Cambridge Audio A1 MK3 SE repair (TDA1514 chips burned) - Page 2 - diyAudio
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Old 29th May 2012, 10:22 AM   #11
diyAudio Member
Join Date: Sep 2011
I was thinkin that I would measure look if the fuses wont burn with new capacitors and no TDA1514A:s in place. If all would be well and I could measure the voltage at the points where the TDA1514A:s are placed I'd solder them in place.

The bim bulbe tester seems interesting enough for future projects too, but I have no spare parts for it now.
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Old 29th May 2012, 10:59 AM   #12
AndrewT is offline AndrewT  Scotland
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Join Date: Jul 2004
Location: Scottish Borders
The bim bulbe (Dim Bulb) is needed now.
Don't power up any modified or new project without it !!!!!!!!!
regards Andrew T.
Sent from my desktop computer using a keyboard
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Old 29th May 2012, 11:11 AM   #13
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Join Date: Feb 2011
check for possible short circuit of output.
allso, use a fast blow fuse, half of the original rating.
prior installing a new chip do mesure the voltage on the board, and see if everything is normal.
use a 10K resistor as load, and mesure output voltage.

I find it hard to burn a TDA chip. actualy i use them verry often to power all short of things.
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Old 29th May 2012, 02:49 PM   #14
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Thanks for the tips! I replaced the caps and tried powering on. The fuse still blows on the left side. Started withouth chips, so no damage done. I measured the big power caps, the right one shows 25v as the left one is at zero.

Measured the output terminals for shorts but found nothing.
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Old 29th May 2012, 04:06 PM   #15
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Location: Kuala Lumpur
Check the transformer secondaries for continuity and the bridge. Working with one rail shorted is not going to do the chips any good.

I always have a look at the output network resistors, 3R3 if they followed the data sheet. Typically these are under-rated and fail open. This makes the chip oscillate and fry.
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Old 29th May 2012, 04:12 PM   #16
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With no chips in, you still have a short. The main supply is also regulated down to +/-15V to supply opamps. You will see a pair of 7815/7915 on there - check those.

You may also find that the board itself is damaged. Are there any burns on the PCB ? Burnt PCB can become conductive.
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Old 29th May 2012, 05:50 PM   #17
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Thanks guys! I might have found the culprit, a transistor marked U3 is split in half:

Might take it out and see if the fuse still goest. Need to order a spare for this anyways so there will be a wait.

Shot of the power amp board, all boards seem intact so no burns.
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Old 29th May 2012, 06:23 PM   #18
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Transistor U6 at the other opamp also has a busted leg, also needs replacement. Different types of transistors, both hard to get here in Finland.

NTE1905 and NTE951 would be working replacements it seems.
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Old 29th May 2012, 07:28 PM   #19
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Allright, I think Im done tinkering for today. I took out U3 and U6 transistors, plugged out the preamp board and fired up the power amp board.

This resulted in C29 and C31 caps exploding (or putting out smoke). A good sign maybe? Atleats the voltage is going on both rails, it didn't like being fired up without the TDA1514 in place and no preamp board plugged?
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Old 29th May 2012, 11:30 PM   #20
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They aren't transistors - they are 78L15 and 79L15 15V 100mA regulator IC's, in TO92 package, used to power the opamps.

ELFA Finland have them, and I expect other suppliers will too. They don't have to be the Motorola versions, and they are quite commonly second sourced by many manufacturers.
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