JW

Mk47 problem. Help a fool out
« on: September 10, 2019, 05:12:12 PM »
I accidentally tripped on the cable and over flew my MK47 mic onto the basement concrete.

Dead mic. Tried a tube swap (with good tubes) a capsule swap, a power supply swap (I have a second) and no difference. I checked B+ voltage at the mic pcb and 290V!!!

Any ideas what this might indicate? Something in the circuit is blocking current? Solder breakage? But where?


Khron

Re: Mk47 problem. Help a fool out
« Reply #1 on: September 10, 2019, 05:13:57 PM »
Cracked solder joint on either end of the plate resistor? And/or cathode resistor?

[Later edit]

Going by what schematics Google Image Search came up with, the dropper-resistor in series with the heater may well be a potential culprit. No wonder the unloaded voltage is that high, the heater's gonna be pretty power hungry (at least 50mA, if the two 6028 heaters are in series), and since everything's powered from the B+ with quite some hefty RC filtering...
« Last Edit: September 10, 2019, 05:21:53 PM by Khron »
Khron's Cave - Electronics - Audio - Teardowns - Mods - Repairs - Projects - Music - Rants - Shenanigans

JW

Re: Mk47 problem. Help a fool out
« Reply #2 on: September 10, 2019, 06:25:27 PM »
Nevermind. . . Got it of course right after posting. Somehow the B+ input's connection to the 1.5K resistor was non existent. Weird . I actually had to manually bridge the two, (pcb trace comprimised? Wha? )  Anyway, seems to be all good now. Thanks for the help Khron.  You were right on it!

Khron

Re: Mk47 problem. Help a fool out
« Reply #3 on: September 10, 2019, 06:35:25 PM »
There ya go, then ;)
Khron's Cave - Electronics - Audio - Teardowns - Mods - Repairs - Projects - Music - Rants - Shenanigans


 

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