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C-Audio RA3000 Amplifier in protect mode
C-Audio RA3000 Amplifier in protect mode
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Old 3rd November 2017, 11:26 PM   #21
amptech is offline amptech  Scotland
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Have you done cold checks on the transistors to see if there leaky?

Also changing parts with full checking will waste time and money only change parts that are faulty plus to much solding and desolding will weaken them tracks.

Check all voltages from ht to the +/- rails that feed the op-amps.

Last edited by amptech; 3rd November 2017 at 11:32 PM.
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Old 4th November 2017, 05:13 PM   #22
ChrisTech is offline ChrisTech  United Kingdom
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Ok, getting there! I would suspect that if you measure at the end of the lifted resistors you will get 7v....

I think that the DC offset that is higher on channel 1 will be due to the low voltage across Z3 it may be that you will need to get a new Zener if replacing the capacitor doesn't bring back up the voltage to near enough 6.2v make sure you get a good tight 1% tolerance Zener.

The protection circuit is probably Q20-23 or D9-10 something in that area at fault. Measure voltages. Around components. There is an 18v veneer is there 18v across it?
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Old 4th November 2017, 05:22 PM   #23
ChrisTech is offline ChrisTech  United Kingdom
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Ps the protect is still engaged because it thinks there is a fault but the fault is the input to the protection detection.
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Old 4th November 2017, 07:48 PM   #24
Devinder is offline Devinder  United Kingdom
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Evening guys,

I tracked down a scope and a signal generator. I stuck a 1V 200hz square wave in, traced it to the TL074, to the gain pots and then to the chokes. To my non experienced/skilled self the wave on the scope looked great. I also stuck a sine wave in and the looked good to.

Click the image to open in full size.

I've not done any cold checks on the transistors and the rail/supply voltages look good.

Chris, I thought that I'd find that 7v but it didn't turn up, I'm guessing it might be coming from the display board??

I'll see what happens when the display board goes back on, that connector 5 is just a ribbon cable soldered to the board.

My theory is that something failed on the display board and has taken out the protect circuit in the process, but that's all guess work.

I've got the zeners on order to but they're are 2%, bugger

It looks like RS don't do them in 1%, These are the ones I ordered Link

Those Q20-Q23 (BF422 and BF423) are no longer available any idea on what alternative I could use if there is a fault with them?

I'll have a measure of that 18v zener in the morning
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Old 4th November 2017, 08:11 PM   #25
amptech is offline amptech  Scotland
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The handy light bulb limiter is handy..

The standard sine wave used when testing amplifiers is a 1k at 1volt. Watch the scope for clean wave..also a mini bench amp with probe to track the signal.

See how you get on with things.

Try on line for zeners..
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Old 4th November 2017, 08:13 PM   #26
amptech is offline amptech  Scotland
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Mpsa42 npn or mpsa92 pnp can be used..
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Old 5th November 2017, 10:00 AM   #27
ChrisTech is offline ChrisTech  United Kingdom
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Just double check that 7v isn't at the anode of D9.
2% should be fine,
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Old 5th November 2017, 10:27 PM   #28
ChrisTech is offline ChrisTech  United Kingdom
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Just to follow up on the as I may not be about.

If with Connector 5 removed, the resistors lifted and the protection circuit says it is going into protect then the fault is purely in the protection circuit. For the circuit types and the voltages involved I don't think the fault would have been caused by anything other than a part failing and I should think it will be located purely within the protection circuit.

The offsets you are measuring on the PCB for the amp circuit is back to a reasonable level.

The input opamp is not happy and needs replacing for sure.

I think that because you have lifted the end of the resistors closest to the protection circuit that when you measure on the resistor you get no 7v, this is why I suggest checking the anode of D9.

I wouldn't spend any time worrying about the rest of the amp (though a recap of the small value capacitors and Zener diode mentioned before would be sensible.
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Old 5th November 2017, 10:44 PM   #29
Devinder is offline Devinder  United Kingdom
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Thank you Chris for your help and taking the time to write your thoughts on this.

Unfortunately I didn't get chance to do anything today.

I'll report back here on my findings.

I'm in no rush to get the amp back up and running and I don't mind on waiting for your guidance if I get stuck.

Hopefully the parts will arrive tomorrow, I think there are enough caps to replace most if not all as RS only sold them in large quantities.
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Old 5th November 2017, 11:56 PM   #30
The Chump is offline The Chump  England
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MPSA42/92 have different pinouts from BF422/423.
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