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phones out sometimes kills audio, pitch-CV? wot?
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phones out sometimes kills audio, pitch-CV? wot?

by controlvoltage on Thu Jan 18, 2007 12:55 am

So I have been doing my head in trying to fix this x0x... sometimes when it is humming along doing its thing, there will be a >cronk< sound and the audio will go dead. Unplugging/replugging the headphones often brings it back, or knocks it out again if it's working.

When it is "out," the sequencer is still running and the gate-CV is still working normally, but pitch-CV falls to a steady ~2V. When it kicks back on (due somehow to plugging/unplugging the phones, although nothing is physically loose on the jackboard), the pitch-CV immediately returns to normal.

I am using a pair of large, low-impedence headphones, Sennheiser HD280 pro (68 Ohm). Could they be pulling too much current thru the headphone output, perhaps causing something else to sag or latch up? If so, what might it be?

I don't know what is the root cause of the failure, but I am going to try increasing the limiting resistors on the headphone output to see if it helps. Any insight into this one would be sincerely welcome.

Cheers!
-=trimmer tweeker=-

controlvoltage
 
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by controlvoltage on Thu Jan 18, 2007 1:50 pm

Well increasing the headphone output resistors from 10 Ohm to 100 Ohm seemed to make no difference... I will try 330 Ohm next but this is leading me to doubt my "too much current draw to the headphones" theory. Once again I was just listening to a sequence, not even touching anything and it crumped out. Unplugging/replugging any jack brought it back. I have looked for questionable soldering in the audio path and not found any... I replaced all of the flimsy ribbon cables with nice insulated stranded wire of my own, so all of the IOboard connections to the mainboard are solid. Something wacky is afoot. Any guesses? I have plenty of measuring equipment but a poor idea of what to look for in this case.
-=trimmer tweeker=-

controlvoltage
 
Posts: 242
Joined: Tue Aug 09, 2005 12:17 am
Location: Between 0 and +5V

by controlvoltage on Fri Jan 19, 2007 1:21 am

OK it seems I was mistaken about the powersupply voltages all being OK, or it had come back to life while I was measuring them previously. I took more careful measurements and found that the problem is intermittent power.

When it goes down, the +12V drops to about 2.4V and the +5.333V goes to zero. Tracing back, I see about +24VDC at the junction of R2 and R1, but only about +2.5V on the other side of R1 (common to IC23-8) where there is normally about 22v. Poking at the R2-R1 side of R1 seems to cause it to stumble or latch up.

I'll doublecheck the solder to everything... that's all I can figure at the moment. If anyone else has any bright ideas or wild guesses, I could use some help.
-=trimmer tweeker=-

controlvoltage
 
Posts: 242
Joined: Tue Aug 09, 2005 12:17 am
Location: Between 0 and +5V

by controlvoltage on Fri Jan 19, 2007 1:37 am

verdict seems to be cold solder joint on R1. Relief/annoying as I didn't stuff that board myself... I was worried that somehow I had created the problem due to my modding, so that is a relief, but I seriously doubt that I would have left a bad solder joint like that in the first place. Hey, at least it's fixed.

Thanks for reading along... hope this is informative for someone.

/\/cv
-=trimmer tweeker=-

controlvoltage
 
Posts: 242
Joined: Tue Aug 09, 2005 12:17 am
Location: Between 0 and +5V

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