Nakamichi PA-7 output current testing - diyAudio
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Old 9th January 2013, 09:17 PM   #1
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Default Nakamichi PA-7 output current testing

I have a Nakamichi PA-7 I'm working on and I would like to know the best way to test the output current protection. Does anyone know what the current trip point is on this amp? The owner of this amp asked about bypassing the protection circuit because he thinks it trips too soon. This unit came in with the main 15A fuse blown. I found no blown parts and I was able to power it up and get it to pass signal. The line current was ~4A. I found that the right channel bias was reading ~100mV and the left channel was at ~45mV. After adjusting the bias pots to the 40mV spec, the line current is down to ~2.5A. The bias pot are very touchy, I'm thinking about replacing them with 10-turn parts.

Thanks for your tine.
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Old 9th January 2013, 09:46 PM   #2
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There is a very good thread on this forum that discusses your issues. It seems that Nakamichi had a upgrade to help with the sensitive trip (see post #23 ). Also it seems that others have recommended replacing the pots with sealed 10 turn pots.
Good Luck
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Old 1st February 2013, 09:35 PM   #3
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Default More Questions

I found that the right channel will trip at ~12A of output current. But, the left channel trips at about ~5A of output current. The left channel goes into a oscillation and causes the protect to trip. Anyone have a idea why this channel would oscillate?

Also, I found that the right channel has a 2SC1473 installed in place of a 2SC3333 for the bias transistor Q212. MCM does listed the 2SC1473 as a sub for the 2SC3333 but I'm not sure if this is a idea in this bias circuit. Does anyone know of a source for the 2SC3333?

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Old 2nd February 2013, 10:07 AM   #4
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Plenty of Chinese companies supply anything you want - in volume and likely just near copies.
2SC1473 looks a pretty god sub. in a non-critical task like typical bias transistors. Almost anything should be OK. but if it failed in the past, there has been an issue there with VAS current demand. They are low gain, low current (50mA) devices- it wouldn't take a lot to "pop" them.
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Old 17th April 2013, 03:07 PM   #5
billyk is offline billyk  United States
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Hey folks, this is the most recent post I can find anywhere about this issue. Does anyone have a solution? I just sold mine and the buyer is complaining it is setting off the protection. Never really did that to me. Could bias be an issue, or is the protection just that touchy?
Thanks for any input!
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Old 18th April 2013, 08:57 PM   #6
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I was able to fix the issue I had with the left channel oscillation issue by placing 0.1uF caps from the +/-V rails to GND on the power amp PCB. After this mod, I was able to get 15A of output current before tripping the protection circuit.

While working on the left protection issue, the right channel started to draw excessive current randomly. I eventually found that a 2SA1120 was used to replace a 2SA968. After replacing this part, the excessive current issue has not reared its ugly head.

I changed the bias pots to ten turn type which helped. But, I find that the right channel bias voltages hits ~160mV at turn on when cold. The left channel bias reads ~80mV. Both bias level out to 40mV after about 90 minutes (see chart below). The next problem I'm finding it that both bias readings will drop to ~25mV once the top cover is placed on the unit. Is this OK? The service manual does not state anything about checking with the cover on or what the room temp should be.


Click the image to open in full size.
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Old 23rd April 2013, 06:00 AM   #7
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Default PA-7 Bias graph

The picture of the bias chart does not seem to show up in my last post. I have attached the file this time around.


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File Type: jpg PA-7 Bias.jpg (89.7 KB, 20 views)
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