Fender Ultimate Chorus chassis removal

[geofo]

How do you remove the chassis so you can troubleshoot the amp. Besides the top and side screws do you have to remove all the knobs or they come out with the chassis.
I get a poping noise when turning the power switch off and there is no output. Also a low hum which is effected by changing the reverb knob, but it is not loud.
Thank you

[soundguruman]

Look underneath, in the rear, there are more screws holding the chassis to the top plate of the cabinet.
You have to see them upside down.

[52 Bill]

The knobs do not need to be removed to get the chassis out. There are usually 4 screws on the top at the corners and 2 side screws, one on each side of the case. The chassis will sometimes get almost glued in from assembly and it will also get caught on the self adhesive foil that lines the inside of the cabinet.

One trick that I use sometimes is to use one of those squeeze clamps that can be reversed to spread instead of squeeze. I use it on the back of the cabinet to push the two sides apart so that the chassis can be removed easier.

[geofo]

Soundguruman and 52 Bill,

Thank you for the tips I will try it tonight and will be back with whatever the problem it is with the Amp.
Geofo

[geofo]

Got the chassis out, thanks for your help. The Amp was putting out a hum which would get worse when the volume was turned up. I removed the board to check solder joints and saw that one of the two input jacks was lose. I soldered all the pins but the one pin that is by itself was soldered but it was broken off from the inside of the input jack. So even though I soldered all the pins there was still humming and moving or pressing on the jack it was effecting the hummimg. So I ordered new input jacks. By the way the input jacks had no nuts holding them on the chassis so I imagine that pluging or pulling the cable was applying force on the jack which finally broke lose from the board. The other input jack was not lose but being it didn't have a nut either I will replace that as well. I think the one pin that is by itself on the jack is the ground pin but I will double check that. I hope after replacing the jacks the problem will go away.

When I shut the Amp Off it makes a popping/squilching noise (not very loud). Is the popping noise normal when you shut off the Ultimate Chorus Amp (model PR204)? If not, do you have any suggestions?
Thanks again.

[soundguruman]

Got the chassis out, thanks for your help. The Amp was putting out a hum which would get worse when the volume was turned up. I removed the board to check solder joints and saw that one of the two input jacks was lose. I soldered all the pins but the one pin that is by itself was soldered but it was broken off from the inside of the input jack. So even though I soldered all the pins there was still humming and moving or pressing on the jack it was effecting the hummimg. So I ordered new input jacks. By the way the input jacks had no nuts holding them on the chassis so I imagine that pluging or pulling the cable was applying force on the jack which finally broke lose from the board. The other input jack was not lose but being it didn't have a nut either I will replace that as well. I think the one pin that is by itself on the jack is the ground pin but I will double check that. I hope after replacing the jacks the problem will go away.

When I shut the Amp Off it makes a popping/squilching noise (not very loud). Is the popping noise normal when you shut off the Ultimate Chorus Amp (model PR204)? If not, do you have any suggestions?
Thanks again.

Yeah, those jacks break all the time. I usually just change them all, as long as the board is out.
The jacks have internal contacts that fail from oxide or abuse. Could easily be a bad jack. And another thing is that the control solder connections crack too.
The popping noise is supposed to be muted by an FET, but it fails quite frequently, and in a lot of amps, it's just been removed altogether.
The FET fails and mutes the amp 100% of the time. So, many people just take it out and leave it out.

[geofo]

Yeah, those jacks break all the time. I usually just change them all, as long as the board is out.
The jacks have internal contacts that fail from oxide or abuse. Could easily be a bad jack. And another thing is that the control solder connections crack too.
The popping noise is supposed to be muted by an FET, but it fails quite frequently, and in a lot of amps, it's just been removed altogether.
The FET fails and mutes the amp 100% of the time. So, many people just take it out and leave it out.

Soundguruman,

Thanks for your response. Would you know which FET it is that you are refering to? what Q# (number) if possible or where it is located. I guess if I know the Q# I can find it on the schematic and then on the board. Also if I chose to do nothing about it, is the popping sound going to effect anything else, or it has to be fixed?
Thanks.

[soundguruman]

Soundguruman,

Thanks for your response. Would you know which FET it is that you are refering to? what Q# (number) if possible or where it is located. I guess if I know the Q# I can find it on the schematic and then on the board. Also if I chose to do nothing about it, is the popping sound going to effect anything else, or it has to be fixed?
Thanks.

I have to dig up the schematic to see which Q it is.
The FET mute has been an issue on many Fender amps. The factory techs used to just cut them out. (I don't know what they do these days)
The pop may be caused by an FET that is failing. Or if it goes bad, the amp stops working, it's muted all the time.
Might be Q6, It's been a while...I remember it is a J111 FET. That's why I have a bag full of them. hahahahahahahh!
Two more things I remember:
The reverb pan may have broken wires or springs inside. Take out the reverb pan and check everything. Check the coils for open circuit.
When you reassemble, coat the heatsink and chassis with more heat sink paste. The factory did not use enough to get the air gaps out.
Be SURE to clean all the controls, and Jacks thoroughly. Use something like Caig Deoxit D 100 or Caig Fader Lube.

Funny, but when you get passed the plastic jacks, FET problem, this is a really great sounding amp. And they seem to work forever...just about.
I hate them because they are such a PITA to disassemble. But I love them because they sound good.

[geofo]

I have to dig up the schematic to see which Q it is.
The FET mute has been an issue on many Fender amps. The factory techs used to just cut them out. (I don't know what they do these days)
The pop may be caused by an FET that is failing. Or if it goes bad, the amp stops working, it's muted all the time.
Might be Q6, It's been a while...I remember it is a J111 FET. That's why I have a bag full of them. hahahahahahahh!
Two more things I remember:
The reverb pan may have broken wires or springs inside. Take out the reverb pan and check everything. Check the coils for open circuit.
When you reassemble, coat the heatsink and chassis with more heat sink paste. The factory did not use enough to get the air gaps out.
Be SURE to clean all the controls, and Jacks thoroughly. Use something like Caig Deoxit D 100 or Caig Fader Lube.

Funny, but when you get passed the plastic jacks, FET problem, this is a really great sounding amp. And they seem to work forever...just about.
I hate them because they are such a PITA to disassemble. But I love them because they sound good.

Soundguruman,

Thanks again for your prompt response.
I noticed there are a number of J111 FETs on the schematic but I am not going to touch any unless I am 100% sure.
As far as the reverb pan, when I replace the input jacks, if there are still issues I will open the reverb pan to check it out. I have read stories about coil(s) opening up or needing soldering so I will keep it in mind if the humming does not go away. I guess if the humming is there only with the reverb pan connected, that would mean that something is going on inside the reverb pan so I will see how it behaves. I am a believer of "if it is not broken don't touch it"
I guess it doesn't hurt to clean all the controls, I just have to get the cleaning spray you have recommended. I haven't done many AMP repairs so I am kind of new to this but I am an Electrical Engineer and I understand a lot of things. Having said that, theory and practice are two entirely different things. I used to troubleshoot and fix car radios many years back but I stopped. I like the troubleshooting part of this whole thing. I always find it very ineteresting and a great learning experience. This is the only way to learn.
I will keep you posted on my progress

[52 Bill]

You didn't tell us which model UC you have with or without DSP.

The non-DSP version uses fets to mute and to channel switch. The two fets involved with the mute are Q3 and Q5. Both of these are dual purposed in that they mute the signal when there is no plug in the input jacks and they are part of the channel switching circuit. If they had been removed the channel switching would no longer work.

Does the channel switching work okay?

[geofo]

You didn't tell us which model UC you have with or without DSP.

The non-DSP version uses fets to mute and to channel switch. The two fets involved with the mute are Q3 and Q5. Both of these are dual purposed in that they mute the signal when there is no plug in the input jacks and they are part of the channel switching circuit. If they had been removed the channel switching would no longer work.

Does the channel switching work okay?

52 Bill

I am sorry I wasn't very specific, it is not the DSP. I have to apologize because I may be an Electrical Engineer but I am new to troubleshooting Amps so I am not familiar with the lingo. Like I said before, theory and practice are two different worlds. When you say "Does the channel switching work okay?" what exactly are you talking about, what are you referring to.
I found Q3 and Q5 and there is a signal name "INH" at the gate and another signal name "SELA"; I also found "SELB". What do these mean, is it "INHIBIT", SELCTOR A" and SELECTOR B" or something else. If they are, what part of the Amp do they refer. I found the INH signal to come off the Input Jack 1 and the SELA and SELB off the Footswitch circuit (U10 pins 1 & 7). Could you explain their function and how they work?

I appreciate it
Thank you

[52 Bill]

I found Q3 and Q5 and there is a signal name "INH" at the gate and another signal name "SELA"; I also found "SELB". What do these mean, is it "INHIBIT", SELCTOR A" and SELECTOR B" or something else. If they are, what part of the Amp do they refer. I found the INH signal to come off the Input Jack 1 and the SELA and SELB off the Footswitch circuit (U10 pins 1 & 7). Could you explain their function and how they work?

I appreciate it
Thank you

Yes, the INH circuit inhibits or mutes the amp audio signal when there is no plug inserted into either of the input jacks. Q3 and Q5 are turned on by the INH voltage, which makes them act as a high resistance to the audio signal that they each carry.

These two field effect transistors (fets) are also connected to the SEL A buss, which select the clean or the drive channels of the preamp.

Notice that Q1 and Q3 are connected to SEL A buss as well as to the INH buss. When the A channel is selected Q1 turns off and un-grounds the audio signal coming from the clean channel at the junction of R20 and R21. Q3 turns on and increases in resistance to block the audio signal coming from the distortion channel at R52. When the B channel is selected, the opposite happens and the clean signal is shunted to ground by Q1 and the distortion signal is allowed to pass through Q3.

The same basic action happens at Q4 and Q5, but there the selection is for the two reverb circuits.

SEL B buss controls the chorus on/off function.

[geofo]

Yes, the INH circuit inhibits or mutes the amp audio signal when there is no plug inserted into either of the input jacks. Q3 and Q5 are turned on by the INH voltage, which makes them act as a high resistance to the audio signal that they each carry.

These two field effect transistors (fets) are also connected to the SEL A buss, which select the clean or the drive channels of the preamp.

Notice that Q1 and Q3 are connected to SEL A buss as well as to the INH buss. When the A channel is selected Q1 turns off and un-grounds the audio signal coming from the clean channel at the junction of R20 and R21. Q3 turns on and increases in resistance to block the audio signal coming from the distortion channel at R52. When the B channel is selected, the opposite happens and the clean signal is shunted to ground by Q1 and the distortion signal is allowed to pass through Q3.

The same basic action happens at Q4 and Q5, but there the selection is for the two reverb circuits.

SEL B buss controls the chorus on/off function.

52 Bill,

Thanks for your super fast response, I appreciate your explanation. Things start to make sense now, it is like connecting the dots.

Thanks again

[soundguruman]

Just remove the FET. If you found the bad one, the problems stops.

[geofo]

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