EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: cpposteve on March 09, 2020, 03:05:13 pm
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Hi all.
Wondered if someone could lend me a hand to fix this faulty amplifier I have here.
It started off that it powered up and then the green light flashes. (Usual thing with these if something has failed) it was going into safe mode. I found the issue which was just a small to-92 npn on the amplifier board. There was also a cap that had bulged next to it also, replaced both of these and all was good. Powered on and then 5 seconds later I was greeted with the magic smoke and then it turns off.
GREAT!!!
I whipped it apart again to be greeted with 20 burnt out resistors and 3 big npn and 3 big pnp transistors which I believe to be the main amplifier transistors. When powering on the resistors feeding these glowed red and switched off. I removed all resistors and transistors and replaced all defective components as per parts list in the service manual.
So after spending a fair amount on more components I then went ahead and soldered them all into place.
Checked for shorts on the power lines, output for the speakers, pretty much every where that looked like it could be an issue. And all tested fine.
Put it all back together, powered it up and POP...!!!
Oh FFS 🙈
This time it blew the mains fuse in the plug. Weird.
So put a 5amp fuse in there instead which is what should be in there and then it blew the internal 3.15a fuse thy powers the transformer.
Weird...so I checked continuity accross the transformer for the mains input and I’m getting 3.5ohms accross the winding. I working it out and to me that far to low!!! As when calculating it should draw around 60 amps at that resistance.
Now I will admit I don’t know much when it comes to transformers so could someone please tell me if this seems nailed? I have tried finding another transformer to no avail what so ever.
Any help or guidance would be really appreciated.
Thanks
Steve
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Busting the power transformer is kind of rare. But from what you say it is possible in this case. The only way to check the transformer is to disconnect the secondary side of the transformer and apply the power to test if it has any failure. Then you can always check the output voltages if it does not blow the secondary side fuse (if any) or the fuse in the plug. At no time however should you put a fuse that is larger than the specified fuse. If there is no disaster at this time, check the primary power circuits like the bridge rectifiers and those beefy caps.
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A low ohm resistance reading for the primary of a large transformer is quite normal. I have 40 years of experience in audio and 25 years or so with Denon products.
Trust me on this.
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Thanks for your input. Do you happen to know much about said denon amplifier?
What would you test next if it was you? Blowing fuse for primary winding of the transformer suggests to me a short circuit somewhere on power up (measuring the plug on the power board where the secondary winding plugs in reveals no shorts.
This has been a steep learning curve for me and I’m almost ready to give up on it and chuck it out. But I just can’t let it beat me.
Any help would be very appreciated.
Thanks
Steve
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I would look for a hard short circuit, maybe a main B+ or B- shorted to chassis, shorted main filter cap, shorted output transistors, etc.
ALSO, since the fuse blowing started after other work was done, my first move would be to double check that I had not done something wrong to cause it.
For example, the small transistor you replaced might have had the wrong basing, or inserted backwards - like that.
Of course now there is other damage also, so that will need to be tended to as well.
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hi,
thanks for your reply.
i did however fix it in the end.
i had written on the pcb in which orientation the large npn and pnp transistors go, and then screwed them down to the heat sink in that orientation, however that is where the mistake was, as when you flip the board over to screw it back onto the heat sink it was then all reversed and back to front, meaning npn where were the pnp should be and visa versa.
after alot of desoldering and soldering once again on these very fragile and cheaply made pcbs, it all sprung back into life once again, although i do notice with only 2 speakers plugged in that it likes to thermal cut after about 30 minutes of mid range volume of speakers labeled up as 6ohms (although measuered at 5.3 ohms) although after a 48 minute firmware update it apears to be slightly better!
cheers
steve