Nikko Alpha II no idle current left channel - diyAudio
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Old 18th February 2015, 03:05 PM   #1
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Default Nikko Alpha II no idle current left channel

Amp powers up and operates. Realized one heat sink was hot while the other was cold during idle. Check of the idle current on the left side measured 0mV. Adjusting R714 1K pot makes no change.

Though I was able to confirm that the pot is adjusting by monitoring Q1 base and watching Q1 emitter.

Can anyone help in what can be suspect?
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Old 18th February 2015, 03:25 PM   #2
rayma is offline rayma  United States
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Quote:
Originally Posted by rfitts46 View Post
Check of the idle current on the left side measured 0mV.
Maybe one of the 0.47 Ohm emitter resistors is open.
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Old 18th February 2015, 05:58 PM   #3
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Rayma, it looks like 2ea .47 ohm emitter resistors in parallel bringing the resistance to .24 ohms. If one opened the emitter resistor value then becomes .47 ohm? Can these be checked for open without having to remove from the board?
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Old 18th February 2015, 06:20 PM   #4
rayma is offline rayma  United States
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Quote:
Originally Posted by rfitts46 View Post
Rayma, it looks like 2ea .47 ohm emitter resistors in parallel bringing the resistance to .24 ohms. If one opened the emitter resistor value
then becomes .47 ohm? Can these be checked for open without having to remove from the board?
Sure, there's no need to unsolder them. After the power supply is discharged, just measure the resistance directly across both pairs of emitter resistors.
The meter should read 0.24 Ohms or so. Zero the meter first because of the low resistance.
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Old 19th February 2015, 01:34 PM   #5
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Good news. Tested the emitter resistors for resistance.

left channel .5 ohms
right channel .5 ohms

the resistance of the clip leads is .26 ohms

so that puts the resistance at .5 - .26 = .24 ohms, half of the paired paralleled .47 ohm resistors.

Decided to give the wire wrapped connections on the left channel amp board a twist using the needle nose pliars.

Now have idle current and was able to adjust to proper setting of 19mV 3mV.

Let it sit for 20 minutes and each heatsink was warm.
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Old 9th January 2016, 11:34 PM   #6
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Do not use this amp. Got it back out and the symptoms are back?
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Old 11th January 2016, 03:26 PM   #7
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The schematics I have the test voltages are not legible. If you have a schematic that the test voltages are legible please email to me (rfitts46@comcast.net ). amplifier functions and sounds good but the left channel has no idle current. TP 14 to TP 15 measures 0V. Heatsink is cold to the touch when idling. The base voltage on Q1 is low. Can only get up to 400mV or so when adjusting R714.
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Old 11th January 2016, 03:28 PM   #8
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The only resistors checked were the emitter resistors. I have not spent a lot of time on this amp. I have never used regularly but am interested in getting the idle current corrected. As stated the last time I played with this the idle current miraculously appeared...in hindsight that in itself is interesting. I thought as a result of twisting the wire ties on the left channel amp board resulted in the idle current reappearing. I have retried retwisting but to no avail. I removed Q1L and Q2L, they are on the same heatsink board so new heat compound could be applied to Q1L. My next step was to disassemble the chassis to a point so the two output boards can be inspected and allow probing from one to the other to determine what differences there are in the predriver area. Currently traveling until Wedn so I have access to nothing. But looking at the posted measurements it appears that the Q1L emitter is low measuring -1.05 should be -1.2. Q1L collector looks good at 1.2V. The schematic has various expected factory measurements but the schematic copies I have these measurements are tough to make out. I am a novice so any direction to focus on are appreciated.
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Old 13th January 2016, 04:11 PM   #9
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It sounds like you have a bad solder joint somewhere, which was temporarily fixed by moving stuff around.

Going through the main and output boards with some flux and reflowing all the solder joints would not be a bad idea.

Also could be corrosion at the end of a link wire from old flux eating away at the copper.
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Old 13th January 2016, 04:51 PM   #10
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Peart, thanks for your thoughts...as soon as I get back into town...am planning on disassembling chassis so that both output boards can be gotten to. Will do thorough inspection and start probing both boards for similarities and dissimilarities. Will report back on observations /measurements.
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