[sdiy] EFM MIDI-Gate?

[Dave Kendall]

Hi.

regardless of anything the rest of the circuit is (or isn't) doing, you 
should see a negative-going pulse on pin 6 of the optocoupler when MIDI 
data is sent to it.
I would check the area around the opto, and make sure there are no 
trace errors there on the board.

Try temporarily removing R5, thus disabling the MIDI THRU part of the 
circuit which may have a bearing on the problem, as the 6N138's pin 6 
is also connected to this part of the circuit.

The scope's scan rate will need adjusting to see the neg pulse at pin 
6, as it's pretty fast.


cheers,
Dave


On Feb 25, 2010, at 01:31, Chromatest J. Pantsmaker wrote:

> Ah.. sorry, that was old information and was indeed a problem with the
> scope.  I'm using a really nice scope now and I still can't get the
> circuit to work.
>
> It seems that I'm not getting any output on the 6n138.  I just get 5V
> and it doesn't appear to be pulsing or anything like that.  I've
> compared the schematic with the traces, and everything goes where it's
> supposed to.
>
> On Wed, Feb 24, 2010 at 6:22 PM, Jerry Gray-Eskue 
> <jerryge at cableone.net> wrote:
>>  Pins 5, 6, 7, and 8 all show the same
>>> waveform which doesn't seem right to me at all.
>>
>> According to the schematic You should have +5v on 8 and Ground on 5.
>> Check your power supply, and be suspicious of your o-scope software, 
>> as
>> several people recently pointed out the sound card inputs are usually 
>> AC
>> coupled and so cannot see DC voltages.
>>
>> Use a Voltmeter or DVM (if you have one) to check out the DC voltages.
>>
>> Also the Input side of the opto should have a floating ground 
>> relative to
>> the rest of the circuit.
>>
>> -----Original Message-----
>> From: synth-diy-bounces at dropmix.xs4all.nl
>> [mailto:synth-diy-bounces at dropmix.xs4all.nl]On Behalf Of Chromatest J.
>> Pantsmaker
>> Sent: Wednesday, February 24, 2010 5:52 PM
>> To: Dave Kendall
>> Cc: Synth DIY
>> Subject: Re: [sdiy] EFM MIDI-Gate?
>>
>>
>> I'm still working on this thing, haven't figured out what's wrong yet.
>>
>> I bought a stack of 10 6n138 optocouplers from mouser.  I've tried
>> several now with the same results each time.  I'm wearing a ground
>> strap.
>>
>> So, using the scope, I'm seeing 5V data bouncing on the inputs of the
>> 6n138 (pins 2 and 3) but on the output, I'm just getting a slightly
>> fuzzy +V and the scope says that it's detecting DC voltage on that
>> pin.
>>
>> I've pulled out every other IC (all socketed) and the results are the 
>> same.
>>
>> Thoughts?
>>
>> The Schematic is here:
>> http://chromatest.net/EFMmidi-gate.pdf
>>
>
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