Nikko A-300D My bedroom amp developed distortion - diyAudio
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Old 13th August 2012, 10:18 AM   #1
Ronj is offline Ronj  Singapore
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Default Nikko A-300D My bedroom amp developed distortion

My main bedroom amp, a vintage Nikko A300D, (which has been recapped a year ago) started having a distortion on Left channel since yesterday. Both channels have sound only left channel sounds saturated/distorted.

More on the recapping and pictures of the amp in below thread
Nikoo A-300D, recapping dilemma...

It has been working perfectly until yesterday. After a cold start, this audible distortion takes 20seconds or so before it becomes noticeable. The distortion, sounds like the amplifier input getting saturated/over ridden. No distortion/crackling noise or anything when Input signal is not applied. It's only with a valid source input signal the distortion happens.

I have a NEC 2SC853 on the left channel getting hotter than it's Right channel counter part. I have removed it and chcked using a DMM and it read fine(No shorts/ No opens). I didn't remove the otherside one as removing it from the PCB was a bit tacky because of its age. So not sure whether this transistor, is the culprit. I may swap this one with the working Right channel today.

All the electrolytic caps in this amp are just 1year old. DC offsets on both the channels are normal. Pre-out/Tape REC out signal when fed to another amplifier is not distorted. So I guess the distortion is limited to the power stage.

Any common problems that may cause this? I don't think this is a solder joint issue as the distortion has a "lead" time before it starts. I assume solder-joint issues should be audible the moment the amp is switched on. Once it starts it stays through out the time Amp is on. And no touching or tweaking on the PCB make any difference.

A transistor, resistor or a cap(remaining ceramics)? Which among them would be the most likely candidate? As always your replays are highly appreciated...
Ron

Last edited by Ronj; 13th August 2012 at 10:22 AM.
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Old 13th August 2012, 10:23 AM   #2
Ronj is offline Ronj  Singapore
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Quote:
Originally Posted by Ronj View Post
My main bedroom amp, a vintage Nikko A300D, (which has been recapped a year ago) started having a distortion on Left channel since yesterday. Both channels have sound; only left channel sounds saturated/distorted.

More on the recapping and pictures of the amp in below thread
Nikoo A-300D, recapping dilemma...

It has been working perfectly until yesterday. After a cold start, this audible distortion takes 20seconds or so before it becomes noticeable. The distortion, sounds like the amplifier input getting saturated/over ridden. No distortion/crackling noise or anything when Input signal is not applied. It's only with a valid source input signal the distortion happens.

I have a NEC 2SC853 on the left channel getting hotter than it's Right channel counter part. I have removed it and chcked using a DMM and it read fine(No shorts/ No opens). I didn't remove the otherside one as removing it from the PCB was a bit tacky because of its age. So not sure whether this transistor, is the culprit. I may swap this one with the working Right channel today.

All the electrolytic caps in this amp are just 1year old. DC offsets on both the channels are normal. Pre-out/Tape REC out signal when fed to another amplifier is not distorted. So I guess the distortion is limited to the power stage.

Any common problems that may cause this? I don't think this is a solder joint issue as the distortion has a "lead" time before it starts. I assume solder-joint issues should be audible the moment the amp is switched on. Once it starts it stays through out the time Amp is on. And no touching or tweaking on the PCB make any difference.

A transistor, resistor or a cap(remaining ceramics)? Which among them would be the most likely candidate? As always your replays are highly appreciated...
Ron
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Old 13th August 2012, 11:57 AM   #3
Tajzmaj is offline Tajzmaj  Slovenia
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It seems like bad solder joint somewhere or even more like bias problem. Have you checked voltages across emitter resistors at the output stage. I mean those one from each o/p device..It might be that trimmer for idle adjust have some problems. Try to move it just a little in both directions and than leave in exact the same position....and do it when amp is SWITCHED OFF. Otherwise outputs might go...careful. If you are not 100% what you are doing rather find some good technician.
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Old 13th August 2012, 01:03 PM   #4
Ronj is offline Ronj  Singapore
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Oh by the way I switched the NEC 2SC853 and the problem persists. There are no trimmer pots on the board for bias or offset adjustment!
The picture below was taken before recap...
Click the image to open in full size.

Last edited by Ronj; 13th August 2012 at 01:07 PM.
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Old 13th August 2012, 01:11 PM   #5
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Quote:
It seems like bad solder joint
Tajzmaj is right. Since the distortion comes as the amplifier gets warm, I would look for a bad solder joint. Try tapping carefully with the grip of a screwdriver on the components and PCB. Maybe you are lucky to locate the component with the bad solder...
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Old 13th August 2012, 02:40 PM   #6
Ronj is offline Ronj  Singapore
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Originally Posted by Jan Dupont View Post
Tajzmaj is right. Since the distortion comes as the amplifier gets warm, I would look for a bad solder joint. Try tapping carefully with the grip of a screwdriver on the components and PCB. Maybe you are lucky to locate the component with the bad solder...
I did look one round... may be I will do it again. I thought bad solder joints will always have distortion unless moved or touched? I also thought bad solder joints wont develop problems while the board is stationary? "Warm" is a relative time as the by the time the distortion starts, even the heat-sinks are hardly warm...

Anyway I will do another round of checking
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Old 14th August 2012, 02:20 AM   #7
sregor is offline sregor  United States
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Sounds like a good job for an oscilloscope. I'll throw out two more possibilities. One (but not likely) would be center voltage drifting. I don't have schematic but appears to be a cap coupled circuit. Voltage at the emitters should be at around half the supply voltage. Try comparing voltages between channels. A more likely possibility would be open output transistor - used to very occasionally see transistor go open after they heat up. You also could have a loose screw on one of the collectors of the outputs-the electrical connection is through the tab and screw. Careful use of freeze spray could also help find the problem. Good luck with it.
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Old 14th August 2012, 03:01 PM   #8
Ronj is offline Ronj  Singapore
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Bad luck... My CRO is out, need to buy another one cheap then Is there anyway to buy a schematic for this amp at least? It looks very similar to Nikko TRM-300 too
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Old 14th August 2012, 03:17 PM   #9
sregor is offline sregor  United States
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If you can find a complete owners manual, Nikko used to put the schematic in at the end of the owners manual. I've seen an OM for the trm-500, but I don't think it is that close.
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Old 14th August 2012, 03:29 PM   #10
Ronj is offline Ronj  Singapore
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Unfortunately most of the google searches on Nikko A-300D returns my own posts And one audioheritage.jp link that's almost about it... And there is this post by a guy looking for TRM-300 Schematic too
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