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C-Audio RA3000 Amplifier in protect mode
C-Audio RA3000 Amplifier in protect mode
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Old 31st October 2017, 05:00 PM   #1
Devinder is offline Devinder  United Kingdom
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Location: Dunstable
Default **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.

================================================== ===========================

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.

Last edited by Devinder; 9th December 2017 at 06:33 PM.
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Old 31st October 2017, 07:24 PM   #2
Devinder is offline Devinder  United Kingdom
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The guts.
Click the image to open in full size.

The voltages I got when doing some quick measurements.
Click the image to open in full size.

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.
Click the image to open in full size.

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.

Last edited by Devinder; 31st October 2017 at 07:43 PM.
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Old 31st October 2017, 07:39 PM   #3
Devinder is offline Devinder  United Kingdom
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Here are photos of the circuit diagrams I've found.

I hope they come in handy for anyone else that have issues in the future with their amplifier.

Channel 1
Click the image to open in full size.

Channel 2
Click the image to open in full size.

Power supply
Click the image to open in full size.

Protect circuit
Click the image to open in full size.

Re-entrant protect
Click the image to open in full size.

Level display card
Click the image to open in full size.
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Old 1st November 2017, 09:16 AM   #4
Devinder is offline Devinder  United Kingdom
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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.
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Old 1st November 2017, 09:34 AM   #5
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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Check C25 and C56. If they are leaky, you will get a DC offset.
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Old 1st November 2017, 09:59 AM   #6
ChrisTech is offline ChrisTech  United Kingdom
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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.
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Old 1st November 2017, 11:02 AM   #7
Devinder is offline Devinder  United Kingdom
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Hi guys,

Thank you for replying and your help.

Jon, I'll take a look at those caps thank you.

Chris, Yes you are right sorry in my rush to upload the images I missed the one for the input stage circuitry.

Click the image to open in full size.
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Old 1st November 2017, 01:36 PM   #8
Devinder is offline Devinder  United Kingdom
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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?
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Old 1st November 2017, 06:50 PM   #9
Devinder is offline Devinder  United Kingdom
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I removed C25 (100uf 25V) and it seemed ok.

I've ordered two new ones just in case and a cheap ESR meter for future help.

I find it strange that the two channels offsets on the input stage is so different and channels on the main output are so similar?
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Old 1st November 2017, 08:29 PM   #10
ChrisTech is offline ChrisTech  United Kingdom
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I would next check the voltages across Z3, 4, 10, 11 C17, 18, 65, 66
The voltage at pin 1 and 14 of IC1.
Make all measurements with the input level controls at zero.

The power supply's should be pretty much spot on symmetrical, from my experience the transformers are well wound.
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