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Old 9th March 2016, 02:50 AM   #1
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Default Nakamichi PA-7 turn on problem

Hi

I just discover right side of the amp failed to turn on a few times, I have to turn the amp off and on to get it going. The last few times it seems to be ok.

Also, at the beginning, the left side always turn on first, the right side took like 15sec to click on.

I just bought it on ebay, the seller offer return and refund. I just want to run by you guys is it worth fixing. Also any advice will be appreciated.

I attached the schematic of one side.

Thanks
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Old 9th March 2016, 04:19 AM   #2
anatech is online now anatech  Canada
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Hi Stephen,
Measure the DC voltage on the output collectors while the unit is waiting to come on. You might be safer to connect your meter leads safely from that point to common and just watch it as you turn the amp on. Wait to see if the amp will come out of protect on it's own. It's not just a turn on delay, it is also watching for a safe condition before closing the relay. I suspect that you have a DC offset issue that needs to be addressed. This could possibly be to the iffy filter capacitors you have.

-Chris
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Old 9th March 2016, 05:24 AM   #3
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Quote:
Originally Posted by anatech View Post
Hi Stephen,
Measure the DC voltage on the output collectors while the unit is waiting to come on. You might be safer to connect your meter leads safely from that point to common and just watch it as you turn the amp on. Wait to see if the amp will come out of protect on it's own. It's not just a turn on delay, it is also watching for a safe condition before closing the relay. I suspect that you have a DC offset issue that needs to be addressed. This could possibly be to the iffy filter capacitors you have.

-Chris
Thanks

When you say the collector, do you mean the collector of the output big transistors.....which is just the output before the relay?

I had the DVM with one side at the black speaker output(ground) and the other one on the collector of one of the big transistor (eg. Q117) of both channel when the amp was off. Then I turn the amp on while the probe is on the collector. Both channel went to zero volt right away. I can't tell the moment it power on as the meter is slow, it jumped to like 0.4V and back to zero in split second.

I don't think that's the problem.

One thing I forgot to say. The sound always turn on with the relay click, I don't even think the relay is sticking. It's not like the relay clicked and no sound comes on. I believe there are separate relay for each side as I can always hear the left relay click and left side comes on most of the time.
Thanks
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Old 9th March 2016, 04:51 PM   #4
anatech is online now anatech  Canada
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Hi Stephen,
Yes, the collectors of the outputs. Anywhere on the speaker connection before the relay is fine.

You're right, it isn't a DC offset problem. It's probably just a timing capacitor thing where it has gone leaky. Normally that would cause the relay to click much sooner as they usually "dry out" and their capacitance drops. I haven't checked, but sometimes there is a capacitor - diode network to detect AC power. If that cap isn't in good shape it can cause exactly what you are seeing. That is more likely.

Yes, that is the protection relay. It cuts the speaker off on power up and in case a channel develops a fault. So it is perfectly normal.

The other relay is the soft start relay. It shorts out a resistor and you should here that one click soon after pushing the power switch on. I think that amp uses a 2R2 resistor for the surge limiter (soft start). The first relay shorts it out, the second is for the speakers to connect.

-Chris
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Old 9th March 2016, 05:22 PM   #5
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Hi,
Can you check the voltage at the collector of Q133. I think a zero voltage will hold the relay off. That will ground the base voltage of Q135 disabling the relay 101.
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Old 9th March 2016, 06:43 PM   #6
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Good morning to all.

I bought my amp for $779 including shipping and all. I made an offer to the seller I would keep it if he refund me $200. So before that, I cannot open the amp to test. I already found the filter caps. It's about $120 plus shipping and tax. I am just waiting for the rely from the seller.


I have been reading the schematic last night. C110 is suspicious. I will look at the collector of Q133 to make sure the voltage is high enough to turn on the relay.

Are they using Q134 as a zener? That's not very nice!!! If I get into it, I would change to a real zener. It is not guaranty how accurate the reverse break down voltage of the Vbe from part to part. that's what the zener is for!!! Do you know the break down voltage?

Thanks

Last edited by StephenR; 9th March 2016 at 06:46 PM.
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Old 9th March 2016, 08:44 PM   #7
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Hi,
I agreed with you about cap C110. I think on power ON the Q133 it is turned ON to hold the Q135 off. When Q133 it is tuning off that will allow the cap to charge up holding the Q135 off for a short time. About Q134 they are using it as a blocking diode.
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Old 9th March 2016, 09:06 PM   #8
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Q134 is the only source to supply a voltage to turn Q135 and Q134 on. I think it has to work as zener. It is common in IC to use reverse biased Vbe as zener. If you look at datasheet, they usually spec the reverse break down voltage between 3 to 10V.

I think the problem can be either the leaky C110 or an unusual high break down voltage of Q134.

But I am on a holding pattern waiting for the seller to reply. I am not willing to pay full price for a defective amp from the start. If he is not willing to refund $200, I might as well return it and start again. This likely requires to take the board off the heatsink. I have to change all the heat conductive pads as it would not work just screwing the board back.

Last edited by StephenR; 9th March 2016 at 09:12 PM.
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Old 9th March 2016, 09:30 PM   #9
anatech is online now anatech  Canada
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Hi Stephen,
You're arguing over $200 for an amplifier in good condition? It may be the most expensive $200 you ever chased. I would highly recommend that you split the difference with the seller.

This amplifier is a known quantity for one, and in really good cosmetic condition on top of that. I guess I'm old, and that brings with it another way of looking at this.

Transistors usually have E-B breakdown voltages of 6 ~ 7 VDC. The muting transistors can go to 12 VDC or higher. Using a transistor as a diode is a time honored method. One thing to consider is that J-Fets make very low leakage diodes. So in that instance, a designer who needs that characteristic is a smart person if they use a J-Fet.

I haven't looked at that schematic for a long time, but I do know that the engineers at Nakamichi were a smart bunch of guys who didn't cheap out and generally were above average in circuit design. I knew them at the time these amplifiers were being made and only ever had one break down under warranty (destroyed a copper box staple that fell inside).

Okay, Q134 is being used as a zener to provide a snap action to turn the relay on quickly, keeping Q135 from being destroyed due to excessive dissipation. C110 is your timing element, but R146 may have gone higher in resistance. Leakage through Q133, Q134, D105 or C110 could most definitely slow things down. Remember that we are talking about uA of current - the base current for Q135. This is a lot lower than you are used to seeing. Low leakage is important in this circuit.

-Chris
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Old 10th March 2016, 11:52 AM   #10
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I bet they knew this which is why they sold it. They probably hoped no one would notice or some think it was normal. I'd ask more than 200. I'd cost you a lot more than that to have a tech look at it. Your time and effort is worth money.
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