Intermittent mid control

[Don]

Recently, I noticed that channel 2 on my Express 5:50 combo sounded lifeless and flat and had less gain, fatness and punch than channel 1!

I suspected pre-amp tubes. Swapping the original tubes around did nothing. Replacing them with a new Tungsol and JJs that I had on hand made the amp sound better overall, though channel 2 was still wimpy compared to channel 1.

I decided to revisit the manual and fool around with the amp settings tonight. While doing this I noticed that 2's mid control did nothing! When I turned the mid control all the way up and the treble and bass all the way down the amp was silent (unlike channel 1 where it still produced sound with these settings)!

I opened up the amp and measured the pot with my DMM. The readings were good. I looked at the cap, it looked good so I put the amp back together figuring that I'd call the tech tomorrow.

The mid control works fine now! Channel two has it gain, fatness and punch back! I haven't heard the amp sound like this in quite a while! I hate it when I "fix" things without "fixing" them!

I think I'm going to open the amp back up tomorrow, spray some cleaner in the pot, check for continuity between the pot, leads and board and re-flow the solder joints at the pot (one of them doesn't look great).

 

[grc911]

Hi Don
Well you obviously touched on something loose and "fixed" it.
Now to see how long your repair lasts!
Healing hands. Well done.

Greg

 

[Don]

Unfortunately, it's not repaired. I've just kept it unreliable. I'll look at it again later.

BTW, I don't want people to get the wrong idea and poke around inside their amps because some guy on the internet said that he did. I've been repairing amps ('50s -'80s Fenders) for many years. I'm not formally educated when it comes to amp repairs- I learned from amp techs, reading books and experience, but one thing that I know is safety. I know how to drain caps, what not to touch and what to touch (and how to touch it).

 

[Don]

The soldering of the lead on channel 2's mid pot wiper was horrible! It looked like a beginner did it! There was a large blob of a cold joint and the wire's insulation was melted slightly. This was done at the factory. I removed this amp from the sealed carton myself back when I bought it a year and a half ago.

I de-soldered and re-soldered the joint, cleaned the pot and tested it. I should be all set. I won't know until I hear the amp tonight.

 

[Don]

It sounds great! This amp hasn't sounded like this in a long time!