My Accuphase E303 show oscillation on both channels.
I recapped both output boards but the oscillation hasn’t changed.
When I disconnect the power to the output transistors the gate signals are fine.
The oscillation occurs when I apply voltage to the output Mosfets.
I have checked the biasing current which is ok,
I have tested the 4x mosfets with the simple tester (without load) and the measure OK.
I suspected the 4x 47pF mica caps but I de soldered one and it was spot on.
Could it be that the mosfets got old and cause these phenomena ?
Needless to say they are obsolete and I haven’t been able to find a suitable replacement
but perhaps it would be more sensible to maintain those original Fets
and do something about the oscillation.
I would love to see if anyone has some advice on this issue.
Many best wishes from Singapore
Bjoern
I recapped both output boards but the oscillation hasn’t changed.
When I disconnect the power to the output transistors the gate signals are fine.
The oscillation occurs when I apply voltage to the output Mosfets.
I have checked the biasing current which is ok,
I have tested the 4x mosfets with the simple tester (without load) and the measure OK.
I suspected the 4x 47pF mica caps but I de soldered one and it was spot on.
Could it be that the mosfets got old and cause these phenomena ?
Needless to say they are obsolete and I haven’t been able to find a suitable replacement
but perhaps it would be more sensible to maintain those original Fets
and do something about the oscillation.
I would love to see if anyone has some advice on this issue.
Many best wishes from Singapore
Bjoern
Attachments
I think posting some more info like oscillation frequency, maybe scope pictures, would make it easier to determine if it's a global loop oscillation, or some local oscillation.
I sometimes found that touching different parts of the pcb with my finger would kill the oscillation, and that has given some clues to the problem.
I sometimes found that touching different parts of the pcb with my finger would kill the oscillation, and that has given some clues to the problem.
Are you sure that it is oscillating also without any probes connected - i.e. the output transistors warm up, excess current is drawn?
Sometimes the probes can invoke oscillation and service manuals recommend connecting them e.g. via 1k resistors.
What is amplitude of oscillation?
7MHz should appear with full amplitude across Zobel resistor, R39. Make sure both both R39 are C20 are good parts and properly connected. (You see the same amplitude across R39 as at speaker terminals, and R39 should measure 4.7 ohms with meter.) Be sure to use x10 probes with shortest practical ground wire at probe. Look for presence of 7MHz on supply rails; scrutinize C21 and C22 and integrity of their connections to supply rails and to ground.
Good luck!
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