Nakamichi PA-7 recap

General
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21

So I ordered the caps needed except the power supply caps. I took the transistors off the heatsink and the insulators are silicone without thermal paste. They were hell to unstick the transistors from them. I thought I was going to break them. Everything looks fine and I will probably just reuse the insulators unless someone says otherwise.

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22

Those look fine to me. Iā€™d personally forgo thermal paste with those as well, if there was an intentions of doing so.

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23

Since you say they were stuck on I bet the pads have been squeezed into the aluminum pretty well over time to make good contact. Iā€™d just leave em untouched.

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24

Thermal paste might work as a good ā€œanti seizeā€ if you plan on doing this job again in the future.

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25

Thanks guys.

26

So, Finally got the caps replaced and also replaced the relays even though they worked fine.

Well, after turning it on, the right channel relay doesnā€™t seem to work. I hear the protection board relay kick on and the left channel comes on.

Question is, did I screw something up? Is the new relay bad? Or, something else?

Thoughts.

Good news is, no magic smoke escaped!:sweat_smile:

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27

Hopefully something simple. Verify the coils are getting voltage, possibility trace back to whatever is powering the coils. Compare with the left side circuit.

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28

Double check for solder shorts.

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29

Yeah, I was thinking about that. Hoping that is all it is. I will have to break out my lighted magnifing glass.

30

See what your DC offset looks like on that channel. If itā€™s good, that points to the protection circuit. I doubt itā€™s a bad new relay, but you never know.

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31

Thanks Tom. Will check on Sunday. My day off.

32

Get on of theseā€¦

Find the 2.5X version. Thank me later.

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33

That would be better than what I have.

34

Ok, got 0v on the good channel 76v on the bad one.

Going to take that channel apart and check solder joints.

Any other checks I can do while it is still together?

35

Yikes! You might want to inspect the IC insulators you reused, too.

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36

I thought of that too. I did give them a good once over before putting it back together.

Maybe I tightened the screws too much? How much torque should be on them?

37

Ok, just checked and everything looks really good. Any suggestions?

38

Since you have one good channel, you can use that to see where things are going wrong. I would start at the driver transistors - see if the base voltages are the same. I would look specifically at your neg voltage rails. Something is pulling that channel to the positive rail.

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39

Thanks Tom. Unfortunately, I donā€™t know how to do that. I would imagine the power would have to be on to do these checks?

40

BTW, when I measured the 76v it was -76v. Donā€™t know if that matters.