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Author Topic: U87 symptoms. FET, caps, or something else?  (Read 673 times)

BluegrassDan

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U87 symptoms. FET, caps, or something else?
« on: June 10, 2020, 02:07:18 am »

I have an older (with battery compartment) U87 that has a bizarre symptom. It has no output until you switch between polar patterns. Then it has momentary signal that dies away.

Does this symptom point to any known common cause? FET? Tantalum caps? Something else?

The voltage on pins 2 and 6, for example, are around 35v and 39v respectively, whereas the schematic calls for 47v. Does that provide any clues?

Thought I would start here. Thanks in advance.

Dan
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Dan Boner
Associate Professor/Director
Bluegrass, Old-Time, and Country Music Studies
East Tennessee State University

klaus

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Re: U87 symptoms. FET, caps, or something else?
« Reply #1 on: June 10, 2020, 02:26:44 am »

Hello Dan,
Aside of possible capsule contamination, the most common problems associated with the symptom you describe are, in order:

1. Defective FET
2. Defective source cap (Tantalum)
3. Defective Coupling cap (Tantalum)

If teh capsule is ok, I would first replace and rebias the 2N3819 FET, then see what happens.

(Your voltage measurements are not conclusive, as the inner resistance of most voltmeters shunt part of the high impedance between the two probes, thus giving a lower reading than expected.)
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Klaus Heyne
German Masterworks®
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BluegrassDan

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Re: U87 symptoms. FET, caps, or something else?
« Reply #2 on: June 10, 2020, 03:13:38 am »

Thanks, Klaus. I’ll work on it and report back.
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Dan Boner
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Bluegrass, Old-Time, and Country Music Studies
East Tennessee State University

RuudNL

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Re: U87 symptoms. FET, caps, or something else?
« Reply #3 on: June 10, 2020, 03:35:08 am »

In the past I have noticed that also capsule pollution can cause this.
After switching the pattern, there is a large voltage 'jump' that also reaches the gate of the FET.
This might bring the FET momentarily in an good working condition.
(Especially because after a short time the audio fades away.)
Voltage measurement at the drain of the FET will provide more information.
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klaus

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Re: U87 symptoms. FET, caps, or something else?
« Reply #4 on: June 10, 2020, 05:30:32 am »

Good point, Ruud, and time to remind everyone again of the simple test to tell whether the failure is capsule or electronics-related:

If the mic is just put down, and not spoken into and drops in volume = electronic failure
If it's spoken into at close range and THEN drops in volume = capsule contamination
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Klaus Heyne
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BluegrassDan

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Re: U87 symptoms. FET, caps, or something else?
« Reply #5 on: June 10, 2020, 03:09:44 pm »

The FET drain is consistent at 325mV. When flipping the switch, it jumps up to around 7V, then decays back to 325mV.

Attached is a visual of the capsule front. Could this degree of dirt be the likely culprit?

The "breath" test cannot be accomplished as it never gets up to the proper voltage.
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Dan Boner
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Bluegrass, Old-Time, and Country Music Studies
East Tennessee State University

BluegrassDan

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Re: U87 symptoms. FET, caps, or something else?
« Reply #6 on: June 10, 2020, 03:10:25 pm »

And here is the rear. Pretty filthy.

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Dan Boner
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Bluegrass, Old-Time, and Country Music Studies
East Tennessee State University

BluegrassDan

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Re: U87 symptoms. FET, caps, or something else?
« Reply #7 on: June 10, 2020, 03:24:54 pm »

I connected the older U87 head to a newer U87ai and it worked. I'll start looking toward the FET.
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Dan Boner
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Bluegrass, Old-Time, and Country Music Studies
East Tennessee State University

klaus

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Re: U87 symptoms. FET, caps, or something else?
« Reply #8 on: June 10, 2020, 04:35:47 pm »

The capsule is in dire need of restoration/cleaning.
This is what accumulation of flaking from a defective foam sock and years of leaving it out in the open without a plastic bag over it looks like.


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Klaus Heyne
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BluegrassDan

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Re: U87 symptoms. FET, caps, or something else?
« Reply #9 on: June 10, 2020, 04:53:02 pm »

Indeed, a bad example brought about by the mic living in the felted Gotham box with no protection, plus whatever crud it accumulated in use. This mic belongs to a colleague who says it gradually stopped working. He thought something was shorting out.

Swapped the FET and nothing changed. On to the tantalum caps before sending this off for cleaning.
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Dan Boner
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Bluegrass, Old-Time, and Country Music Studies
East Tennessee State University

RuudNL

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Re: U87 symptoms. FET, caps, or something else?
« Reply #10 on: June 11, 2020, 03:54:35 am »

A capsule in this condition is asking for problems...
And 325 mV (DC) on the drain of the FET is way too low!
Have the capsule cleaned by a professional, and don't even think about cleaning it yourself!
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