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Faulty breath/MIDI circuit

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Faulty breath/MIDI circuit

PostPosted: Wed May 15, 2019 6:41 pm
by BJG145
A while back I picked up a Yamaha BC2 breath controller, and an old MIDI adapter called an Anatek Wind Machine.

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There's something wrong with this adapter though. It sometimes works great, delivering a sensible range of values from the breath controller input. But then it'll suddenly drop out, producing only a very limited range, or no data at all. Cycling the power a few times might bring it back, or change its response.

There's an article here by someone who pulled one apart to fix it, and I was just wondering if anyone could glance at the layout and components and suggest which bit might be failing, to cause this kind of erratic output. (I'm pretty sure it's the box that's faulty and not the BC2 since, as I say, cycling the power can sometimes revive it for a bit.)

I'm hoping someone might be able to glance at this and say, oh, you need a new crystal, or something. (This is a stock photo; I haven't sawn the top off mine yet.)

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Here is a parts list of the ICs and crystal:

U1 - MC68HC05C8FN 8 Bit Microcontroller

U2 - 74HC4538 Dual retriggerable precision monostable multivibrator

U3 - 78L05 5 Volt Regulator

U4 - 78L05 5 Volt Regulator

U5 - 1741C XNBR 741 Op Amp

X1 - 4.00MHz Fox040A CRYSTAL; 4.000000 MHZ; + 30 PPM @ 25 DEGC; 150 OHMS; -20 DEGC; DEGC

Re: Faulty breath/MIDI circuit

PostPosted: Wed May 15, 2019 8:05 pm
by Folderol
First thing to do is look over the board (both sides) with a strong light and a good magnifier to see if there are any crystalline joints, or hairline fractures.

That big cap is a possible suspect - both from the point of view of it's leads having been stressed at some time or if it is quite old.

A squirt of contact cleaner on the switches wouldn't go amiss - also flicking them back and forth a few times.

Bad connections near (or in) the DC voltage input socket could also suppress the control range, but wiggling it would likely prove the point there.

If you have a voltmeter (of course you do!) check the +5 and +10V rails, and also measure the DC input when working correctly, and see if it drops when faulty - that will at least prove whether the breath controller itself is OK.

Re: Faulty breath/MIDI circuit

PostPosted: Wed May 15, 2019 8:17 pm
by BJG145
OK! Thanks Folderol... :thumbup: