Hello all,
I have been struggling for some time to bring a Yamaha M-60 back to life.
After fixing the mechanical problems, changing the electrolytic capacitors (except the giant ones on the power supply), redoing absolutely all the soldering on the amplifier board, I still have a problem that I can't get rid of.
I have on one channel a white/pink noise of approximately 15-20mV at the output and on top of this, is superimposed random a static noise of amplitude hundreds of millivolts.
The other channel works OK.
I locally heated the components on the input stage, sprayed with freeze spray, changed some transistors on the input stage, starting with the double TEC-J (in English it is usually referred to as J FET) , but without result.
By connecting a 20µF capacitor between the output of the VAS stage and GND the noise did not change.
What I can do more, except to exchange the components form one channel to the other one?
I have been struggling for some time to bring a Yamaha M-60 back to life.
After fixing the mechanical problems, changing the electrolytic capacitors (except the giant ones on the power supply), redoing absolutely all the soldering on the amplifier board, I still have a problem that I can't get rid of.
I have on one channel a white/pink noise of approximately 15-20mV at the output and on top of this, is superimposed random a static noise of amplitude hundreds of millivolts.
The other channel works OK.
I locally heated the components on the input stage, sprayed with freeze spray, changed some transistors on the input stage, starting with the double TEC-J (in English it is usually referred to as J FET) , but without result.
By connecting a 20µF capacitor between the output of the VAS stage and GND the noise did not change.
What I can do more, except to exchange the components form one channel to the other one?
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Feed a clean signal into the input stage and check with scope moving from point to point, you may have a component that is about to fail or underperforming due to age. If you have not done so make sure you remove all dried glue residue, this causes conductivity & corrosion.
The noise is usually below or at the limit of my oscilloscope noise. Only the static noise is high enought to be easy visible on the scope but is not present all the time.
I will post some image from the scope.
I will post some image from the scope.
This sounds like a very vexing, frustrating problem.
If using you are using a X10 scope probe, you could switch to X1 probe at amp output for improved sensitivity, but X1 probes can be problematic when probing internal nodes because of capacitive loading/possible oscillation.
Do you have a preamp that could be used for aded viewing gain?
What is "TEC-J" ?
I have downloaded a service manual from manualslib.com. Do you know of a better source?
Good luck!
If using you are using a X10 scope probe, you could switch to X1 probe at amp output for improved sensitivity, but X1 probes can be problematic when probing internal nodes because of capacitive loading/possible oscillation.
Do you have a preamp that could be used for aded viewing gain?
What is "TEC-J" ?
I have downloaded a service manual from manualslib.com. Do you know of a better source?
Good luck!
Probably one half of the input diff pair transistors has a noisy junction. Can also be a diode in the VBE.
The schematic is identic with the M65, M65 has additionally a switch cu power off the Vu-meter.
Now I made a test and it seem to react at heating and cooling the diode D105. For me is strange but I will exchange between channels this diode.
This si a schematic made by me from a divided schematic from manual:
IC101 and IC102 are a integrated pair of MOS tip TEC-J transistor. The MOSFET use an layer of SiO2 to isolate the gate from channel and the MOS TEC-J use a PN "Junction" to isolate de gate from channel.
Now I made a test and it seem to react at heating and cooling the diode D105. For me is strange but I will exchange between channels this diode.
This si a schematic made by me from a divided schematic from manual:
IC101 and IC102 are a integrated pair of MOS tip TEC-J transistor. The MOSFET use an layer of SiO2 to isolate the gate from channel and the MOS TEC-J use a PN "Junction" to isolate de gate from channel.
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I already exchanged the transistor between channels. The noise remain in the same channel.
Don't do that, you could end up with two bad channels.
Better to just replace the part under consideration with a new one.
It is enough that only one channel is defective and I cannot use the amplifier. So if they are both, it won't be the end of the world.
I have some experience, I'm not afraid that I will destroy the other channel as well. I have already exchanged about 6-7 transistors and some diodes, from the input stage.
I have some experience, I'm not afraid that I will destroy the other channel as well. I have already exchanged about 6-7 transistors and some diodes, from the input stage.
I solved! It was D105. I replaced the diode in booth channels with 1N4148. Now the noise of the amplifier are below I can see with the oscilloscope.
