Tech Question on Bad Mic/Line Input on Seck 1282 Console...

[MarkGifford-1]

Hi, could anyone offer an opinion on what component may be causing this issue?

 

I'm trying to fix this myself, as repair by a pro would cost more than the mixer is worth. I'm an OK solderer - I've done some component level repair before, just don't have the knowledge to troubleshoot this kind of stuff.

 

Thanks in advance!

 

- Input on Mic/Line is very weak and distorted

 

- Tape input is fine, which would lead me to believe that it's localized to the area of the schematic I've uploaded.

[agedhorse]

The first thing I would look at is the insert jack bypass switch. Try a 1/4" TRS bypass plug with tip and ring tied together.

 

You would really need to use a scope and signal generator to go any farther.

[Pro Sound Guy]

Are both Mic and Line distorted?

 

If this is so then suspect Mic/Line switch or further on down the road Op Amp circuit or beyond.

 

If your OP Amp is in a socket then reseat it.

 

Otherwise as Aged posted you need to inject a signal and use a scope.

 

 

 

[MarkGifford-1]

Are both Mic and Line distorted?

 

If this is so then suspect Mic/Line switch or further on down the road Op Amp circuit or beyond.

 

If your OP Amp is in a socket then reseat it.

 

Otherwise as Aged posted you need to inject a signal and use a scope.

 

 

 

Switches and Insert jack are OK - I cleaned them.

 

Mic/Line are both low level, but sound OK and gradually get distorted as I get towards the top of the Input Gain level - just like if it was functioning properly and getting the gain set too high. It's just way down in level. Would this point to the opamp being fine, especially as the other half is working as it should?

 

Tape input is fine - plenty of level and functions as it should.

 

Would this place the problem after the output 7 on the opamp, and before the mic/line - tape selector switch?

 

I've edited the photo with a red box in that section - there's not much there, I could probably change out those 4 components and see if it worked...

 

Thanks again!

 

MG

[agedhorse]

Impossible to say, could be something hanging up in the pad awitch, a failing coupling cap, bad op-amp (though I doubt that), etc.

[wesg]

Op-amps rarely fail in my experience, it always seems to be something around the op-amp that makes it behave incorrectly...switches, jacks, connectors, passive components, cracked PCBs, poor soldering, orange juice...

 

I'm strictly an amateur here, but it's likely I could fix this with this time-honoured technique: inject a signal into two channels. One channel where it sounds good, and one channel where it doesn't. Follow the signals through the channels with a 'scope until you find that it's different. Puzzle out why it's different by comparing lots of related things between the two channels until you know what bit is bad. Then repair or replace that bit.

 

If you can't troubleshoot the problem, then you could replace random components until you're blue in the face and still not figure it out. I know this, because I have turned my face blue many times. If you don't have a scope, check your local Craigslist etc. You'd be surprised at how inexpensive used decent-for-audio scopes have become. My test gear is mostly from the 1970s or 1980s, but audio still works the same as it did back then. You can use a computer as a signal generator for this application in a pinch.

 

Wes