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Old 13th December 2012, 07:18 PM   #11
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Hi,
Nigel normally I check the rails voltage using the voltmeter in AC. This will tell you how much ripple you have in the PS voltage. If you read the DC in AC it should read zero. That means the capacitors are good. The reading should be close or almost zero. Higher that 4 mv means that you have problem with the capacitors. The best way it is use an scope but when you do not have one this is the cheapest best way to check it. So I recommend j4976 to check the voltage in AC and if the voltage it is too high replace the capacitors.
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Old 13th December 2012, 07:22 PM   #12
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Quote:
Originally Posted by tauro0221 View Post
Hi,
Nigel normally I check the rails voltage using the voltmeter in AC. This will tell you how much ripple you have in the PS voltage. If you read the DC in AC it should read zero. That means the capacitors are good. The reading should be close or almost zero. Higher that 4 mv means that you have problem with the capacitors. The best way it is use an scope but when you do not have one this is the cheapest best way to check it. So I recommend j4976 to check the voltage in AC and if the voltage it is too high replace the capacitors.
Almost all meters don't work in that way - they read DC on the AC ranges, and give wildly inaccurate results (as was found here). A small number of meters do have DC blocking capacitors in them, but not many, as it's not really something that's needed at all. But as I said, it's not something any electronics engineer would ever try and do.

It's also a pretty useless test anyway - if the capacitors are that bad, then the DC readings will be wrong as well.
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Old 13th December 2012, 07:24 PM   #13
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Quote:
Originally Posted by Simon B View Post
Of course I'm sure I'm missing something here, but if he knew what the "actual fault" was, apart from the already mentioned very loud hum, then.........
You mean apart from missing the fact that's he's made no mention of any loud hum?
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Old 13th December 2012, 07:38 PM   #14
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I mentioned "fuse and tda issue" ... my amp is blowing fuses and tda7293...Im a guitarrist with some iron solder skills...the fact is when I connect poweramp board link cable to main board ( +32, -32v, output from tda) the main fuse and poweramp chip are blowed!!!

and the only thing that I considered wrong( Im not sure!) was the presence on AC voltage on same point that 32V DC are present...

Any other component that I changed ( ZD, BR ) were fine...I suspected of BR because that behavior ( AC voltage where it not supposed to be, I guess) but tested out of the circuit and it is fine...but, I changed BR, ZD5, Zd6 because I bought new ones before tested..


Sorry, if I couldnt be clear, thanks in advance!!

Last edited by moonmark; 13th December 2012 at 08:05 PM.
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Old 13th December 2012, 11:09 PM   #15
Enzo is offline Enzo  United States
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Nigel, I respectfully disagree. Except for the most inexpensive junky meters, MANY meters these days if not most have a DC blocking cap for AC voltage readings, and putting the meter on AC volts to read ripple on a DC supply is a standard technique. My FLuke 77 has been with me for 20 years and works that way. The $2 meter I got at Harbor Freight works as you describe. Of course one has to know which he has, but if I measure a 9v battery on AC and get 20v, then I know it is wrong. If I get zero, then the method works.


One thing about these TDA7293 boards, you MUST make sure the power supplies are discharged before connecting the cables from the modules to the rest of the system. And do not disconnect them while powered. NEVER plug them in while powered.
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Old 14th December 2012, 12:28 AM   #16
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Question What is wrong with my AVT50

Quote:
Originally Posted by Enzo View Post
... putting the meter on AC volts to read ripple on a DC supply is a standard technique.


One thing about these TDA7293 boards, you MUST make sure the power supplies are discharged before connecting the cables from the modules to the rest of the system. And do not disconnect them while powered. NEVER plug them in while powered.
What Do U think is wrong with this amp? Maybe C63 and C94 from schematic are damaged? Im not changed or tested those caps yet... Is that ripple (70c AC) on DC supply wrong?
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Old 14th December 2012, 01:15 AM   #17
Enzo is offline Enzo  United States
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I was disagreeing with Nigel in the general case, but when you measure 70vAC on a 32vDC rail, then he is right, your meter is fooling you.

DO you have a largish cap like a 0.1uf? Clip it in series with your red meter probe and take that AC reading again. The cap will block the DC. Excess ripple on the power supplies should not blow up the ICs however.
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Old 14th December 2012, 10:27 AM   #18
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Quote:
Originally Posted by Enzo View Post
Nigel, I respectfully disagree. Except for the most inexpensive junky meters, MANY meters these days if not most have a DC blocking cap for AC voltage readings, and putting the meter on AC volts to read ripple on a DC supply is a standard technique.
Like I said, some do, most don't - and all the old analogue meters didn't.

As for a 'standard technique', I've never heard of any engineer doing it, and I've been a professional service engineer for over 40 years.
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Old 14th December 2012, 10:30 AM   #19
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Quote:
Originally Posted by moonmark View Post
What Do U think is wrong with this amp? Maybe C63 and C94 from schematic are damaged? Im not changed or tested those caps yet... Is that ripple (70c AC) on DC supply wrong?
It isn't there - as I explained previously, you can't check AC on DC rail on most meters.

Does the fuse blow if you power up with the chip out of circuit?.

Assuming it doesn't?, it 'might' be that it's oscillating (usually ultrasonically) and that can blow the chip almost instantly.
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Last edited by Nigel Goodwin; 14th December 2012 at 10:33 AM.
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Old 14th December 2012, 11:18 AM   #20
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j4976, do you have access to an oscilloscope? If not, I would suggest using your volt meter on the AC scale (the hum is likely 60 or 120Hz (or 50/100 depending where you are)) with a capacitor on the end of the probe to block any DC. Something like 47nf will be good, make sure it's a high enough voltage to withstand the highest voltage in the amp. You can probe the signal path through each stage. This will tell you if the whole signal path is contaminated. If the hum is present at the first stage then you definitely have a root power supply problem. You could also just start by checking each DC voltage rail (with the technique above) to see which rail is faulty.

Diagnostics, not tryagnositcs.
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