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Nakamichi PA-7II won't power on, limiter resistor blown
Nakamichi PA-7II won't power on, limiter resistor blown
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Old 12th June 2015, 08:24 PM   #1
tenelson is offline tenelson  United States
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Default Nakamichi PA-7II won't power on, limiter resistor blown

I've come across a non-functional Nakamichi PA-7II amp. It will not power on. I found that R504, the 20W resistor on the limiter PCB is blown. So I replaced it, and it immediately blew again. The thermal fuse in series with it is intact. Does anyone have any ideas as to what might be causing the resistor to blow?
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Old 12th June 2015, 09:50 PM   #2
gasgeek is offline gasgeek  United States
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Most likely the power transformer is shorted. R504 is acting as an inrush limiter until the relay kicks-in which is supplied by a secondary voltage from the transformer.
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Old 12th June 2015, 10:10 PM   #3
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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Or a short circuit on a secondary winding.
Disconnect the secondaries and power it up with the 60W light bulb to replace the resistor.
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Old 13th June 2015, 01:07 AM   #4
tenelson is offline tenelson  United States
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Wow, the power transformer would be difficult to replace I imagine. Can I just check for shorts on certain wires to/from the transformer?
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Old 13th June 2015, 06:56 PM   #5
wayne is offline wayne  United States
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Nakamichi PA-7II won't power on, limiter resistor blown
Disconnect secondary from rectifiers as a test.
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Old 3rd March 2020, 02:56 AM   #6
tenelson is offline tenelson  United States
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I'm returning to this project after years of letting it sit. I replaced the blown R504 with a light bulb, as suggested. When powered on, the voltage back from the secondary seems to vary from 0 to 1.3 VAC. If I disconnect the secondary and measure it, I get 1.7ohms. Is this telling me the transformer is toast?
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Old 3rd March 2020, 12:02 PM   #7
Chamberman is offline Chamberman  United States
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Does the light bulb you installed in place of the current limiting resistor stay brightly lit or dim?

1.7 ohms across one of the secondaries sounds about right.
According to the schematic the transformer primary has an internal thermal fuse inline with it. If you're only seeing 1.3VAC across one of the secondaries under power then you may want to ensure that the xfrmr primaries are not open due to this fuse.
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Old 3rd March 2020, 01:41 PM   #8
tenelson is offline tenelson  United States
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The bulb didn't seem to light at all, though it definitely allowed the 120 VAC to reach the other side of the resistor pads (I measured it), so it effectively took the place of the resistor. It was a 75W incandescent, for what it's worth.

Do I understand the purpose of the bulb correctly, in that if it didn't light up, there is no short in the rest of the connected circuit?

Chamberman, thanks for the suggestion. The next step for me is to desolder the 2 wires to the primary from the limiter PCB and measure those. If they are open, that implies the thermal fuse is blown? If the fuse is ok, should I get something in the low single-digit ohms like on the secondary I measured? Are there any options for a replacement transformer if the thermal fuse is blown?
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Old 3rd March 2020, 02:55 PM   #9
Chamberman is offline Chamberman  United States
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You are correct in that if the lamp doesn't stay lit then there is no load on it. The more brightly it is lit the higher the load. Typically a class ab amp with low idle bias and a decent amount of filtering capacitance will brightly illuminate the lamp for a few secs then it will dim way down to a low level. You have all of the electronics after the transformer disconnected so you should not see any illumination unless there's a problem.

Also, yes if the primaries measure open then the thermal fuse opened.
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Old 3rd March 2020, 03:12 PM   #10
Chamberman is offline Chamberman  United States
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BTW there is a second thermal fuse that is on the slow start board. It should be contacting the body of the slow start resistor(s). It's in series with those slow start resistors.

When you measured the 120vac was it present at the wires feeding the xfrmr primaries?
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