Fender Ultimate Chorus hum on clean channel.

[Daddykool]

Greetings from a new member! I am fixing one of these amps for a friend. It hums enough on the clean channel to be annoying. The hum goes away on the drive channel. I have been working on it for days and have searched forums without finding exactly my problem or the solution to it. I have replaced the main power supply caps as they looked old. I have replaced both the input sockets and smoothing caps for the 16V supply. I replaced the U1 IC and about 7 small electrolytic caps. I've also checked for dry joints all over the PC board, resoldering many poor joints and checked and replaced bridges where tracks were previously bridged. Being a 1990s solid state amp, it's been on the operating table many times. Many diodes have been snipped and resoldered for checking from the top of the board. I have checked continuity of repaired parts of the board. There is a 50 cycle wave on the output transistors, which disappears when switching to the drive channel. I have the schematic and theory of operation, which I got from the Fender agents. There seem to be many possible causes of this problem. I'm running out of ideas and would be extremely grateful for any help. I've spent many hours with my meter and scope and cannot seem to pinpoint the source of the problem. It sounds as if there is a lead plugged into the input, but not connected to a guitar, that is an open input causing the hum. Thanks in advance!

[DrGonz78]

One troubleshooting idea comes to mind. Follow the signal path in the clean channel and short various pathways to ground. The idea is that you are trying to determine where the hum might be amplifying in the circuit. So this might help determine exactly what part of that circuit that affects the hum.

[g1]

The drive channel works completely ok? Even set as clean as possible (vol. full, gain minimum) ?
If so, attached image shows the only parts exclusive to the clean channel.
I would start by checking all the pots nuts are tight and are all stock types as pot brackets are sometimes used to complete ground circuit.

[Daddykool]

The drive channel works completely ok? Even set as clean as possible (vol. full, gain minimum) ?
If so, attached image shows the only parts exclusive to the clean channel.
I would start by checking all the pots nuts are tight and are all stock types as pot brackets are sometimes used to complete ground circuit.

Hi g1, thanks for the great suggestions. I had already checked or replaced most parts in this section and have now replaced the remaining caps which were hard to check. All the control pots seem good. There is no sign of the hum signal on the output of the IC, which I have replaced to make sure it didn't have an undetectable fault. I've been all over the board with my scope and the hum doesn't appear on any of the IC outputs, although there is some on pin 6 of U10B, in the channel switching section. I wonder if it could be something as simple as a faulty channel selector switch. Have you worked on one of these amps? I wouldn't have chosen to, but am doing it as a favour for a friend, which has led to several days' work with no successful outcome. I'm open to more suggestions. Thanks.

[g1]

Apologies if you know this, but I want to be clear in case you misunderstood.
You said the pots seem good. I'm not concerned about their control function right now, but how they are used for grounding.
Chassis must be fully assembled and all pot nuts tight or you can have ground/hum issues.
Also, the pots all have brackets, or 'wings' on them. Those brackets are soldered in and some go to ground traces and are used to make ground connections. If the solder is not good you can get hum. If someone replaces them with the type that do not have the 'wings', you can get hum.

If you look at the foil side of the board, you can follow the trace along and see how the brackets are sometimes used as jumpers.
The switch can not cause the hum as it is just triggering the switching circuit. (U10 area)

[Daddykool]

Apologies if you know this, but I want to be clear in case you misunderstood.
You said the pots seem good. I'm not concerned about their control function right now, but how they are used for grounding.
Chassis must be fully assembled and all pot nuts tight or you can have ground/hum issues.
Also, the pots all have brackets, or 'wings' on them. Those brackets are soldered in and some go to ground traces and are used to make ground connections. If the solder is not good you can get hum. If someone replaces them with the type that do not have the 'wings', you can get hum.

If you look at the foil side of the board, you can follow the trace along and see how the brackets are sometimes used as jumpers.
The switch can not cause the hum as it is just triggering the switching circuit. (U10 area)

Hi, sorry I wasn't more specific, but I did mean all the pots have a good earth connection. That's one of the first things I made sure of, after resoldering many solder joints, including the ones on the pots. I realised that the switch only sends a current to trigger the switching transistors after writing about the possibility of a faulty switch. My next areas of investigation are the channel switching area and the effects loops. Thanks.

[Daddykool]

Hi guys. I'm still baffled by the symptoms this amp is displaying. It hums on clean, but not on overdrive. The input lines to the power amp section do not change when switching from clean to dirty. They are clean and hum-free. It's hard to understand why the hum comes and goes with channel switching, when the signal to the power amp does not change. On clean, there is about 1V p-p on the power tranny bases, on dirty, only 0.15V p-p. Any ideas?

[Daddykool]

The voltages are hum....

[g1]

If you turn the tone controls on clean ch. all the way down, does it affect the hum? How about the clean volume control, can it kill the hum?

[Daddykool]

If you turn the tone controls on clean ch. all the way down, does it affect the hum? How about the clean volume control, can it kill the hum?

No, none of the tone controls or volume control have any effect on the hum. I'm really looking for far-fetched answers to this problem. It looks like hum-free signals go to the left and right power amp sections, whether on the clean or dirty channel. It seems to defy logic that when the clean channel is selected, the power amps are putting out hum, so I am considering all sorts of possible explanations. Is there any known history of these PCBs becoming partly conductive, as I have experienced in valve guitar amps, of course with higher voltages more likely to cause carbon tracking or whatever. It's a head scratcher...

[g1]

It looks like hum-free signals go to the left and right power amp sections, whether on the clean or dirty channel.

So the hum is not coming into the power amp inputs? You should verify this by putting a stereo shorting plug into the FX Return jack.
With plug inserted, if the hum is still there only with clean ch. selected, then yes, time to look for oddball causes.

[Daddykool]

So the hum is not coming into the power amp inputs? You should verify this by putting a stereo shorting plug into the FX Return jack.
With plug inserted, if the hum is still there only with clean ch. selected, then yes, time to look for oddball causes.

Yes, well the hum does disappear with a plug in the loop return. I worked my way back to the mono effects loop and via U4 to the junction of R20 and R21. If I earth this point, the hum goes away. I guess if I pull out Q1, it will show whether the hum originates from the U1B area or from Q1 itself?

[DrGonz78]

What happens when you remove the reverb tank?

Edit: Sorry I read wrong on the schematic. Now I see you traced back further in the circuit to R20-21.

[Daddykool]

What happens when you remove the reverb tank?

Edit: Sorry I read wrong on the schematic. Now I see you traced back further in the circuit to R20-21.

Thanks for trying to help. I did forget to plug the leads into the tank one time and it made a huge hum.

[g1]

I guess if I pull out Q1, it will show whether the hum originates from the U1B area or from Q1 itself?

Yes try this.
If the hum is still there (with Q1 removed), it must be in the area ahead of Q1, shown in post #4. That is the only part of the circuit not shared with the drive channel.