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Old 9th March 2005, 05:27 AM   #1
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Default amp been in for repair, not fixed need advice to trace faults

ok, i have also got protection mode on my harman kardon avr507 amp,

really cheesed off!!

my front left & right was not working but my surround left and right was as well as center channels,

thought id take it into this tv repair shop, he said it would cost about 90 pounds to repair, i thought ok what the hell least it will be all done,

waited 5 days and phoned, he said good news and bad news :/

i asked whats that then,

he replyed" good news is the front left and right are working now,
bad news
he replyed" the left surround is not working.

then he said" i replaced the blown transitors, however the left surround channel, its ok for about 5 mins, but then gets red hot and keeps blowing the transistor,
he then replyed" only thing i can suggest is to see how it goes for 3 months and if its still ok bring it back and i will try and fix the left surround :/

now is it me or dident i not put this in for repair??? dident he say it would be 90 pounds to repair it? i dident think repairing a amp mean half repairing it??
i think either he couldent be bothered or wasent experenced enough for the job, as that come back in 3 months sounds like a fob off to me what you guys think?


also, whats strikes me strange is, my surrounds were ok before it went in but my fronts was not, now only 1 channel on the surround works, and the 2 fronts are working?

protect only comes on if i plug a speaker into left surround.


im very unhappy as its still not fixed :/ so im going to have to look at it myself hope someone can advize me what to look for etc

what do u guys make of this?
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Old 9th March 2005, 08:06 AM   #2
jaycee is offline jaycee  United Kingdom
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Probably because he is a TV repair man - the inside of a TV is very different from an amplifier.

It sounds like to me the channel is incorrectly biased too high, so the thing heats up when working with a load and goes past the SOA for the transistor = *bang*
Either that or something is causing the channel to break into oscillation.

Unless you know what you are doing, and have a schematic, don't try and repair it yourself as you will most likely just cause more damage. Power amps are not very forgiving of mistakes.
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Old 9th March 2005, 03:10 PM   #3
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well trouble is i cant find a schematic anyware for this model? or a service manual? ive emailed a few places that sell service manuals but they dont have it in stock which strikes me kind of strange??

the amp is a harman kardon avr 507 ltd edition, also heard its similar specs to the Harman Kardon AVR-4550, however i cant even find this schematic or service manual,

if anyones got them please let me know,

just curious why you think it might be the bias setting, as before it went into the shop this channel was perfect,

i have just shelled out alot of money to get this fixed, which is a lot for me as im a poor student, and cant afford to pay another 100 pounds, thats why i said i will have to look at it myself, plus theres only about 45 components of each channel, consisting of resistors, transistors,caps, and couple of diodes, so cant be that hard to test them, as i said i have alittle electrical no how, used to repair consoles, however amps are diffrent, so just need some pointers on the best way to test these with a dmm.
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Old 9th March 2005, 04:13 PM   #4
jcayer is offline jcayer  Canada
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Quote:
Originally posted by mrlennon
... so just need some pointers on the best way to test these with a dmm.

May I suggest http://sound.westhost.com/troubleshooting.htm#comm1 for a starter...

Good luck

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Old 17th March 2005, 03:32 AM   #5
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ok been busy with my trusty multimeter,

found a few components gone,

can i just double check with you guys, i have got the values right for the resistors, below.


mini resistor, 4 band,

brown,red,red,gold, 1200 ohms
.................................................. .................................
mini resistor 3 band,

brown, orange, gold, 13 ohms
.................................................. ....................................
mini resistor, 4 band,

brown,silver,red,gold, 1800 ohms
.................................................. .................................
mini resistor, 4 band,

red,red,orange,gold 22000 ohms
.................................................. .................................

normal resistor, ( light green body,)

silver,silver,brown,silver,blue, - unsure on value for this one?

also found a ceramic cap, (not charging) but im not sure its value as it only has a number12 printed on it, so i guess im gona need the service manual for this one, however trying to get it is another thing . cant even find anyware that sells it, and harman kardon wont sell manuals to outsiders only registered repair centers.


also concidering i dont have a proper cap tester handy i might just replace every compnent on the channel, as its only gona cost about 10 pounds, that way i guess i know everythink checks ok then, and alot cheaper than a repair.
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Old 18th March 2005, 01:22 AM   #6
anatech is offline anatech  Canada
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I think your 13 R resistor should be 1R3 given the colour code you gave.
I would just try to repair the channel systematically. Look at the other working surround channel. By just replacing like parts you may just repeat the shop's mistake. The small value caps like ceramics are probably okay. Look for a short as a failure with those.
-Chris
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Old 18th March 2005, 02:07 AM   #7
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thanks anatech for the reply mate,

im pretty sure looking at it the: green bodied, silver,silver,brown,silver,blue, type resistor is a inducer,

also i really think the ceramic cap is gone as i checked the value with the other working channels, the value rose very high, but the blown channel ceramic rose a tiny amount and stopped ,

so from what i can tell it looks as if its gone, plus 3 resistors, and a transistor.

i said i was going to replace all the components on the blown channel as theres a few trans, and there is a far few on each channel, but if you want i could type out the tested components values on the good channel compaired to the tested components values on the blown channel see what you think.

let me know, maybe you could tell more which ones look dodgy.
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Old 18th March 2005, 03:01 AM   #8
anatech is offline anatech  Canada
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Hi mrlennon,
Might be an idea. There is nothing worse than repeating someone else's errors. Do not use ECG or NTE parts. Replace any you do see with the correct part.

Ceramic caps normally will fail with a short across their terminals. You likely measured some other aspect of the circuit behaviour. Of course, nothing is impossible.

TV repair guys are normally very poor at servicing audio. A completely different world. They just don't know it. The state of your set doesn't surprise me at all - sorry about that. You can probably do a better job.
-Chris
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Old 18th March 2005, 10:02 AM   #9
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well from what i can tell, all the tv repair guy changed was the power transistors on the top and bottom,

also im finding it hard to find a place in uk that sell all the parts i need
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Old 18th March 2005, 10:53 AM   #10
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Quote:

Might be an idea. There is nothing worse than repeating someone else's errors. Do not use ECG or NTE parts. Replace any you do see with the correct part.


Chris is 100% correct here. Never use NTE or ECG replacement parts for audio. Two reasons, 1st, the parts come close to the original parts but are NOT a match. Second, the price is about 10times higher than the actual part cost.

As a general rule never take audio to a TV repair shop. They are good (sometimes) at what they do but are absolutely no good with audio. I have seen this time and time again.

Joe
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