Fender Ultimate Chorus chassis removal

[geofo]

Soundguruman

You mentioned to remove the FET(s) and the problem will go away. What abouit what 52 Bill mentioned as far as the channel switching will not work if the FET(s) is removed.
Also what is your take on the previous post that I put up regarding the humming when the guitar's volume is up
Thank you

[soundguruman]

I replaced the two input jacks and the problem was fixed, but I hear the AMP humming when a guitar is plugged in and the guitar's volume is up. There is no humming when the guitar's volume is down or when there is nothing connected to the input. When the guitar is playing it sounds normal.
The squilching sound when I turn power off is still there. Should I check Q3 and Q5.

52 Bill
You had mentioned in one of your earlier replies " If they had been removed the channel switching would no longer work." I am not clear what you mean by "the channel switching would no longer work" Could you explain a little more. How would I check if the channel switching works or not.

Thanks

The hum when the guitar is turned up is normal.
It comes from the guitar pickup.
Removing an FET to see if it stops a problem, may stop the channel switching, but you will find out which one is going bad, and replace it.
So you are on the right track.
Certain FETs in these Fender solid state amps can be removed, without causing any loss of functions. They are for muting only. It's usually the muting FET that fails.

[52 Bill]

I mentioned before that Q1 and Q3 serve two functions, mute the audio when there are no plugs in the input jacks and mute the audio when the channels are switched.

On the front panel there is a switch that selects clean or drive channel correct? Does the amp switch between clean and drive? If it does, then the reason the clean channel turns off when you switch is because the fet turns off the signal. If the fet was removed, then the clean signal could not be grounded out when the drive channel is selected and both clean and drive signals would be present at the same time.

The hum and noise you get when the guitar is plugged in generated by the guitar. When you turn down the volume on the guitar it goes away which proves that point.

[Kayjay73]

Hello
I have in my workshop a Fender Ultimate Chorus Amp, which has a very Low Level Hum, but only on the Clean Channel. This happens when You plug an earthed 1/4" Plug into either Input Socket.
Other Forums have suggested changing out all of the Electrolytic Caps in the Power Amps, and the Main Power Supply Filter Capacitors, plus the 2 that filter the Preamp Power Supply Rails.
I have done all of this, to no improvement. If the Dirt Channel is selected - as it does at first Power Up, no problem, but it appears when You Insert the Earthed Input Plug. Power Supply Rails appear
clean, and the 16 Volt Zener Regulators are doing their job. If the Dirt Channel is reselected, the hum then vanishes. Same if you Isolate the Preamps from the Power Amps by plugging in to the Effects Return socket. - The Volume and Tone Controls on the Clean Channel do NOT effect the hum level -- ?
As we all understand, the Dirt Channel has much more gain, yet is quiet --
Has anyone ever come across a problem like this? - as I'm sure that Amp was not released from the makers like this. I am able to post 'Scope Traces if this will help --
At the moment, I cannot see how to post a New Topic --
Thanks, and Kindest Regards
Kayjay73

[Jazz P Bass]

If the Clean controls do not affect the hum then the issue is most probably after the controls.
Not much there.

I would suspect the Clean channel switching jfet Q1.
Or a weak solder connection.

Schematic attached.