Microphone Oktava 219A stopped working

[skunks]

Yesterday I turned on my condenser mic Oktava 219A, but there was no sound. This mic served me from ~2001. It was a surprise for me.
I disassembled it. One of 1uF caps looked a little bit like extra fat, so I replaced it, but it didn't help.
Please could you advice my next steps.
May be buy and replace all the caps?

[Khron]

Re: Microphone Oktava 219A stopped working

For what it's worth, i've managed to track down the schematic, as well as some circuit descriptions / upgrades:

http://www.recordingmag.com/download...schematic2.jpg

http://www.recordingmag.com/resource...etail/316.html

The schematic has some voltage readings at several points - you might want to start by checking if voltages in your microphone are anywhere near those values.

Regarding the capacitors, even though the article recommends tantalums, i would keep away from those, and just use some decent quality electrolytics, or even film caps, if they can physically fit

[stj]

Re: Microphone Oktava 219A stopped working

there is so little to it, if he resistance of the mic and the transformer windings are good then it's probably the fet.

[SteveNielsen]

Re: Microphone Oktava 219A stopped working

Mic's get a lot of handling and potential abuse, check for broken wire connections and bad solder joints too. Broken wires can hide really well when they're that small.

[skunks]

Re: Microphone Oktava 219A stopped working

Khron666, thank you for direction. However this schematic is different because number of resistors and condensers differ.
I measured voltages:
1.7v instead of 6.8v on C3 (47uF).
3.4v and 0v on FET instead of 2.1v and 22v.
Because schematics differ I could not find point of 0.8v to measure.
So, that's all what we have now.

there is so little to it, if he resistance of the mic and the transformer windings are good then it's probably the fet.

The resistance of capsule is more then 2000k.
Transformer's resistance is
~760 Ohm on primary winding
8 Ohm + 8 Ohm ~= 15.4 Ohm on secondary (btw point 4 is not connected to ground on my PCB, it is connected to other stuff)

And (maybe it is suspicious) resistance between primary and secondary widding is 36 kOhm (due to the rest of schematics may be, I didn't solder out the transformer)

[stj]

Re: Microphone Oktava 219A stopped working

well a mic with 2meg impedance sounds strange.

and obviously the primary and secondary on the transformer should be isolated.

[Khron]

Re: Microphone Oktava 219A stopped working

"Technically", the capsule should show infinite resistance, since it is, basically, a capacitor.

The resistor coming from the positive supply rail is usually in the range of 1Gohm, and so is the gate bias resistor.

[stj]

Re: Microphone Oktava 219A stopped working

"Technically", the capsule should show infinite resistance, since it is, basically, a capacitor.

it's an electret mic?
it's huge - i thought it was a moving-coil type.
even electret mic's usually have a 2.2k resistance.

[Khron]

Re: Microphone Oktava 219A stopped working

No, this is what's called a "true condenser" type, ie. one that needs an external bias voltage (in the order of 40-60v). Specifically, a large-diaphragm ( ) condenser.

Those usual (1/4" or 1/2") electrets have the jfet inside the capsule, and that 2.2k resistor goes to its drain.

[skunks]

Re: Microphone Oktava 219A stopped working

So, should I try to change that JFET also?
"Replace the FET with a Toshiba 2SK170BL" as suggested in article (from 1st reply made to this thread)?
BTW original jfet has 4 legs, "Toshiba 2SK170BL" has 3 legs.

I cannot fully proceed with mod described in article because
1. lack of experience in such things
2. the schematics differ
so I'd like to proceed with some kind of newbie non-destructive repairing.

[Khron]

Re: Microphone Oktava 219A stopped working

What's the stock JFET, then?

If you really wanted to, lifting the schematic shouldn't be all that difficult, there's only a handful of components

PS: I think i might've found the original(?) schematic, as well: http://www.sdiy.org/oid/mics/Oktava-MK219.gif

That JFET symbol looks like it has its case grounded, i believe?

PS2: According to the article, there are four variations of circuit board:

"The design of these mics has changed a lot over the years, and you’ll find that there are four basic PC board layouts. The third and fourth match the schematic shown here.

Revision 1: You can tell the first type of board because the capsule leads are soldered directly to the PC board without any standoffs. Board traces go from the FET to C2 capacitor and to the capsule. C7 on the schematic is replaced with three 1 uF capacitors in series.

Revision 2: This layout uses plastic standoffs to which the capsule wire, C2, and the FET are soldered. This results in lower noise and less trouble with flux contamination. This board can be recognized because C7 is replaced with three capacitors in series but there are standoffs on the front end.

Revision 3: This layout matches the schematic exactly. There is one capacitor for C7, but the reed switches (glass barrel things) are still mounted directly to the rear of the PC board. Most of the mics out there seem to be this revision.

Revision 4: This board does not have the reed switches on the main PC board but has a small daughterboard with the reed switches on it. Avoid this revision if you can, and if you can’t, remove the daughterboard—and therefore the pad and bass-cut functions."

[Khron]

Re: Microphone Oktava 219A stopped working

What voltage are you getting on pin 4 of the transformer (or across C9)?

And what about between pins 2 (or 3) and 1 on the XLR input of your preamp / audio interface?

The voltage readings you got are terribly low...

[skunks]

Re: Microphone Oktava 219A stopped working

Khron666, you are right, I can put an effort and draw schematics. But I'm not too much experienced in electronics to know what this 4-leg JFET(?) is. It is "7БК8" written on it, but googling for "7БК8" didn't help. The scheme you guessed unfortunately isn't this either (mine has 8 resistors, 11 condensers (4 of them electrolytic), point 4 is connected not to the ground but somewhere else, my FET has 4 legs). Maybe because my mic is called "Oktava MK-219A", not "Oktava MK-219".

My PCB's Revision seams to be 1 (because of the number of caps. Number of resistors differs though). I wonder what are that plastic standoffs that make a difference between revision 1 and 2? I guess I don't have these.

Voltages:

Transformer:
pin 4: 44v
after C9: 0v
XLR:
between pin 1 and 2: 44v
between pin 1 and 3: 44v

What about C9 - I replaced it, as I said. It was 1uF 160v, but I had only a 1uF 50v condenser. It's hotter then other 3 electrolytic condensers.

I wonder what could be bought to replace FET...? I'm confused with 4 legs - all 4 legs leads to different places in PCB, so I don't know if it's possible to replace it with 3-legs "Toshiba 2SK170BL"

[Khron]

Re: Microphone Oktava 219A stopped working

It's right in the description: "Revision 2: This layout uses plastic standoffs to which the capsule wire, C2, and the FET are soldered. This results in lower noise and less trouble with flux contamination."

Hint: read the last sentence there


It's a pretty safe bet that the leg that goes between the blue capacitor and that thin "tall" resistor is the gate of the JFET (like in the schematics).

https://www.badcaps.net/forum/attach...2&d=1442664256

The lowest leg of the JFET, in this picture, seems to go to a trace that goes straight to the ground "plane". The drain would most likely be the top leg, and the source is the one that's left. Simple and logical

How certain are you that the C9 you replaced was good? You CAN use a larger value capacitor as well, as long as it physically fits. It's there to filter the voltage coming from the phantom power supply, so really, "bigger is better" 47u/63v capacitors are usually in 6.3mm diameter / 11mm tall cans, same height as the originals (which look like 5mm / 11mm).

EDIT: I'm attaching a photo with my suspected part names, according to the old (russian) schematic, and from what i could try to see, regarding the traces, through the board. Feel free to correct those, and i hope it helps

[skunks]

Re: Microphone Oktava 219A stopped working

Thank you for your help, Khron666. I really appreciate that.

I thought that those 2 little things you marked as R1 and R2 are condensers . I've measured it's impedance - it's more then 2000k Ohms, article proves they has to be that big (1000M (1G) resistor).

Saying this - I must admit that you were right point me to this schematics:
http://www.sdiy.org/oid/mics/Oktava-MK219.gif

Here are 2 more photos - one of them with back light in order to see traces (and all 4 legs of FET are seen clearly).
What about FET legs - that 4th leg was ground! . Is it necessary? Can I use 3-leg FET and not solder ground?

[Khron]

Re: Microphone Oktava 219A stopped working

A-haaaa! So i was indeed right - the old russian schematic showed the JFET's case as grounded, as i "decoded" it

I've gotta get ready for rehearsals, but i'll get back to this later tonight, and "re-annotate" this better-lit circuit.

Have you tried replacing the capacitor you had already changed, with another one? If the current one is faulty (or damn near shorted internally), no wonder you're barely getting any voltage on the JFET (and thus, no signal).

[stj]

Re: Microphone Oktava 219A stopped working

full pansonic recap and a new fet wouldnt cost the price of a bottle of beer.

[Khron]

Re: Microphone Oktava 219A stopped working

Well, i think i've managed to figure out which component's name is which I hope this, and the article i linked above, will help.

C9 and C7 can easily have greater values, that's only going to help. C9 should be rated for (at least) 63v, although 50v might be enough. C7, for safety, should be 50v. Value-wise, as big as you have available, anything up to 47uF should be good

If you have one, and it fits, you could swap C8 for a film capacitor (worst case, you could tack it on, on the solder side of the board, maybe with a bit of double-sided tape).

But first of all, change C9, because it sounds like that might (still) be the cause of the problem. Then see how your voltage readings look.

[redwire]

Re: Microphone Oktava 219A stopped working

If the JFET bias is OK (22V), and you still have zero output, I'd look very closely for capsule contamination.
I have seen capsules shorted from spit, spiders, dust, metal chips etc. at the edges.
The output connector wiring/pins look like they are shorting too? Pry them apart a bit.

[skunks]

Re: Microphone Oktava 219A stopped working

The quest if over! The mic is working now.
Thank you once again, Khron and all the guys who shared their thoughts to help me.

All 4 electrolytic caps were bought yesterday on local market. And today I started to replace them. First of all I replaced C9 (once again:-D) and C7. And it helped! Then I replaced other 2 caps just for the case.

BTW, when I turned my mic ON without the case (which acts as a shield) there was a VERY loud hum (50Hz I suppose), much louder then my voice. Though I didn't touch ground while holding it. When I touched pre-amp case - the hum level became lower and I could hear myself a little. When I assembled the case around my mic - that terrible hum disappeared completely.

I'd like to replace JFET also (if it will improve the sound), but the Toshiba 2SK170BL has 3 legs and I'm not sure if it will work without ground leg.

What about capsule - it's rather dusty. I'd happily clean it, if there is a correct way to do it.