input source with no speakers connected and turn up the gain, if the level lights go up then it said that the fault will be with the protection circuit.

I did this and the lights did increase with the gain.

I lifted one leg of R63, R64 and removed connector 5.

The protect light still came on, I got 8mV on choke 1 and 32mv on choke 2.

I also measured the output posts and the 7V on both has now gone to 302mV on channel A and 50mv and rising on channel B.

Hi amptech,

Yes the tracks aren't very good on these at all. On looking around the board as well as the corroded tracks I've seen a couple that have lifted.

I do really like this amp for some reason, think it looks like a good basic design and layout. The heatsink could do with improvement, hopefully if I can get it back up and running with you guys help I'll look in to attaching the little individual heatsinks to the tops of the mosfets.

I see from your profile you are in to solar power, shame you're not closer as at work we do some MS12 uplift testing on PV panels and sometimes have some spare panels after testing.
 
The amp was running fine one minute, then stopped and went in to protect.

That's one reason I thought not to try adjusting the bias via the pots as it going out like that I thought there must be fault due to a failed component.

By leaky input transistor do you mean the opamp? I've ordered a new opamp IC just in case.

From what I've found online and the measurements taken with Chris's help it points to the protect circuit.

What I found on here on a thread from 2007 about a test on the protect circuit.
Hi Kevin,
The first thing to do is figure out which part of the amp is faulty.

you say it runs up but doesn't work do you mean it's in protect?

If you put in a signal (off load, no speakers connected) and turn up the volume does the bar graph light up as notrmal, check on both channels. if it does then the fault is likely to be in the protection circuit.

if not then you have probably got a DC offset somewhere on the offending channel.

thanks

Lee Basham

I've put an order in to RS for caps and the opamp IC.

I was thinking about replacing the transistors on the protect circuit but I've found they are no longer available (BF422 and BF423).

I've also noticed today that one of the BF423 has been swapped out for an A872 item (Q24) and Q45 has been swapped out from an A958 to an A968.

The Q24
zvLZ4n5h.jpg


The Q45
sgzcG3Dh.jpg


The opamp
uGvPUbBh.jpg
 
Devinder, do you have a scope and signal gen handy or even a amp that you can use to trace the audio signal out?
The long tail input pairs are the small signal transistors.

Use mpsa42 npn or mpsa92..these are 300v devices say 100ma or so ideal for input stages.

The protector is doing it's job via shutting off the relay due to high offset.

Signal trace the op-amp with music in but keep it low..now the signal will follow to the volume pot and feed out of the middle leg...Do not load the output yet.

See how you get on..
 
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