Yamaha M80 oscillation

Hello

I have been adjusting the bias on a (repaired) Yamaha M80. I have realised that one channel has an oscillation at the output (sine waveform, about 1Vpp at 500 KHz) with no load, and before biasing it (pure class B). It happens occasionally, but when it happens if I start to bias the channel, it disappears quickly. Once biased, seems that this oscillation is not present anymore.

Output transistors are 2SC3858/2SA1494 (Digikey)

Should I worry about this? What kind of test could I do to check stability?

Thank you.

This sometimes happens after repairing. Where the driver transistors also replaced? Original parts or substitutes? Had this problem once when the bandwidth of the drivers was much higher than with the originals.

Where the driver transistors also replaced? Original parts or substitutes?​

Yes, drivers, predrivers and output transistors, in both channels:

- Predrivers: 2SC3416/2SA1352. They are original spare parts from Yamaha.
- Drivers: 2SC4793/2SA1837. Yamaha sold them as substitues from 2SC3298/2SA1306. They have the same bandwidth as originals.
- Output transistors: 2SC3858/2SA1494. I used them as substitutes from 2SC2773/2SA1169. They have the same bandwidth as originals.

I think that the good news is that it only happens occasionally when it's not biased, if I'm not mistaken in this mode the transfer function is theoretically less stable.

Anyway...what do you think about add a small capacitor connected between the collector and base of the output transistors (or drivers) to improve stability at high frequency? Any drawback?

arm2,
I used all the same part as you in my M-80 repair except for the predrivers 2SC3416/2SA1352, I used KSC3503/KSA1381. I hope I don’t have the same 500kHz oscillation you’re seeing (don’t have an oscilloscope available to check). I’d be surprised if it actually affects the performance of the amplifier in a significant way.
So have you been using your repaired M-80? I use mine daily and have been very happy with it especially with Auto Class A turned on. It does run warm after it been on for hour or two, around 115 degrees F (~ 46 degrees C), but I’m not worried this is well within the specification for the output (TJ 150 degrees C max). Over the last month or so it has really started to sound much better to me, I don’t know why: break-in/burn in maybe; or my hearing?
Good Luck,
Barry

Hello Barry

I hope I don’t have the same 500kHz oscillation you’re seeing (don’t have an oscilloscope available to check)

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I don't think so. It seems quite stable once biased, but I'm wondering if I can improve stability.

So have you been using your repaired M-80?

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Yes, but not daily. No problem so far, although I still have to do a "stress test" with a dummy load.

I use mine daily and have been very happy with it especially with Auto Class A turned on.

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I can not hear any significant difference with Auto Class A on or off. In fact, I have bypassed the switch...

bmanone said:

I used all the same part as you in my M-80 repair except for the predrivers 2SC3416/2SA1352, I used KSC3503/KSA1381.

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KSC drivers are twice faster than 2SC. This could be a problem, although I used them in a previous repair and they worked well.