Yamaha G50 112 (MK1) noise

[Dpiddy]

Hi all,
First time poster here.

I have a Yamaha G50 112 (MK1 - so no parametric eq etc)

It's a fantastic amp, but it hisses loudly, which is fine in a band setting but not suitable for recording (which is what I bought it for!)

I'm familiar with soldering and electronics repair (not so much with amps though) and have read this thread https://music-electronics-forum.com/...-repair/35951-

So, I bought replacement transistors and replaced both FETs with new 2sk30As, which worked but didn't reduce the noise at all. I then replaced all 2sc1000 with NOS Toshiba 2sc1815y's as per the other thread. This caused the amp to stop working altogether, so I put back the original transistors and it works again, but still with the noise.



I'm a bit baffled as to why this happened - but one lead I have is that old Japanese transistors can have different pinouts? I have looked at the datasheets and the pinouts seem to match.

Has anyone got any ideas how to reduce the hiss for this otherwise great sounding amp? I've included the schematic as a link.

[Pixel]

I completely recapped my VX55 which is an 80s Yamaha amp. It was awfully noisy before but is really quiet now. YMMV. I didn't test the caps for esr until after I removed them. I recall all of them were "okay" only a couple were a little bit high.

The small electrolytics were all replaced with film caps. For electrolytics I used Nichicon PWs except in the power supply which were HE, GU and GY.

[g1]

I then replaced all 2sc1000 with NOS Toshiba 2sc1815y's as per the other thread. This caused the amp to stop working altogether, so I put back the original transistors and it works again

Did you check the transistors with your meter before installing?
Any chance the 'NOS' parts came from China? (ebay, etc.) If so, they could be fakes.

[Dpiddy]

Did you check the transistors with your meter before installing?
Any chance the 'NOS' parts came from China? (ebay, etc.) If so, they could be fakes.

They came from a very reputable (if not, the ONLY reputable) over the counter electronics shop in London - Cricklewood Electronics https://www.cricklewoodelectronics.com/

[Helmholtz]

It's easy to identify the gate with an Ohmmeter or diode tester. Drain and source are typically interchangeable.

But the problem with jfets is the huge parameter variation of maybe 500%.
Means they often have to be selected for the circuit.
That's the main reason why the industry doesn't like them.

[Helmholtz]

It was awfully noisy before but is really quiet now.

What kind of noise? Hissing?

[g1]

They came from a very reputable (if not, the ONLY reputable) over the counter electronics shop in London - Cricklewood Electronics https://www.cricklewoodelectronics.com/

Then likely they are genuine as stated on the website. But there is no mention of them having been tested, and it is not all that uncommon to find defective new parts. Always best to test new semiconductors before installing, when possible.

(edit: these comments are about why the amp with new 2sc1815 parts did not work, and have nothing to do with the noise issue)

[Pixel]

What kind of noise? Hissing?

Like water spraying from a hose. This was a $5 craigslist amp that I really only wanted the speaker from because it matched the one in my G50-112 III. It works quite well now though the distortion is not very good. I found a bunch of picks and a dollar inside it so I guess it was really a $4 amp.

[Dpiddy]

What kind of noise? Hissing?

Yeah, just a lot of hiss