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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 7th March 2020, 06:47 PM   #21
tenelson is offline tenelson  United States
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Never mind, I found some nice sealed Bourns 4 turn trimpots on Digikey.
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Old 8th March 2020, 12:45 AM   #22
Chamberman is offline Chamberman  United States
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You might want to measure the hole pattern to verify anything you're buying will fit. Someone recently did this change on Audiokarma, here.

I wouldn't do it the way he did it though. I've done this before with the Bourns 3296P pot he mentions and bent the legs such that it will flip the pot to where the adjust screw is facing up. There's generally enough lead length to allow this.
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Old 8th March 2020, 03:44 PM   #23
tenelson is offline tenelson  United States
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I wound up going with these: 3339W-1-102LF Bourns Inc. | Potentiometers, Variable Resistors | DigiKey.

And Iím planning on bending the pins front to back and back to front such that the adjuster is facing up, like you mention.
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Old 20th March 2020, 05:33 PM   #24
tenelson is offline tenelson  United States
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Iíve received my replacement transistors and have started removing the old ones. Should I use a little thermal paste between the transistors and the pad mounted to the heat sink, or is it ok to just mount them dry? The old transistors seem to be a little stuck to the pad, but I donít see any obvious paste.
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Old 20th March 2020, 06:37 PM   #25
Chamberman is offline Chamberman  United States
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You should use a new silpad or equivalent pad dry. Or use mica & grease. It's messier but works great. Don't reuse the old thermal pad.
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Old 23rd March 2020, 01:26 AM   #26
tenelson is offline tenelson  United States
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I have removed and replaced all of the main transistors on the right channel. It was pretty each once I got going.

Now, when I power on the amp through the DBT, the bulb is lit for a few seconds then dims. With both channels connected to the power supply, I see that both have an LED lit on the amplifier boards indicating overload. Adjusting the trimpots for bias has no effect, on both channels it doesn't budge from around 4mV DC. What are the reason the power amp protector circuit can indicate overload? The service manual doesn't seem to say anything about this circuit.
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Old 23rd March 2020, 02:19 AM   #27
Chamberman is offline Chamberman  United States
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The overload circuit is probably seeing high DC or the DBT may be dropping too much voltage and messing with it. What wattage bulb are you using?

Whats your DC offset measure across pins 1 & 3 on TP12R & TP12L?

If the DC offset is high then adjust it down, try to get it < 100mV anyway.

If the DC offset is low as measured across those test points and the DBT is dimming appropriately a few seconds after turning the amp on then try removing the amp from the DBT and see if the overload on both channels goes away.
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Old 24th March 2020, 07:44 PM   #28
tenelson is offline tenelson  United States
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With the DBT removed, I powered on the amp, measured, the offset, and got -81.1v on both channels! This must be implying another part of the circuit is not working, but I'm surprised that both channels would have the same problem. Ideas of what to check next?
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Old 24th March 2020, 07:50 PM   #29
Chamberman is offline Chamberman  United States
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That's very odd that both channels are doing the same thing. Have you checked to be certain the positive DC rail voltage is present?
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Old 24th March 2020, 07:54 PM   #30
tenelson is offline tenelson  United States
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Yes, I get +82.3v for the positive rail and -82.2v for the negative rail, as measured on the large resistors between the rail and the main transistors.
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