Oktava MK-219 Mod Help

[funkymonksf]

I just modded an Oktava MK-219 to match Scott Dorsey's article Oktava MC-219/319 Upgrades

Sound goes in and out until I place my finger on the joint shown in the picture below. Do you folks have any idea what is going on? Most importantly, what is the solution?



Thanks for your time!

 

[Khron]

Uncertain solder joint?

 

[funkymonksf]

The joint is sound. I’ve double checked, resoldered, and tested via MM.

I thought maybe I needed to ground that joint since my finger seemed complete the circuit somehow. But grounding did nothing but silent it.

 

[Khron]

Is R2 still in one piece?

 

[funkymonksf]

Yes

 

[Khron]

And getting at least close to the indicated drain & source voltages on the JFET?

 

[k brown]

Resistor maybe internally fractured? Is it carbon, or metal film?

I'd also replace C2, especially if it's a ceramic - their leads can become detached internally or heat damaged.

 

[funkymonksf]

Yes, from I can tell.

I also noticed that my right hand must be holding the base of the mic body when my left hand’s index is touching the joint. Otherwise it squeals.

Sometimes the mic starts to pick up sound but it fades in and out.

 

[funkymonksf]

Ok I’ll try!

 

[Khron]

Yes, from I can tell.

I also noticed that my right hand must be holding the base of the mic body when my left hand’s index is touching the joint. Otherwise it squeals.

Sometimes the mic starts to pick up sound but it fades in and out.

Sounds like the JFET gate is indeed floating; the resistance through your body is some number of megohms, just enough to bias the gate where it's supposed to be. Swap out R2 with a known good one, and even C2, and see what happens then.

 

[k brown]

Suppose it's possible there's also a not-obvious break in a trace; Oktava boards aren't the greatest.

 

[funkymonksf]

I literally replaced all the components to match Russel Tech’s mod and it now works. Very likely the 1gig resistor though, although I never tested it.

One bummer is that I accidentally poked a hole in the capsule. Fortunately, it works and actually sounds fantastic!

Thanks to everyone who shared!

 

[DazzboDuck]

I literally replaced all the components to match Russel Tech’s mod and it now works. Very likely the 1gig resistor though, although I never tested it.

One bummer is that I accidentally poked a hole in the capsule. Fortunately, it works and actually sounds fantastic!

Thanks to everyone who shared!

Glad you got your issue resolved with new components, what was the difference in sound ? If you were upgrading another 219, what components would you use ?

 

[KZhack14]

Hi there, hopefully I can use this thread to ask some questions regarding to the Oktava variants (referring to Henry Spragen's Schotava):
(Original schematic by Henry @ Audio Improv here)



Was just wondering since only line 2 is actively driven, could I just use a center tapped transformer to make the circuit a true balanced output by replacing the impedance balancing resistors. Can I step up the output as well if I use something like a 1:2 tranny? Will probably try this circuit with a 34mm LDC.

(EDIT: I might have missed out one polarized capacitor in between the input of tranny and phantom line going into Q2, is it needed to prevent DC from spilling back into the transformer input?)

 

[k brown]

Output in orig schematic is "truly balanced" (impedance balanced); it's just not 'active balanced', in that there is no inverted signal on pin 3. Still utilizes the balanced input of a preamp to cancel noise picked up by the cable.