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Another C-Audio RA3001 DC Offset

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shaun1264 View Drop Down
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    Posted: 01 April 2024 at 10:07am
Trying to fix the last RA3001 I have.  This one is being difficult... This one has a 7.4v DC offset on channel 1 and channel 2 passes no audio.  Focusing on the DC offset first. 

On a limiter dim bulb it starts up fine.  All relays click and protect light goes out but DC offset appears.  Surly the protect circuit would sense this and the protect light would come on?

This one has the flatpack MOSFETS .  All seem to test fine.  I removed the four fuses for the MOSFET power rails. Fired up again.  Protect light goes off, relays clicks but still have a DC Offset on output.

Digging further, changed opamp out, replaced diff pairs and mirror current.  No change.  A stubborn DC offset still appears.   Replaced all transistors in protect circuit, still no luck.  Measured opamp DC, high and low.  both measure very close to each other...

Checked all diodes.  All seem fine.  I don't detect any open resistors.  Running out of ideas now.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote simonp1100 Quote  Post ReplyReply Direct Link To This Post Posted: 01 April 2024 at 10:34am
Check the PCB tracking around by the fan, have had a few of these for repair were the tracking has been eaten away by smoke fluid.

Is the DC offset + or - with reference to ground ?. Does the DC offset change when you adjust the preset pot ?.
I would expect the protection circuit to shut down with this amount of DC offset, check the 47K resistor for each channel & make sure these are connected to the relavent places as per the circuit (again possible tracking issues). 

Check all preset pots, possible open circuit or failed due to heat.
Check feedback resistors for each channel R59 & R77 all should be 39K.
Check R32, R38, R97, R102 all should be 220K.
Measure across each pair of 6V2 zener diodes on each channel driver circuit.

Let me know how you get on. Clap
 


Edited by simonp1100 - 01 April 2024 at 10:43am
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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: 01 April 2024 at 5:44pm
-Checked tracks near to fan. Continuity on all.
- Preset pots have variable resistance.  With the 7 volts DC i can make go down and up a little.
- Feedback resistors all seem correct value
- 47K resistors are fine
- 220K are fine.
-Voltage across Zener Diodes measure just over 6.2v on each channel.

DC offset is positive with reference to ground as a potential.

-Bias set at just over 300mV between positive and negative gates resistors . When bias is increased, amp draws more power obviously, bulb glows dim and DC offset drops to around 5v.

I'm starting to think it maybe something Ive missed with mosfets.  I may pull them on that channel to see if offset disappears.

I just find it odd that protection doesn't kick in with the DC offset.  That offset can do some damage to speakers!


Edited by shaun1264 - 01 April 2024 at 5:47pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote simonp1100 Quote  Post ReplyReply Direct Link To This Post Posted: 01 April 2024 at 7:43pm
Could be a leaky FET.

De-solder all three leads on each FET one at a time on the faulty channel and see if the DC offset reduces. 

I would still check the protection circuit as that amount of DC offset should set it into protect. You could lift each side of the 47K resistors on each channel and apply a small voltage from a power supply (+ and -) and see if the protection circuit activates.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kedwardsleisure Quote  Post ReplyReply Direct Link To This Post Posted: 01 April 2024 at 8:40pm
if you run a sine wave through it, what does it look like on the silly scope, are both halves of the waveform driving ok? An offset going towards one rail might just be a lack of drive on the opposte side for whatever reason.

I would imagine any fet problems would cause heavy current draw. My money's on the signal stages ie error amp or current limit (but no more than 50p)
Kevin

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Post Options Post Options   Thanks (0) Thanks(0)   Quote shaun1264 Quote  Post ReplyReply Direct Link To This Post Posted: 01 April 2024 at 9:00pm
I'll run a sinewave and scope it sometime this week.  

I forced it into protect by opening thermal sensor.  Switched on, protect light comes on.  No relays click on main board but I noticed I had 7vdc offset on both channels! Connect thermal sensor wire back, DC offset back on channel 1.  
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Post Options Post Options   Thanks (0) Thanks(0)   Quote simonp1100 Quote  Post ReplyReply Direct Link To This Post Posted: 01 April 2024 at 9:12pm
Remove both the small thick wires (think they are yellow and grey) that go from each channel MOSFET row on the PCB board to the protection circuitry / output relays, then power up the amplifier and check the offset voltage at the MOSFET source end on each channel. Please measure and tell me what this voltage is

Sounds like there maybe a problem with the protection circuitry as this should have kicked in with the DC offset on the problem channel and the fact that you had 7vdc offset on both channels then connected the thermal sensor wire back & now the DC offset is back on channel 1. ?


Edited by simonp1100 - 01 April 2024 at 9:15pm
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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 10:38am
Well, i've managed to fix channel 2....

Using my scope to trace.  Nice sinewave out of opamp.  But no signal at R88.  Traced it back to bridge/stereo switch.  Looks like the switch was pushed in.  Very loose to switch. Wafer coming away.  But even after putting back and and spray with switch cleaner it still didn't work so I bypassed the switch.  Now have nice sinewave out of output. 

In the process of tracing channel 1 now. 
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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 12:10pm
Making progress...  Tracing from input 






Edited by shaun1264 - 02 April 2024 at 1:15pm
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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 1:45pm
Unsoldered both grey and green ground wires from mosfet end.  Measured from mosfet end to ground I have 5v on both channels.  When I connect wires back up and measure from same point offset jumps down to very low millivolts .  25mV on good repaired channel (bridge switch problem) and 159mV on bad channel (bear in mind offset preset pot not at right value )

Green and yellow LEDs on too for bad channel .

With grey wires still disconnected I get 7v dcoffset still measuring from speaker terminals 



Edited by shaun1264 - 02 April 2024 at 1:53pm
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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 5:50pm
Horrid looking sinewave probed from speaker terminals. 

With wires disconnected from MOSFETS end.  Scoped here and got a nice clean sinewave.  DC offset was much lower.  With wires still disconnected measured from speaker outputs still had the 7VDC positive,   Connected wires back up and measured at same place (at MOSFET end and still got low DC offset.  But at speaker terminals that offset was still there.  Very strange! 

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Post Options Post Options   Thanks (0) Thanks(0)   Quote simonp1100 Quote  Post ReplyReply Direct Link To This Post Posted: 02 April 2024 at 7:28pm
Looking at your last picture, can you attach the earth / ground of your scope to the 0V volt rail (where the two thick wires meet in the middle and use this as your reference ground) not the casing. 

Also reconnect the two thick ground wires that go from the speaker terminals to the ground star earth point on the PCB and re-measure the output DC volts.

Can you also check the earth resistors on the PCB board for each channel R71, R105 (both should be 47R).
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