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C-Audio RA3000 Amplifier in protect mode
C-Audio RA3000 Amplifier in protect mode
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Old 2nd November 2017, 11:31 AM   #11
Devinder is offline Devinder  United Kingdom
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I had chance this morning to make those measurements.

Z03, 6.08V
Z04, -6.22V
Z10, -6.20V
Z11, 6.16V
C17, 90.30V
C18, -90.00V
C65, -90.10V
C66, 90.10V

IC1
P01, -0.90V
P14, -0.30V
P04, 18.83V
P11, -17.45V

I also measured the supply to IC1.
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Old 2nd November 2017, 02:17 PM   #12
ChrisTech is offline ChrisTech  United Kingdom
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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.
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Old 2nd November 2017, 03:42 PM   #13
Devinder is offline Devinder  United Kingdom
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IC1 pin 7 measures -11.3mV

Where would you recommend to buy components as I've been using eBay of late and I worry about cheap low quality components.
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Old 2nd November 2017, 08:06 PM   #14
ChrisTech is offline ChrisTech  United Kingdom
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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.
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Old 2nd November 2017, 08:10 PM   #15
ChrisTech is offline ChrisTech  United Kingdom
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Also try the same thing but disconnect connector 5.
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Old 3rd November 2017, 10:10 AM   #16
amptech is offline amptech  Scotland
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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.
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Old 3rd November 2017, 12:37 PM   #17
Devinder is offline Devinder  United Kingdom
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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.
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Old 3rd November 2017, 01:30 PM   #18
amptech is offline amptech  Scotland
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Have you checked the amps baising as per the schematic?
There is a trim pot to adjust offset volts also, you might have leaky input transistors and the protection circuit is doing it's job of holding off the relay.
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Old 3rd November 2017, 09:21 PM   #19
Devinder is offline Devinder  United Kingdom
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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.
Quote:
Originally Posted by lee.basham View Post
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
Click the image to open in full size.

The Q45
Click the image to open in full size.

The opamp
Click the image to open in full size.
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Old 3rd November 2017, 11:04 PM   #20
amptech is offline amptech  Scotland
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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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