Nakamichi PA7II: HF noise-oscillation issue - diyAudio
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Old 18th April 2016, 12:59 PM   #1
jvhb is offline jvhb  Europe
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Default Nakamichi PA7II: HF noise-oscillation issue

Hi, I have a problem with one channel in a Nakamichi PA-7II (220v european version), and I was hoping someone familar with these could chime in..

I tried to attach the service manual, but it is too large. The complete document can be found here:
Nakamichi PA-7 Manual - Stereo Power Amplifier - HiFi Engine
But I extracted the page with schematic of main amp, and that is attached here.

There is a high frequency noise on the right channel, which seems to be around 2 Mhz, with varying strength. I have attached a few pics of output on scope. Test signal is 1Khz sine.

One picture shows just the HF noise present with the amp idling and no input signal. (Time on scope is set at lowest here: 0.5 uSec per division.)
During this test I found a clear connection with Iq (bias) level: The noise would disappear when as I lowered Iq to around 20mV (specification states 40mV). As I raised Iq towards 40mV again, the noise would come back!

At a second test (today) I was unable to replicate this exactly. Now I could just see the problem from the high distortion level at 1w output, and also my oscilloscope actually makes a high pitched whine when I connect the right channel - which does not happen with the left (good) channel.

One photo shows distortion residual from HP339 distortion analyzer below the 1w 8ohm output (1 khz sine test signal). The HF noise is clearly visible on the sine output. DMM shows RMS value of amp output.
DC offset is around 4-5mv, so that seems fine.

For comparison, I have also attached a pic of the the left channel output, and with distortion residual below. Very nice.

So the fault seems to be related to Iq level, but varies periodically in serverity.

At this point I am thinking defective output transistor, so unless someone has a better idea, I will desolder the source resistors one by one (in pairs), and try to eliminate the fault that way.
But if anyone has seen this issue before (perhaps even on a Threshold of similar design) I would really appreciate your input. Thanks in advance.
Attached Images
File Type: jpg Right channel problem.jpg (94.2 KB, 207 views)
File Type: jpg Right channel 1w result_bad.jpg (68.0 KB, 200 views)
File Type: jpg Right_2 Mhz noise app. 0.45V P-P idle (40mV Iq).jpg (73.2 KB, 199 views)
File Type: jpg Left channel 1w_good.jpg (72.5 KB, 199 views)
Attached Files
File Type: pdf Nakamichi_PA-7II_page 14.pdf (685.5 KB, 36 views)
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Old 18th April 2016, 01:14 PM   #2
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anatech is da man , with more than enough experience with these amps
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Old 18th April 2016, 01:15 PM   #3
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Hi ...

I have briefly read through your description and my guess (not an expert on this though) would be that the amplifier becomes temporarily unstable due to the likely higher transistor bandwidth when the current is increased (either with the 1 kHz tone or the idle current). Suggestions could be an old electrolytic capacitor (i.e. less efficient HF filtering) or to place some small ferrite toroids on the base(s) of relevant transistors.

Again - not an expert on this so just a suggestion.

Hav en god dag ;-)

Jesper
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Old 18th April 2016, 11:13 PM   #4
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I would check to see if the output "zobel" RC network to ground has a burned
resistor. Sometimes you see this when the amp has been abused in some way.

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Old 19th April 2016, 06:44 AM   #5
jvhb is offline jvhb  Europe
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Thanks for the suggestions, yes I hope Anatech will come by

> Mr Pass: The R in the Zobels are fine (unfortunately) - in both channels it measures 5.1, exactly as specs. But your input is much appreciated.

I should add, that the amp looks to have let a relatively "sheltered" life: There are no visible signs of overheating or miscoloring on PCB or any resistors. And all the 2w source resistors measure very close to 1.05 ohm (they are 1R nominal).

Is there a (simple) way to test if the problem lies in the 4 "Stasis" transistors? - as opposed to the "slave" current dumping output transistors? The latter I can test by disconnecting them one pair at a time, but this won't work for the Stasis quad.
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Old 19th April 2016, 11:10 AM   #6
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Check to see if any transistors have been replaced with more modern transistors.
I decided to upgrade the drivers in an amp from TIP devices to more modern faster, higher beta toshibas. The thing oscilated and I had to put the TIP devices back in there.
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Old 19th April 2016, 03:43 PM   #7
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Yes good point, but all transistors look original. They are all the correct Sanken types in any case.
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Old 19th April 2016, 04:09 PM   #8
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Hi jvhb,
I think Nelson nailed that issue down pretty well.

These are normally very stable amplifiers, but losing the zobel would make most amplifiers very unstable. Look at the zobel for overheated resistors. The capacitor needs to be measured as they generally can reduce in value or even open up without showing any outward signs of trouble. For the cost of them, you might be further ahead just replacing them. Don't use a wire-wound resistor in a zobel network as the inductance will take it out of circuit right where it's needed the most.

Absolutely have a good look at the output and driver transistors Look for different date codes on these parts as there might be a fake in there. When these blow up, I replace all the outputs on that channel, and the driver transistors as well. If you install faster drivers, you have to figure out what the new compensation capacitor's value should be. Probably easier to install the originals as 2 picoDumbs discovered.
Nice amp BTW. Extremely reliable and they can sound pretty good. One day I'd like to try one in my new system - just to see how it sounds. It's a keeper though.

Hi Choky,
Many thanks for the kind words. But, I will bet that you knew the answer as well. In this case, being a design that Nelson originated didn't factor into the problem.

-Chris
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Old 19th April 2016, 07:23 PM   #9
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Thanks for the schematic - I didn't have that.

If the output RC networks are OK, then probably we need to look elsewhere.

In Nakamichi's enthusiasm for wide bandwidth, the II series were given even
faster devices with parallel drivers and so on. As you might imagine, this
trends toward instability, especially when transistors are substituted.

I would suggest that you explore stabilizing the output stage with some
resistance in series with the bases of the transistors. Q114 and Q115 are good
candidates at 100 ohms and Q116 - 119 at 10 ohms.

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Old 19th April 2016, 07:37 PM   #10
anatech is offline anatech  Canada
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Hi Nelson,
To be fair to Nakamichi, we never had any series II amps or preamps come in for warranty service. Not even for post warranty service. The only warranty we ever had for the original amps was one (executed by a copper staple for boxes - the giant ones. Staple did not survive the ordeal either.) The other warranty claims we made were for the protection circuit modification to back it off. Nothing ever died in warranty other than the staple incident. I've never, ever seen zero defects like that with any line, not even Revox (who bench tested every single piece before it was shipped out.). We only ever serviced that one PA-7 amplifier, and a CA-7 that met spec, but I had to make it less noisy. That was a challenge as the S/N rating was in excess of 100 dB (can't remember exactly what the spec was).

That was a darned good lineup no matter what people would like to believe. Good design Nelson, and the Nakamichi folks made theirs tough as nails.

-Chris
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