Oktava MC 012 60hz hum *Solved*

Hi all,

So I picked up one of the aforementioned microphones off of craigslist tonight and it has a noticeable 60 cycle hum. I got it for cheap, and it looks like the body has taken a couple of decent whacks from a drumstick. Caveat emptor I suppose. I've pretty much given up even trying to audition gear at other people's houses. Who can hear anything over the computer *and* the refrigerator running in the same room?

Anyway, my meter shows good continuity between the xlr ground and the mic body.

Upon opening it up, this is what I encountered:



Could that be my problem?

Here are the rest of the guts:




Anything else pop out at you?

I know folks have modded these quite a bit, but my searching didn’t really turn up anything on hum with regard to this mic.

Insights and the combined wisdom of this board would be much appreciated.

Thanks!

Jed

I was able to dig up the Dorsey mod that everyone talks about. It contains the schematic for these mics. According to the article, the component which appears to be shedding a bit of its skin is the leak resistor. This would seem to be an unlikely candidate for my problem but I'm a babe in the woods here, so correct me if I'm wrong. I’ll probably replace it as the article recommends doing so anyways.

I’m going to check the grounding a little further with my meter to see if the problem is somewhere else along that path and I’m just not seeing it.

You can find the Dorsey article here if it is of interest:
http://philzone.routeflap.net/audio/uploads/oktava-mod/

Check pin 1, pin2 and pin 3 from the pins of the XLR to the connections on the PCB.

I think that resistor is shorting to the case. The worn spot on the top corner is probably the culprit. Try covering that spot with something, then bending the resistor in a little so it doesn't rub the inside of the case when you reassemble, and see if that fixes the hum. Also, be careful about this if you mod the mic. It's a really tight fit. If the new resistors rub against the inside of the case when you reassemble, it may wear through the coating on the resistor, and you'll be back at square one...but with new parts.

Good luck.

Thanks for the suggestions guys. I finally had some time this morning to take another look at it.

Gus, I metered all the pins to their connection points on the pcb. Everything looks fine on that end. I also checked the ground plane in about 10-15 spots and it seems fine.

Pantsonfire, I tried your suggestion as well. I was really hoping that was going to be the culprit, but it made no difference at all. Disappointing.

I'm going to go back over stuff and see if there is anything else that might be shorting to the case some how.

Any suggestions where I should look after that?

Thanks,

Jed

Jed, That resistor is one of the 2) 680 meg ohm units in the mic that are normally upgraded. The state of the resistor tells me that you should be in the market for the Dorsey mod.

Some questions come to mind. Are you sure that it is the mic? I'm thinking cables, grounding loops and phantom PS. Does the hum level change when you hold the mic?

What happens when you place the mic in a grounded metal box (Mic isolated from the box)

Oh, how loud is the hum?


Then again you might want to dump it quickly. (What do you want for it?)

Regards, Ethan

OK was one or more of the screws inside the microphone? I have repaired one that had a damaged PNP(no output) because someone turned the screws all the way in and powered up the microphone. I used a 2n3906 IIRC to replace the PNP.

Now if you get it working you might want to leave the stock parts. I have changed parts in a few and the sound can change, often the stock caps like in your picture sound better with acoustic guitar IMO. You have what looks like BC electros and the tant like in the SD mod.

Ethan has posted a good question "Does the hum level change when you hold the mic?"

Does another known good condenser microphone SD, LD have the hum issue with the same cable preamp and power outlet? Homemade or bought preamp and cable?

Hi Ethan (& Gus - we were writing at the same time I guess),

I'm inclined to think it is the mic, insomuch as I can swap in other mics into the same chain and the hum disappears immediately. This is going into a little Joe Meek preamp with a store bought cable.

Hum does not change if I'm touching the mic.

Ethan, I'm not sure if I understand your idea of putting the mic into a grounded box...

As for level...it's not a massive hum. It's pretty far below things but it is audible and it would definitely build up over a number of tracks. When I dime my preamp and look at the results on the spectrum analyzer I see background noise down around 90db and big spike centered at 60hz that is up around 50db.

Granted, that is with the preamp at max (so high that taking a breath sounds like a hurricane) and its not a level I record at often. But it is annoying.

I'll grab a handful of giga ohm resistors and do that part of the mod a little later this weekend. I'm not sure it's going to solve this problem though.

Ethan, I see you're in town...if I can't solve this in a reasonable amount of time I could probably let it go for a very reasonable price. Lets assume that would be a last resort. I'd much rather get the use of the mic! But maybe we could get a beer and talk audio some time?

Gus, I'm going to look at the screws. It's clear this mic has been opened up before and maybe that could be a source of the problem.

Best,

Jed

The holding test is to see if if You act as a pickup antenna and feed 60 Hz through the mic body. The Fariday cage (in a box) is to see if it is actually the mic that is picking up the 60 Hz. Not a real good test....as that it does not tell you what the problem is.

I'm still wondering if the damaged resistor has a much larger resistance than the other.

-Ethan

Problem solved!

I got some parts in from mouser today. I replaced the two 680 Mohm resistors (they basically fell apart when I de-soldered them) with 1 G ohm ohmites. Expensive!

I also replaced C2 with the cap Dorsey recommends.

Talk about a tight fit!! I thought I was going to need a shoehorn to get the thing back together, but it *just* worked.

It fired up and we're in business! No hum! And I would swear there is a lot more gain too! (But that could just be a faulty memory on my part.)

Anyhow, I did a little test on classical guitar and it sounds pretty nice. Definitely very usable. A steal, really, when you consider I spent about $40 on the mic, $10 on parts to fix it (and an hour of time).

Thanks to everyone for help and suggestions! :guinness: :sam:

Glad to hear that you solved it. That resistor looked bad!

Ethan

What cap type and value is C2 ?

Also , the link above for Dorsey mod is broken. I would like to view it if possible.

I remember a repair I did on a KM84. It had a leaky tantalum cap and was enough to generate a miserable hum. Holding the mic in your hand would increase the hum level as well as holding the mic near anything that emitted any RFI fields.

I uploaded the file for folks. It's pretty huge (~39Mb). You can get it here: http://www.claireandjed.com/music/oktava-mod.pdf

All the part values are in the document. A lot of the digikey part numbers listed in the article are out of production or are not stocked at small quantities. I found some substitutions at mouser. But you really need to look at the spec sheets and get out your ruler when choosing parts -- it is an insanely small space in there.

I forgot to take photos when I replaced the parts, but if I open it back up for any reason, I'll take some and put them up.

right on! i'm still convinced it was that abrased looking resistor, but as long as it's working now...
i'm glad you stuck with it. troubleshooting is always the most patience taxing stage of any project for me.

I'm sure it was the resistor(s) too. I don't think that the exploded one was shorting to the case (it certainly wasn't after I pushed it in to check), but both of them were in really bad shape. The bias resistor literally fell apart after I de-soldered one leg. Crazy.