Shure SE30 - no compression

I have a otherwise clean and working Shure SE30 mixer/compressor.  But there is currently no compression function.  All other functions are operating as expected. 

There is no gain reduction displayed on the meter, and there is certainly no audible compression.  (Which for this beast, would not be something you could miss!)

Should I suspect the IC 201?  (it's a RCA ca3050).  Is this a common fault that anyone has had experience troubleshooting?

Very good documentation and schematics here:

http://pubs.shure.com/guide/SE30/en-US.pdf

You should test the circuit step by step.  Is there something you can measure before and after the driver transformer T5?

chops said:

Should I suspect the IC 201?  (it's a RCA ca3050). 

Click to expand...

I wouldn't....... at first anyhow.

Have any capacitors ever been replaced in this?

The crimps on the boards interconnecting wires also cause problems. Best fix is to just solder them at the crimp point, then make sure the push on part is nice and tight.

Thanks, folks. The crimp on wires all seem to be making good contact.  No change when pressure is applied, at least one at a time...  (I suppose if there were multiple bad contacts, this method of testing wouldn't be helpful.)

I fed the unit a sign wave and can follow it up to the input of q4, but not beyond. (Just with a dmm, not a scope.).  This is very crude, I realize, and my understanding of the circuit is less than complete.  But does this mean q4 is potentiallythe culprit? 

I haven't seen Q4 in the schematics, which one is it?

Maybe he means Q401?????

How's C403????

And to think I was suspect of C203....

Sorry.  I meant Q404. I still see signal after q401 and q402.

chops said:

Sorry.  I meant Q404. I still see signal after q401 and q402.

Click to expand...

Where are you probing????

Also, have you checked out that voltage table in the manual?? It has a lot of good test points......

Thanks.  I'll definitely get set up and test the points referenced  on that table included in the manual.  Will report back with any wild deviances from the expected measurements.

Thanks again.   

Don't trust the eleco's. Just repaired one and that was the problem.

tommia said:

Don't trust the eleco's. Just repaired one and that was the problem.

Click to expand...

Do you mean the electrolytic caps?  (I'm not hip to that particular slang - "eleco's".)

If that's what you mean, did you replace all electrolytic caps on all boards?  I suppose that something that should be done anyway... 

I've ordered the caps for  c201, c202, c203, and c206.  And transistor q203.

My earlier assumption that q 404 may be bad was incorrect.  I can still
see my sine wave on the scope at the input of the rectifier at diodes d201 and d202, so I think that means all the transistors on board 4 must be fine. (According to my understanding of the block diagram. )

Things seem to fall apart around q203, and I can't see my sinewave heading into the IC M201 or at either leg of c203.  The "gated memory" circuit is being triggered when signal crosses a threshold, but that's as far as it gets.  No gain reduction happens. 

Thanks again. This place is amazing...

Do you mean the electrolytic caps?  (I'm not hip to that particular slang - "eleco's".)

Click to expand...

Yes, sorry typo
I replaced the ones who were in the audio path. A lot of them measured very bad. I've never seen that so bad.

Success.  Replaced q203 as well as c201 and c202.  And now it sounds like the crappy compressor I was hoping for!

Now I need to figure out how to shorten the release time?  (Below  the minimum setting on the pot R5.)

Thanks for the help!

chops said:

Success. Replaced q203 as well as c201 and c202. And now it sounds like the crappy compressor I was hoping for!

Now I need to figure out how to shorten the release time? (Below the minimum setting on the pot R5.)

Thanks for the help!

Click to expand...

Four years later .. EDIT: I have one that has a lot of hiss and low volume on mic input, loads of clean gain on line input, and loads of progressive distortion when the inputs are turned up but no compression as it's supposed to sound - the response rate dial doesn't do anything. Comp. is not disabled on the rear panel! Also only the bulb works on the meter, the needle doesn't move on either setting. Any pointers other than the bin?

Download, and read the manual. https://pubs.shure.com/guide/SE30/en-US.pdf
It has all the voltages of the circuit. Go through the alignment procedure and see where it
doesn't align.