pvision

Shure M267 compressor not functioning
« on: March 31, 2017, 07:58:26 PM »
I have a couple of Shure M267s where the compressor doesn't work. The units passes signal with the compressor on but the threshold control does nothing. It seems as though nothing is happening at all when the compressor is switched in

I haven't done any fault-finding yet but it struck me that a potential point of failure is the 86A8908 optocoupler which links the sidechain to an opamp in the output circuitry

The Shure parts list states Commercial Alternative: None for the part

Is anyone here familiar with the 86A8908 opto isolator?

Schematic
http://cdn.shure.com/user_guide/upload/955/us_pro_m267_english_ug.pdf


Nick Froome


Walrus

Re: Shure M267 compressor not functioning
« Reply #1 on: April 01, 2017, 05:36:36 AM »
I vaguely remember them from my days at Shure, but I never opened one up then. Think they were similar to Vactrols.
I have an M267 at work, I'll crack open th lid and have a look when I'm back in Monday.
Kevin.

pvision

« Last Edit: April 01, 2017, 09:07:12 AM by pvision »

Walrus

Re: Shure M267 compressor not functioning
« Reply #3 on: April 01, 2017, 12:50:49 PM »
OK, I don't need to open mine up then.  :)
A bit of detective work and I was reminded that GDIY'er rp3703  used an old Shure AMS8000 automatic mic mixer to make an API pre  https://groupdiy.com/index.php?topic=62036.msg786983#msg786983 and I see that one of the pictures shows the opto it used and is marked HTV so may be the same one.
Hopefully when he stripped the AMS8000 boards of components, he may have kept them.....
Kevin.

pvision

Re: Shure M267 compressor not functioning
« Reply #4 on: April 01, 2017, 08:31:40 PM »
OK, I don't need to open mine up then.  :)

Yes - I concluded it was a bit lazy of me to post the question without much supporting info!

The failure rate for these is about 1 in 5 which is a bit high for me to support. I'll do some testing of the bad one i have to see if I can track down the fault. I'll check the voltage at the input side of the opto then lift the output side and put a meter across it. That should tell me if it's good or bad

It looks like audio is taken to a bridge rectifier which feeds the input of the opto and the output controls the feedback round the output opamp.

It compresses audio in a wonderfully brutal way!

Nick Froome

PRR

Re: Shure M267 compressor not functioning
« Reply #5 on: April 01, 2017, 11:43:42 PM »
> potential point of failure is the 86A8908 optocoupler

Presumably EVERY part is a potential point of failure; else why is it there?

Poke the DC voltages around the rectifier and opto. Looks like the plan is single +30V supply and U205A makes a 15.7V reference. All around the opto should be this 15.7.

Lift one leg of the LED side of the opto, tack in a good LED, slam signal. Does it light? If not, turn it around. Still no light? Trace through rectifier and U205B, limiter switch, see if signal gets through centered on 15.7V.

Tack a 100K pot across the opto photo-R side and trim it. Does gain go away?

If it is truly blown opto, there' s no ideal fix. They picked the photo-R for specific time-constants, not commonly found in oil-burner or dusk-dawn light electric eyes. LA2a type photo-Rs may work, but are not cheap. (The LED is trivial.)