Hello
I have in my workshop a Fender Ultimate Chorus Amp, which is suffering an AC Hum on the Clean Channel. This happens when you plug an Earthed 1/4" Plug into either Input Socket. If You then change channel with the switch on the front panel, the hum vanishes.
Same if You plug into the effects return socket, which then isolates the preamps from the Power Amps.
Work done so far: Replaced all electros in and around both Power Amps. Also replaced the Main Filter Electros known to be a problem in these amps.
I cannot see excessive AC Hum Signals on the Preamp Power Supply lines.
I will post Scope Traces, taken at the output terminals of the RH Power Amp, LH is similar --
I have in my workshop a Fender Ultimate Chorus Amp, which is suffering an AC Hum on the Clean Channel. This happens when you plug an Earthed 1/4" Plug into either Input Socket. If You then change channel with the switch on the front panel, the hum vanishes.
Same if You plug into the effects return socket, which then isolates the preamps from the Power Amps.
Work done so far: Replaced all electros in and around both Power Amps. Also replaced the Main Filter Electros known to be a problem in these amps.
I cannot see excessive AC Hum Signals on the Preamp Power Supply lines.
I will post Scope Traces, taken at the output terminals of the RH Power Amp, LH is similar --
OK, so it is not power supply ripple hum.
I don't quite follow your description. Hum in the clean channel. Channel switch makes it go away. So switching to dirt channel doesn't hum. Switch back to clean and hum still gone? Or switch back to clean and hum is back?
Plug into FX return and what exactly? Hum vanishes? Or hum changes with channel switch?
Assuming the clean channel is the hum source, do ANY of the controls affect that hum in ANY way? COntrols that do are after the source. Pins 1 and 7 are outputs on th op amps. Scope them for hum.
I don't quite follow your description. Hum in the clean channel. Channel switch makes it go away. So switching to dirt channel doesn't hum. Switch back to clean and hum still gone? Or switch back to clean and hum is back?
Plug into FX return and what exactly? Hum vanishes? Or hum changes with channel switch?
Assuming the clean channel is the hum source, do ANY of the controls affect that hum in ANY way? COntrols that do are after the source. Pins 1 and 7 are outputs on th op amps. Scope them for hum.
Hello
Thanks for all --
It sounds like a Mains Hum. Here in New Zealand, our Mains is 50 Hz.
If you eliminate the hum by switching to the Dirty Channel, once you Re-select the Clean Channel, the noise is back. If you plug an earthed 1/4" Plug into the Effects Return socket, effectively isolating the Preamps, Noise is also gone, hope this describes it better. Attached are 'Scope Traces, taken at the Output Terminals of the RH channel Power Amplifier, LH is similar.
00986 Shows the Background Noise Before Plugging in a 1/4" Earthed Plug into either Input Socket;
00990 Shows the Hum Signal; this is quite Low Level, but very noticeable in the Amp's 12" Speakers.
Maybe someone here has seen all of this before?
Thanks for all --
It sounds like a Mains Hum. Here in New Zealand, our Mains is 50 Hz.
If you eliminate the hum by switching to the Dirty Channel, once you Re-select the Clean Channel, the noise is back. If you plug an earthed 1/4" Plug into the Effects Return socket, effectively isolating the Preamps, Noise is also gone, hope this describes it better. Attached are 'Scope Traces, taken at the Output Terminals of the RH channel Power Amplifier, LH is similar.
00986 Shows the Background Noise Before Plugging in a 1/4" Earthed Plug into either Input Socket;
00990 Shows the Hum Signal; this is quite Low Level, but very noticeable in the Amp's 12" Speakers.
Maybe someone here has seen all of this before?
Attachments
And, Yes, I am aware that there is some oscillation in both Power Amps, Tried to eliminate this before by Replacing all Electrolytic Caps in Both Power Amps, checking all resistor values, Trial substitution of Output Devices. Have not tried substituting the Input Opamp yet.
Also Replaced both Main Filter Electrolytic Caps. None of this made any difference.
Beginning to run out of ideas, as I'm sure that these amps would not be designed like this???
Also Replaced both Main Filter Electrolytic Caps. None of this made any difference.
Beginning to run out of ideas, as I'm sure that these amps would not be designed like this???
The Question that You asked about the Control Effect: The Clean Channel Controls have No Effect at all on the Scope Signal.
One thing I did discover is that the hum signal only appears at the very first insertion of the Earthed 1/4" Plug into either Input Socket, at the point where it switches U10A, which operates the switching Fets.
Regarding the Preamp Power Supply Filter Caps C88 and C89, I at first did suspect them, but after bridging them one by one with a capacitor of larger capacity, there was no change, which means that they are probably okay.
One thing I did discover is that the hum signal only appears at the very first insertion of the Earthed 1/4" Plug into either Input Socket, at the point where it switches U10A, which operates the switching Fets.
Regarding the Preamp Power Supply Filter Caps C88 and C89, I at first did suspect them, but after bridging them one by one with a capacitor of larger capacity, there was no change, which means that they are probably okay.