Nikko STA-8080 Preamp Oscillation After Recap - diyAudio
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Old 23rd February 2015, 05:31 AM   #1
kouiky is offline kouiky  United States
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Default Nikko STA-8080 Preamp Oscillation After Recap

The time finally came for a recap, and of course - I used radial wound poly film capacitors. Antenna or microphone, you be the judge. I don't normally use this old receiver, but it was given to me in good working condition as far as I knew, some 15 or more years ago, and there was a quality about it that I liked at times. After the recap, if my hand came in proximity to several of the newly installed film caps or certain Elna Silmic II electrolytics (with the same effect in the same spots on either channel) the preamp would hum. If I taped them, there was a puck or dong through the speakers. In light of that, the sound was far better than previously. The more or less drastic improvement in bass, midrange and treble clarity after the recent parts update was shortlived.

I left the system running while I removed myself from the room, made some food and ate, and when I came back in, something was...off. I took a few steps back and I could hear high pitched ringing, very high and directional. It was the switch mode power supply in my Samsung big screen, and it was accompanied by a buzz. It does that if I don't use it every day. I've developed a very strong dislike for devices with SMPS because they all seem to do this after a while. That high pitched noise is uncomfortable, and my router and other tv do it too.

That wasn't all, the buzz was coming through the speakers. Unplugging the big screen stopped the loudest source of the noise, but it was still coming through the speakers. I turned off the receiver and said "Ah ha". I had a good notion of what was going on. When I turned it back on, the buzz was gone, but then I noticed something new - the audio rack was emitting a tone, a multiple of 60Hz. This was coming from the receiver vibrating. At first I thought it was another component, but then I got down by the receiver and my ears peaked. There was a high pitched "weeeeeee" actually emanating from the film coupling capacitors in the preamp.

It was singing but apparently only knew one tune...

Oscillation. I previously suspected the receiver of oscillation in the past month, or so. The treble sounded as though it was modulating at a lower frequency, although there has never been an apparent tone present on the output. Even now, nothing is hot. It appears to originate around the coupling between the first preamp active stage Q621/Q622 and the output buffer Q623/624, which are coupled by C625/C627 0.47uF, the tone stack, and C627/C628 0.47uF. These two last positions are the most microphonic to physical stimulus.

How is it this has come to be, and how is it developing positive feedback? Is one capacitor on each channel communicating with the next in line, self sustaining as the result of phase shift? Thank you for your assistance, and there are schematics available on several sites for this model.
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Old 23rd February 2015, 07:15 PM   #2
kouiky is offline kouiky  United States
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The preamp stage with 0.47uF film capacitors.
Click the image to open in full size.

C627/C628 were acting as microphones and after passing a signal through two active devices, C631/C632 were acting as speakers. I am still kind of surprised that this happened, but I have learned something from it. As previously mentioned, it was audible from the capacitors. I had a problem with Mundorf Supreme capacitors producing sound when I brought my soldering iron near them and behaving microphonic in a pair of speakers' crossovers, so maybe I shouldn't be so surprised.

I took a few minutes today to replace these films with Elna Simic IIs electrolytics, which are certainly cheap enough, and the problem has left. They are not as clear sounding as the film, but where I doubled the value the bass is more extended. This isn't main piece of my system anyway, but at least it's better off than it was for backup service.

The preamp stage with 1uF electrolytic capacitors.
Click the image to open in full size.
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Old 24th February 2015, 12:34 AM   #3
sregor is offline sregor  United States
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Thanks for sharing this interesting information. FWIW I would have tried stacked film type capacitors. I never noticed any microphonic caps in my travails, but did have microphonic cables once.
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Old 24th February 2015, 04:58 PM   #4
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I think you ruined the layout by shoehorning those huge caps in there.

It's like you added a bunch of antennae to your circuit, in all the wrong places. It adds a lot of parasitic interference to the layout.

Of course, michrophonics is a distinct possibility too. You're just begging for michrophonic interference with the way you mounted them.

I know that some electrolytic caps in old equipment are really terrible. Modern electrolytic capacitors are way better now than they were in the good old days. You might try them.
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Old 24th February 2015, 07:08 PM   #5
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I am screaming this when ever i have the chance .... you 've learned something the hard way

What you could do is to find MKT 1uf or 2.2 uf rated 63 volt and with a 5mm pitch from wima easy to install and will play better .

If wana have more fun with that and learn something more try to see if something changes while using the huge capacitors but with 180 degrees different orientationthan you had before
( polarity on this size might be an issue and to your surprize exists eventhough not mentioned on the part )
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Old 24th February 2015, 07:48 PM   #6
kouiky is offline kouiky  United States
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Thank you for your replies.

east electronics, I'll keep that in mind for the future.

sregor, I agree. I would have used stacked silver micas in a feedback loop, but I didn't foresee an issue in a signal path with wound films. I was seriously mistaken.

Quote:
Originally Posted by Fast Eddie D View Post
I think you ruined the layout by shoehorning those huge caps in there.

It's like you added a bunch of antennae to your circuit, in all the wrong places. It adds a lot of parasitic interference to the layout.

Of course, michrophonics is a distinct possibility too. You're just begging for michrophonic interference with the way you mounted them.
Oh, I know. There wasn't a great deal of choice due to underlying components, though.

Quote:
Originally Posted by Fast Eddie D View Post
I know that some electrolytic caps in old equipment are really terrible. Modern electrolytic capacitors are way better now than they were in the good old days. You might try them.
I did, I replaced every capacitor in the preamp and amplifier with Elna Silmic IIs (which I had on hand and were very inexpensive), shown in picture two, above. One capacitor is a Nichicon on the regulator. I'm not picky about the brands.

Last edited by kouiky; 24th February 2015 at 07:51 PM.
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Old 24th February 2015, 07:50 PM   #7
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Edit my name please I feel fat already !!!
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Old 24th February 2015, 07:55 PM   #8
kouiky is offline kouiky  United States
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Ha! How is it now?
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Old 24th February 2015, 08:00 PM   #9
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moots better thank you
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Old 24th February 2015, 08:05 PM   #10
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At east electronics we have seen this obviously more than once i could say more than enough ..

Gurus have been "upgrading " solid state amplifiers using in similar positions usually a Dc blocking cap with 4.7uf @630 V tubish capacitors obviously cost a fortune and obviously almost destroyed i nice Japanese amplifier . Owner brought in a nice Sansui and the statement was that ""i pay a fortune and the amp dont play any more "

We actually keep stock of these capacitors that actually make very nice replacement in crossover networks.
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