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R/E/P => Klaus Heyne's Mic Lab => Topic started by: BluegrassDan on June 12, 2018, 09:33:23 pm

Title: KM84 Low output suddenly
Post by: BluegrassDan on June 12, 2018, 09:33:23 pm
Hey folks. I was playing at Bonaroo this weekend and using my KM84s. Everything was fine during mic check.

A bolt of lightning struck VERY close to us. Sounded like a rifle - flash and bang at the same time. One of the KM84s stopped working (the output is very low, although seemingly not distorted). No other equipment was damaged.

I got them home, swapped capsules, and determined the capsule is fine.

I'm not saying that the lightning caused the malfunction - could have been coincidence - but could something have been damaged due to high SPL? Could this cause the FET or one of the old tanatlum caps to fail? Is there a particular area I should zero in on?

Thanks in advance!
Title: Re: KM84 Low output suddenly
Post by: uwe ret on June 12, 2018, 10:34:52 pm
To check for proper operation of the FET measure the voltage at the drain (junction of R4, C4,C2 and S1). It should be close to 10V.
Very low voltage at this point, even near 0V could have various causes:
1.)      C4 = 1F/25v Tantalum leaky or short circuit;
2.)     Zener diode GR1 = BZX45C24 leaky or short circuit,  (any 24V Zener with 0.5W  maximum power dissipation will do)
3.)     C5 or C8 = 5F/25V Tantalum leaky or short circuit
I doubt the transformer may have been damaged, but if in doubt, measure for continuity between the red and black leads of the primary, and between the white and blue leads of the secondary.

A shorted FET (T1 = 2N3819) would result in a voltage of around 2V at this point.
Title: Re: KM84 Low output suddenly
Post by: BluegrassDan on June 12, 2018, 10:58:55 pm
Thanks so much for the info. The voltage at the junction you specified is very low (close to 0).

I'll start with the caps and see how it goes. Thanks again.
Title: Re: KM84 Low output suddenly
Post by: BluegrassDan on June 13, 2018, 01:34:10 pm
If the FET is bad (hopefully not), is there a preferred replacement that retains the excellent sonic character of the KM84?
Title: Re: KM84 Low output suddenly
Post by: klaus on June 13, 2018, 01:42:28 pm
You can obtain the original 2N3819, now made by Fairchild, from Neumann or any well-stocked parts house.
But you must properly bias it, otherwise the mic will have low or no output, will distort or be noisy.
Title: Re: KM84 Low output suddenly
Post by: BluegrassDan on June 13, 2018, 04:42:08 pm
One more question. Is there a preferred type of capacitor to replace the tantalums? Would be hard to find any film capacitors small enough to fit. Would small Nichicon or similar electrolytic be okay?
Title: Re: KM84 Low output suddenly
Post by: klaus on June 13, 2018, 06:22:29 pm
Nichicon, Panasonic, Elna... all fine and, at 63V rating, will fit.
Title: Re: KM84 Low output suddenly
Post by: BluegrassDan on June 13, 2018, 10:50:24 pm
Thanks guy! I really appreciate the help.
Title: Re: KM84 Low output suddenly
Post by: boz6906 on June 17, 2018, 01:14:36 pm
High static charges can puncture the gate insulation on some FETs, but usually not JFETs like the 2N3819.

With a close strike (little delay between flash and bang) and 100meV discharge anything is possible.