Channel Switching Issue

[Phano36]

I will like to know if someone else have the same issue with their MA 100C ?

I have sometime, a very lound POP when switching from my Drive channel to the Clean (always from drive to clean and never from clean to drive). I bought this amp this summer and this issue started during the first week.
I brought it back to the store and they send it back to the distributor for a check and nothing was wrong (they said). I have other Amps and I know that a small Pop is normal when switching channel but a loud Pop like the one i'm experiencing can possible one day blow one of my speakers.

I'm still not convince, this situation is normal. Someone have a sugestion ?
Regards

[SteveD]

Oops, almost missed this one.

Welcome to the forum Phano36.

There is a circuit which creates a 'mute window' that operates a 'J174' switching fet (TR45). If that was faulty then it would not mute.

As for only happening when switching fron Drive to Clean, maybe component tolerances could cause that but it would have been flagged during final sonic and functional testing.

Is there another MA you can compare it with?

[Phano36]

Hey Steeve !

Yeah, I tried some other Amps and they didn't seem to have the same issue. Unfortunately, my problem is intermitent and like when you have a bad tooth pain and you decide to go to the dentist, it stop as soon as you arrive in the cabinet !

I tried to reproduce the issue at the store but it didn't happen in front of them. Anyway, they sent it back to the distributor for a check ( in Montreal Canada) and the tech there didn't found nothing wrong.
They think i'm probably crazy now.

So now I mute my strings when I switch channel to prevent my speakers to get damage. Let say i'm a little desapointed. Also maybe i'm crazy a bit but when this BIG POP happen it changes the sound of the amp.

I have 3 other amps, all tubes... and I wanted a Marshall to replace the first amp that I had (Marshall white serie that I sold )26 years ago. I'm sure do not want to quit this MA of mine but..

[KevinOConnor]

Hi Guys

Hey Phano36: Check if the pop occurs when the boost is not selected, or if it makes a difference.

The jfet referred to by Steve is normally-off. Two comparators are set up as a window detectors, where upper and lower voltages are ignored but a signal within the window causes one or the other comparator to turn the jfet on.The comparators are looking for a pulse from one of two capacitors.

One capacitor tells the circuit when the boost is switched; the other tells the circuit when the channels are switched; both of these connections are asymmetrical inasmuch as only a change in one direction is detected. The caps are tied directly together at the comparator inputs which is not generally a good way to achieve economy of use of the comparators, so the charges stored in the caps can distribute itself between the caps which alters the effective size of signal passed by each.

Variability of the circuit performance can be ascribed to component tolerances, leakage across the board through parasitic capacitance and/or a dirty PCB surface, poor solder connections, or an actual defective component in the comparator circuit. The output muting jfet is not the prime suspect.

A service tech can scope the comparator input to see if there is a pulse at both transitions, or at least at the problematic one.

Have fun
Kevin O'Connor

[SteveD]

... both of these connections are asymmetrical inasmuch as only a change in one direction is detected.

A change in either direction triggers the window comparator.

[Phano36]

maybe component tolerances could cause that but it would have been flagged during final sonic and functional testing.

From my personnal experience in Quality Assurance (20 years for a big Aerospace cie) I know that Inspection and FTP are not always done as per Eng. requirements.

Since I still have this anoying issue, and also because my Local Marshall Service center in Montreal Canada doesn't seen to have a Clue of the existance and the cause of the actual issue (Note that my amp is still under waranty) and that they didn't help me at all, they even charged me for the transport (UPS) without fixing my problem. Would it be possible to Marshall to submit me the spec of the J174 so I can do it myself ? Another sugestion would be that Marshall submit an SB to their authorized distributors & Repair Centers about this potentiel issue. So that the final costumers like me, pass for peoples that are complaining for nothing.

[SteveD]

Sorry you didn't get your problem properly resolved. The J174 is just a standard fet, nothing special.

[Phano36]

Hey Steve, thank for the inputs. I've today today on the net and found some J174.
So i will perform the operation myself. Never better serve but by yourself .... I'll give you guys the result of the operation later : )

[txslowhand]

Yep, I have the same problem on my brand new MA50H. I get a pop when I go from the boost/OD channel back to the clean channel. Doesn't seem to be as bad when the volume on the OD channel is turned down. I can also hear the reverb tank when I switch from OD to clean. It's like someone taps/bumps the tank when I switch from OD to clean. Kinda annoying. Still really like the MA50H though. Glad I got it.

[Phano36]

txslowhand

Yeah, I also ear that reverb tank noise like you. I didn't have the time to perform the repair on the J-Fet (TR45) or at less inspect the soldering before attenting to replace it.

Flow soldering doesn't always give good results and at less, you need to have a good Quality Control to spot the issues at final inspection. Believe me, I know what i'm talking about I've been in the aerospace electrical & electronic industry for 20 years now and I've seen this kind of issue many time in New producing countries. Because even do of all efforts you make money saving doesn't replace experience and skill workers. Usualy it takes 5 years of production to gain enought experience to reduce the Quality rejections but you have to master a very good Quality follow up Processes. Ahhhh enough of that crap lolll It seem that I`m in work now !

Anyway I have some gigs comming so I leave the amp the way it is for now but I plan to take a look inside of it by the end of June.
I should take a couple of pictures and publish them if the repair work.
Ciao

[txslowhand]

txslowhand

Yeah, I also ear that reverb tank noise like you. I didn't have the time to perform the repair on the J-Fet (TR45) or at less inspect the soldering before attenting to replace it.

Flow soldering doesn't always give good results and at less, you need to have a good Quality Control to spot the issues at final inspection. Believe me, I know what i'm talking about I've been in the aerospace electrical & electronic industry for 20 years now and I've seen this kind of issue many time in New producing countries. Because even do of all efforts you make money saving doesn't replace experience and skill workers. Usualy it takes 5 years of production to gain enought experience to reduce the Quality rejections but you have to master a very good Quality follow up Processes. Ahhhh enough of that crap lolll It seem that I`m in work now !

Anyway I have some gigs comming so I leave the amp the way it is for now but I plan to take a look inside of it by the end of June.
I should take a couple of pictures and publish them if the repair work.
Ciao

Thanks Phano36. Let me/us know if you have any luck with the repair.