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a cap polarity question...
Old 7th February 2009
  #1
Lives for gear
 
๐ŸŽง 10 years
a cap polarity question...

Hi again (more recapping mess)-

Can anyone here determine what the proper polarity of the 100uF caps in this picture would be? Seems I've seen electros on opamp outputs going both ways. The symbols show as non-polarized, but the schematic is quite old and may have had changes made in the final design. The stuff that came out was polarized (four axial/two radial) and I've had the thing since the mid-eighties, so I don't think it would have been recapped. I had the old polarities marked out, but the stupid axials had arrow stripes pointing to both ends (+&-) so I'm now second guessing my markings (this thing's been sitting dead a long time).

FWIW- After the re-cap it was pretty much non-functioning. It passes a weak signal flooded with LF buzz and I think a few functions don't work. While restoring it, I replaced an obsolete LM378 amp for the headphones, which was burned out, with a modern sub (NTE804), as well as some cooked power diodes in the same area. It's running a variety of socketed 4558 opamps which I think are original, but I have enough TL072s to swap some in next. After that, I'll go after the old crusty switches, but I'd like to be able to rule out this cap possibility.

Much thanks as usual,

George
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Old 8th February 2009
  #2
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JohnRoberts's Avatar
 
๐ŸŽง 10 years
There is no dc bias, so no preferred polarity for a polar capacitor. Since the opamp is bifet dc offset will be random, so if you feel like micro-managing you can measure actual offset, but more correct to use a polar capacitor.

I'd flip a coin since it sounds like you have bigger problems.


JR
Old 8th February 2009 | Show parent
  #3
Lives for gear
 
๐ŸŽง 10 years
Thanks John!

I hoped that would be the case. I'll leave them where they are for the time being then. I'm about 90% sure that's how they were to begin with anyhow and it did work once upon a time, so I'll rule them out. Not looking forward to the switches. They're those long 90 degree pushbutton things with about ten legs on either side.

Take Care
Old 8th February 2009 | Show parent
  #4
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nosebleedaudio's Avatar
 
๐ŸŽง 15 years
I would use a Bipolar and forget it...
Old 8th February 2009 | Show parent
  #5
Gear Guru
 
1 Review written
๐ŸŽง 15 years
Measure the DC offset at the output of the opamp. If it's positive, place the + pin towards it. If negative, place the - pin towards it. If you use low offset, low bias current opamps you could remove it.

Also, add a 22 pf NPO ceramic cap across R102 and R106.

Jim Williams
Audio Upgrades
Old 8th February 2009 | Show parent
  #6
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nosebleedaudio's Avatar
 
๐ŸŽง 15 years
Quote:
Originally Posted by Jim Williams โžก๏ธ
Measure the DC offset at the output of the opamp. If it's positive, place the + pin towards it. If negative, place the - pin towards it. If you use low offset, low bias current opamps you could remove it.

Also, add a 22 pf NPO ceramic cap across R102 and R106.

Jim Williams
Audio Upgrades
Right, it kills me when designers leave off the very important FB cap...
Have seen some serious oscillation problems because it was omitted...
Old 8th February 2009 | Show parent
  #7
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JohnRoberts's Avatar
 
๐ŸŽง 10 years
Quote:
Originally Posted by nosebleedaudio โžก๏ธ
Right, it kills me when designers leave off the very important FB cap...
Have seen some serious oscillation problems because it was omitted...
Not to quibble, but a unity gain stable Bifet opamp, running at more than 20dB closed loop gain, with the ESL of a 100uF cap and 470 ohm build-out in series with any external load capacitance is probably pretty stable.

I have a personal problem with TL07x Bifets in general to drive external output lines in professional gear since they're only rated to drive 2k total. That said widespread opamp swapping is difficult as better parts may not drop into quad sockets, could require more PS current, and perhaps have less friendly stability margin.

From symptoms described, the OP probably has other more serious problems to worry about.

JR
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