Oktava MK-219 help

[sweetbeats]

Sorry..maybe this is the wrong spot for this and Mr. Moderator please move if needed.

I recently came into possession of an Oktava MK-219 LDC. It is finicky (part of the reason it came to me since I know enough to be dangerous and I seemingly have no sense not to open stuff up and start poking things...)

It'll cut in and out completely at random or get very thin and weak sounding with greatly diminished output. So far I've:

• tested/swapped cables
• tested other mics on the same preamp and phantom supply
• clipped one pole of each of the magnetic reed swtiches for the lo-cut filter and pad effectively removing those functions from the loop

I could get it to come back to life by flicking the bottom area of the body with my finger. I opened it back up again and checked the wiring from the jack to the PCB and its good. Found I could get it to exhibit the problem by gently flexing the bottom area of the PCB so I suspected a bad solder joint or cracked trace so I refreshed the solder joints in that region and layered solder over a couple of the traces...you can see that work here in the upper right of the PCB:

Well, that didn't make any difference. Then I started tapping on the tranny and found that simply tapping on it causes the intermittent performance. I can get it to come back to life by touching one of the two traces I modified (which come from pins 2 and 3 of the XLR) with any metal tool held in my hand.

So now I'm stuck with what to do. Not sure if a faulty tranny is potentially a repairable issue (never worked on transformers) or if it is time to replace it and if so is it possible to get a drop in replacement for that and where.

Can anybody help?

Here are a couple more pictures of the innards:

BTW, I was really impressed with it in stock form when it was working. I've been really dissatisfied with my Studio Projects C1 and the MK-219 confirms my dissatisfaction. It was so much smoother and handled a difficult vocal source (very powerful cutting vocal quality) like no other mic in my collection.

Here is a schematic:

 

[crazydoc]

Open up the transformer and make sure the internal connections are secure. If you have an ohmmeter, check the continuity of the windings (while tapping the transformer if necessary,)

 

[sweetbeats]

Cool...thanks. I've never messed with transformers before so it is uncharted territory for me and I was concered that I'd kill it by trying to open it up...it doesn't look user friendly. I'll figure it out.

 

[sweetbeats]

is the tranny toast?

I pulled the cover off the transformer. After much gentle poking and prodding around I suspect there is a bad winding. Can anybody confirm?

Again, new territory for me.

• When I gently apply pressure with my tweaker on a certain spot on one of the coils the signal dies.
• Sometimes I can get it to come back by varying the location of the pressure, but sometimes not and I resort to touching either the trace from pin 2 or pin 3 of the XLR jack with the tip of a metal hemostat held in my hand. This makes no sense to me.

Here is a picture of my tweaker gently pressing on the confounding spot:

 

[evm1024]

put your scope on it...

So if I read it correctly when you touch it with some metal (pin 2 or 3) it works. Ditto when touching specific places inn the windings.

Are you oscillating?

--Ethan

 

[sweetbeats]

Pssst! Guys! Watchout...here comes the scope newbie!

Thanks, Ethan...my first real need for my new-to-me scope. But...what does oscillation look like? Do I put one probe on pin 1 and the other on pin 2 or pin 3?

 

[evm1024]

Thanks, Ethan...my first real need for my new-to-me scope. But...what does oscillation look like? Do I put one probe on pin 1 and the other on pin 2 or pin 3?

Ground the probes ground to the mic's ground and touch the probe tip to both outputs and anywhere else you want to.

I assume that you know that you can pull the probe tip off to expose a probe tip for touching pads ad so on.

Oscillations.... Well it all is about 1/f

a signal at 20 KHz has 50 uS period (1/20,000 = 50uS) so any signal that has less than 50 uS period is outside of the audio band and should not be there.

A 1 MHz oscillation would have a 1uS period (time of 1 cycle).

Look for crud and things that should not be there. If you do have an oscillation it should be stand out.

--Ethan