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Another C-Audio RA3001 DC Offset

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shaun1264 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote shaun1264 Quote  Post ReplyReply Direct Link To This Post Posted: 02 April 2024 at 7:39pm
Yeah.  Same result when I attached probe ground wires to green 0 wires.  
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Post Options Post Options   Thanks (0) Thanks(0)   Quote shaun1264 Quote  Post ReplyReply Direct Link To This Post Posted: 02 April 2024 at 7:49pm
I'm going to have to pull a leg out on the resistors tomorrow . They both measure 0.04ohms in circuit.  
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Post Options Post Options   Thanks (0) Thanks(0)   Quote shaun1264 Quote  Post ReplyReply Direct Link To This Post Posted: 03 April 2024 at 8:55am
Those resistors all measured fine.

Happy to announce that is now fixed!  Spent a good amount of time looking at the schematic.  Looked at what other voltages could be 'back feeding' into the speaker output and in the end I found if I disconnected the display driver board ribbon cable the DC offset disappeared.  Very unusual!

The LED displays also have shorted LEDs.

Adjusted the DC offset pots to get down to 1mV on both channels. 

So, I find it odd the protection circuit did not detect the DC offset.

Things I checked through the journey...

DC offset pots
Differential pairs and mirror current (replaced all these transistors)
Replaced all transistors in protection
Lifted and measured many resistors.
traced signal using scope right through to outputs
isolated zobel/protection and measured straight from MOSFETs
Checked and checked again all diodes
Spent plenty of time looking at schematic
Finally removed display board and offset gone!

It did look like someone previous had been trying to fix this as one of the diff pairs resistors had been pulled. 

Checking the driver board now. 

On channel 2 no output fault was an easy one.  Tracing signal using scope found bridge/stereo switch defective.  I bypassed this and this fixed it. 

Thanks Simon and Kev for your input in this. 





Edited by shaun1264 - 03 April 2024 at 9:03am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote simonp1100 Quote  Post ReplyReply Direct Link To This Post Posted: 03 April 2024 at 10:09am
Glad you got it sorted ClapClap

Never had a display driver board faulty before in any of the C-Audio RA series amps i have repaired over the years. Please let us know what you find on the faulty board.

Simon
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Post Options Post Options   Thanks (0) Thanks(0)   Quote shaun1264 Quote  Post ReplyReply Direct Link To This Post Posted: 03 April 2024 at 10:54am
I'll go through the board properly later. But a quick scan nothing seemed shorted unless the chip has gone bad.  

A new issue now... 

i can only get bias to 280mv with pot on end stop.  Resistors on voltage divider measured fine but I've noticed some sort of bodge modification done with a small ceramic caps on R42 and R43 piggybacking over another resistor. Is this an official mod?   I was wondering if this was dragging the bias down on that channel.  This mod is also on channel 2 but bias is fine for that one.  Hmmmm.   


Edited by shaun1264 - 03 April 2024 at 11:13am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kedwardsleisure Quote  Post ReplyReply Direct Link To This Post Posted: 03 April 2024 at 2:04pm
Interesting display board fault. You'd think in a tug of war between a bargraph driver and an output stage, the output stage would get its own way. Is the problematic output stage able to deliver plenty of current?

I have a note on the diagram I made a while ago mentioning that the speaker relay contact not closing properly can cause erratic bias problems. Also to check R32/R102 are in spec for strange offset issues. It's in my writing but I dont remember it!!



Kevin

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shaun1264 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote shaun1264 Quote  Post ReplyReply Direct Link To This Post Posted: 03 April 2024 at 5:47pm
Getting plenty of current drawn using a 4ohm load.  My worry was if there was a leaky MOSFET or open/shorted as I doubt I would get much or anything under a load. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote shaun1264 Quote  Post ReplyReply Direct Link To This Post Posted: 04 April 2024 at 10:29am
Just a follow up.  Lost drive at channel A.  Got a good output into relay but nothing out even though I could easily see the contacts switch over...  Dodgy relay I think there.  Bypassed temporarily and soak testing on a 4 ohm load.  All seems fine .  Sinewave looks clean with no oscillations on top of waves.  

I was stupidity adjusting the wrong pot for bias.  Got plenty of bias now into that channel.  

Also there seems to be an error on the schematic or board.  On the schematic. R70 is shown with a zener diode....   I compared it to another RA3001 I have and that has a 180K resistor in that R70 spot.    Not sure what was going on there??  


Edited by shaun1264 - 04 April 2024 at 11:50am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote simonp1100 Quote  Post ReplyReply Direct Link To This Post Posted: 04 April 2024 at 10:37am
R70 is a zener diode. 

The circuit diagram you have is up to date (before C-Audio was taken over) and the board fitted in the amplifier you have is an older version board. Would recommend you do the mod's for the protection circuit as per the circuit diagram.  Clap
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Post Options Post Options   Thanks (0) Thanks(0)   Quote shaun1264 Quote  Post ReplyReply Direct Link To This Post Posted: 04 April 2024 at 10:53am
Thank you.  Taken off bulb limiter.  Still soak testing now on 4ohm. Ballast resistors getting toasty.  Square wave and sinewave all fine. Heatsinks getting warm.  

Cleaned off all the flux.  Plenty of it on the board.  

Someone had a similar issue to me on another forum some years ago where there was DC offset and that was traced back to a open resistor in the protection.   I checked that resistor and it was fine. He found if he disconnected the conenctor to display board the DC offset vanished.  
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