Express 5-50 weird problem

[BMcA]

Hello freinds and greetings from Northern Ireland. I purchased an Express 5-50 from a guy from England on Ebay second hand in new condition with flightcase. The amp is only over 7 monnths old and still under warranty. The amp is working brilliant and well pleased with the sound for the music I am playing. The problem that I have discovered is that when I place my finger on the clean channel gain control there is a buzz as if I am earthing something. How can this be possible through a plastic knob I ask myself. If I remove my finger the buzz disappears. If I hold my finger on it and touch any metal parts with my right hand on the amp eg. power switch or standby or even those parts on the top of the amp with screws that hold the chassis in place it disappears also. So to me there is some sort of earthing problem. I have checked all the other controls on the amp and they are perfect. I removed the chassis and had a look at the back of the gain control in question to see if any of the connections were touching the chassis and they are all clear. The plastic knob gets me or are they really plastic. Has anybody heard of this before or could give me any suggestions please of what I am looking for and might be able to fix myself. The last thing that I want to do is to ship this back to England for repair as the carriage is costly. Then the shop in England has to ship it to the distributor in Scotland and so on. Thanking you for your time.

 

[Charles Reeder]

Hey BMcA:

I have encountered this sort of problem before on several amps. Of course I can't say for sure if your problem with the Express has a similar cause; but it is worth checking before you go through the bother and expense of shipping back for repair.

On both a Marshall and a Rivera I owned; the nut behind the knob on the faceplate were just a tiny bit loose (they could be turned easily by hand). As a result; when I touched the knob it pushed the 'pot' backwards to where it was no longer grounded to the back side of the chassis.

Try removing the knob from the front and tightening that nut underneath. It may not help...but it can't hurt.

Of course there's the old joke: Guy goes to the doctor and raises his arm and says..."Hey doc...it hurts when I do this"
The doctor replies...."Don't do that!"

Is the hum that much of an aggravation if it only happens when you adjust the knob? If tightening the nut won't help...is the buzz or hum enough of an issue to send back? Only you can decide that...but I wouldn't send it back if I only heard it for a few briefs seconds while I was adjusting it.

Best of luck with it, Charles

 

[BMcA]

Thank you very much Charles for your quick reply and suggestions. You were 100% correct about the nut. I took the black knob off and checked the nut but couldn't move it by hand. I decided to take the chassis out and then removed the nut from the front. I pulled the pot out and turned the serrated washer back to front and put the pot back in and give the nut a good tighten from the front. Problem solved. Thanks again Charles.

 

[sotto]

Hi:

Was the star washer on the inside or the outside of the chassis?

Here's my experience:

I have an Express 5:25, and this amp also makes a noise when I touch the clean channel gain control. I live near the Hollywood Mesa store, so took the amp in for them to check. They also had another 5:25 in the store, and this amp did exactly the same thing. Anyway, they pulled the chassis from my amp, removed the gain pot from the chassis, cleaned the chassis paint from around the pot hole (inside and out), reinstalled the pot and tightened everything up. I plugged the amp back in and it did exactly the same thing it did before. Since both my amp and the store amp did the same thing, I assumed it was the nature of the beast and accepted it as a "feature" and not a bug.

I'd definitely like to hear from anyone else about a fix for this if it exists.

 

[Rudee]

Hi:

Was the star washer on the inside or the outside of the chassis?

Here's my experience:

I have an Express 5:25, and this amp also makes a noise when I touch the clean channel gain control. I live near the Hollywood Mesa store, so took the amp in for them to check. They also had another 5:25 in the store, and this amp did exactly the same thing. Anyway, they pulled the chassis from my amp, removed the gain pot from the chassis, cleaned the chassis paint from around the pot hole (inside and out), reinstalled the pot and tightened everything up. I plugged the amp back in and it did exactly the same thing it did before. Since both my amp and the store amp did the same thing, I assumed it was the nature of the beast and accepted it as a "feature" and not a bug.

I'd definitely like to hear from anyone else about a fix for this if it exists.

Exact same problem I'm experiencing with a 5:25 I purchased a couple weeks back. I'm not impressed with the quality control of these amps.

 

[creekhed]

I am EXTREMELY pleased with the Construction Quality of my 5:50 1X12

What an amp!

 

[omghi2u]

Yea...my 5:50 never had such a problem. Maybe just the 525.

 

[BMcA]

Hello Sotto.

Sorry I never replied sooner. That star washer is inside the chassis. It sits between the pot and the chassis so when you tighten the nut on the outside it pulls that washer against the chassis to ground the pot. In my opinion there should be no noise when touching any knob. Have you tried all the knobs to see if any of them are making a noise. Try holding the problem one and hold the standby or power switch with your other hand and if the noise goes away it is definitely a ground problem with that pot. If this is the case I would give the nut another little tighten but not that much that you are going to strip it. In my case I gave it a really good tighten up.