nikko trm-750 humm/buzz problem - diyAudio
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Old 29th January 2013, 08:07 PM   #1
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Join Date: Feb 2011
Default nikko trm-750 humm/buzz problem

Hi everyone.
this is my first post on this forum, so i hope, that i will be heard and i will get any solution to my problem.

So, i'v got an amplifier Nikko TRM-750. when i turned it on for the first time, i was shocked about hum/buzz sound comming from the speakers connected to this amp. so i opened it up and found out that the supply capacitor was burned down. it's a double capacitor in one, that i havent found such one for replacement in local store, so i took two separate capacitors and mounted on universal pcb. after that, i carefully put it in the amp, connected the leads and turned it on. After this replacement the hum/buzz sound comes a lot quieter but still loud enough to be heard in any area of my room. so i decided to change all of the electrolytic caps in power amp stage. after done this, i found out that none improvement was made to the hum/buzz. i tried to finde out from there this noise comes and it is preamp stage. but its very strange, what when i disconnect preamp from power amp, it seems like power amp working fine, but when i connect an mp3 player or any source directly to the power amp it starts hum too, but a little bit quieter than the preamp is connected to the power amp.

First of all, i want to find out what causes the hum in power amp when i connect any source to it directly, cause i changed all the electrolytic caps in it. (i also checked the bridge rectifier which seems to be fine).

Second, what can cause the hum/buzz sound in pre stage (i also changed pre stage main supply caps and checked voltage regulators, they are fine). i found out, that then i regulate bass in pream it changes the intensity of the hum, but than i use volume it doesnt change anything.

Thanks in advance
waiting for replies.
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Old 29th January 2013, 10:03 PM   #2
amptech is offline amptech  Scotland
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Location: Glasgow
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The amplifier in question are you able to load photo of said unit..

Hum in amplifiers can slowly creep in due to ageing parts such as old capacitors breaking down, with noisy transistors you hear crackling popping sounds as the unit warms up.

Hum can be from poor grounding or leaking caps...your above comment shows you've change caps and that your amp works to a point.. Start by checking your low supply preamp supply lines look for discoloured resistors and again look for dry solder joints.

Sometimes the mains transformer will buzz and hum thus vibrations will be heard at the such case's the faulty/buzzy old iron core can be dipped in hot varnish this will coat the inner windings and will do the trick if not, replacement is the only thing.

Earth loops from connecting cable's and equipment can also cause hum problems and a loop breaker can be fitted..
In some power amps they fit a 10 OHM resistor between chassis and earth, this is a earth lift component from the star earth connection thus lifting the electronic psu circuit by 10 ohms..

regards A.
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Old 30th January 2013, 02:26 PM   #3
sregor is offline sregor  United States
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Join Date: Jan 2006
Location: massachusetts
+1 on possibile grounding problems. Sometimes it is just a screw grounding a PCB which is loose. Also, in that same vintage of Nikko they had problems with broken solder joints on connectors going to PCB's (seen it in the receivers - don't know if it applied to the amps.) You may need to get a schematic and check regulated power supply voltages. Had one where one of the regulator transistors was shorted, so voltages were too high with too much hum. Good luck.
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Old 6th February 2013, 10:55 AM   #4
diyAudio Member
Join Date: Feb 2011
Thanks for your replies.
For now, i can't take a look at the amp for more investigations, but when i get to it, i will look for your suggestions.
One more thing is, i fink that it is not ground problem, because i checked all the grounding, and i'm for sure its not a ground loop, because my mp3 player is not plugged to any socket it runs from batteries and amplifier dont change anything if i plug it to grounded to earth or not grounded socket.

P.S Sorry for my bad english
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