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Old 10th October 2012, 07:38 AM   #471
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Quote:
Originally Posted by dtses View Post
sounds like I had the same issue! make sure your toroid is connected correctly, check voltage after rectification across PS caps, then check across C102/C202 it should be around 14vdc on both rails, let us know if it's so. If not check R11 if it's shorted.
Thanks dtses. What caused this problem? How did you fix it?

Thanks
Badri
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Old 10th October 2012, 08:19 AM   #472
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i've cooked R11 for 4-5 times, one of them was likely when I shorted toroid's secondaries. But anyway you should check voltage first of all even if R11 is ok
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Old 11th October 2012, 10:44 AM   #473
Marra is online now Marra  United Kingdom
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I connected up my boards one channel at a time with a 50va transformer with 2x20v secs and a light bulb tester to check that things were in the right ballpark and that nothing smoked.The right channel gave the following readings and looks good to go.
C101/201 +- 28.52v
C102/202 + 14.65v - 14.36v
Offset -4.2mv

Left channel
C101/201 +- 27.5v
C102/202 + 14.2v - 14.65v
Offset 1.25v
The input also has about 0.5v on it.

Any pointers/advise would be much appreciated.
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Old 11th October 2012, 10:55 AM   #474
atupi is offline atupi  Romania
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Too much offset on left. Also you should have 0v on input.
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Old 11th October 2012, 11:18 AM   #475
Marra is online now Marra  United Kingdom
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Yes I realise that just wondered where to start looking.
I forgot to say that on the faulty channel the relay did not click nor the led light up until I started checking voltages namely the voltages across C102/202.
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Old 11th October 2012, 11:18 AM   #476
AndrewT is offline AndrewT  Scotland
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the input will probably not be 0V.
If the DC blocking capacitor has ZERO leakage current then you should see ZERO Volts, if the input has a grounding resistor.
If the DC blocking cap leaks and/or the grounding resistor is omitted, then it is very likely that a non Zero voltage will exist on the input.
If you measure the input after the DC blocking capacitor or omit the DC blocking capacitor then you will measure the input offset voltage of the chipamp.
This input offset can be upto 600mVdc.

output offset ~ 1.2Vdc indicates an error.
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Old 14th October 2012, 12:15 PM   #477
Marra is online now Marra  United Kingdom
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I recleaned the pcb's this morning and reflowed a couple of joints that looked a bit iffy and rechecked the voltages.Most values are as shown in previous posts but the offset now reads 3 volts and input .4v.On the good board the input reads 200mv but drops to afew mv's with the dmm left connected across it and the output reads 2.4mv with the input shorted.Have I fried either the 318 or the 3886 or both?Tia for any advice.
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Old 14th October 2012, 12:18 PM   #478
AndrewT is offline AndrewT  Scotland
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Where is the 0.4V coming from at the input?
Is that 0.4V an AC voltage or a DC voltage?
Is it before the DC blocking cap?
Have you used a DC blocking cap?
What type is it?
Is it leaking?
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Old 14th October 2012, 12:33 PM   #479
Marra is online now Marra  United Kingdom
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Thanks Andrew.The input has a polypropylene connected and the voltage is dc which starts at .26v drops slowly with the dmm connected but then rises back up again.I've just noticed that the heatsink warms up whereas the other channel it stays cold.The relay is slow to latch on and clicks on and off over a few minutes.

Last edited by Marra; 14th October 2012 at 12:40 PM.
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Old 14th October 2012, 12:55 PM   #480
bcmbob is online now bcmbob  United States
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When I have had a similar problem there was a short/solder bridge involved. One place to look is between the input pin bracket and the PCB. It can be solder or even a tiny piece of wire caught underneath the plastic pin housing. A failing resistor or one with incorrect value also bit me on a V1.3. That also caused excessive heat at the heat sink.
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