Octava MKL-2500 *hum* FIXED Completely..

  • Thread starter Thread starter Slowjett
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Slowjett

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I got an Octava MKL-2500 today, and it had this stupid hum. Well most people talked about the cable haveing a bad sheilding connection to the mic. That cuased a BIG hum. I fixed that and the hum went down considerably.

mkl2500cbl.jpg


Turns out the big Issue is the power supply. This is a chinese power supply, known to cause a real bugger of a hum. Basically rendering the mic unusable.

here is the culprit:
mkl2500a.jpg


Its much smaller than the good russian power supply.

I opened up the chassis, and after prodding around inside the power supply a bit with a jumper wire I discovered the issue. It appeared the the mic cables were not grounded propperly at all. Look at my diagram.

mkl2500b.jpg


Piont A is the ground from the wall plug. B is the common ground lead connected through the metal plate to point A, and then goes to the circut board. Point C is also a common ground, and it should all be on the same circut. When I touched C to A The hum went down considerably. So I figured I could just solder a wire from A to C.. Point B didnt give as good results as shorting A to B. Then I figured points A and B werent makeing good connection to each other OR the chasis.

So I took A and B apart and cleaned up all the contacting surfaces to ensure a good common ground over the whole unit. I used sand paper and cleaned up both sides of the plate around the AC converter where the holes were for the screws. Top and Bottom, down the bare metal. Then I cleaned the power supply chassis (houseing) around the screw holes.. Made sure to get right down to the bare metal there also.

mkl2500c.jpg


I put the unit back together and to my amazment, no more hum! It was a very simple fix, didnt even have to fire up the soldering Iron.


Here are the before and after samples.

These samples should not be taken to show the clarity/quality of the mic, since it was in a noisy room and I was about 3 feet from the mic sitting at my desk. Any crackling you hear is me moving around in my chair. Plus I killed it with compression so you can hear it =]

Before


And After


Small samples of each next to each other...



Turns a not so usable mic, into a great mic.

Hope this helps you!

Josh
 
I'm blown away. GREAT job, Slowjett. There are a lot of people out there that bought that exact model from Guitar Center, and many of them had hum problems.

I've got that exact power supply, and although the mic didn't have a hum caused by the power supply (my chassis and tranny frame were already sanded), it DID have a hum caused by an unsoldered shield to the connector body of the mic connector. A touch of the soldering pencil, and it's quiet as a mouse. ;)
 
Great work man!

The other problem I found is in the cable, at the power supply-end XLR. The cheap 7 pin XLR connector barrel was painted on the inside! This isolated the barrel from the internal connecting block sheild ground. So while the mic was grounded to the cable, and the cable was soldered to the XLR block, the grounding was not continuous to the power supply through the XLR barrel.

I ran a small dremel tool cut off wheel down inside the barrel, cleared off the paint down to metal and it worked like a charm.
 
Wow! Great job!! Wonderful info!

Since we're on the subject of the 2500's power supply, I have a rather curious question regarding Oktava's claims as to the power supply injecting 3rd (or what really should be 2nd) harmonics into the tube to produce a fuller sound. Michael, maybe you can answer this. Or Marik, or Harvey...someone!

If I understand my electronics correctly, the two types of voltages supplied to a tube mic are the filament and B+ voltages, both of which are DC and are smoothed by large capacitors. How in the hell can you "inject" or alter the voltage to the tube to produce said harmonics? It just seems to be a sales ploy rather than real technology. Are they somehow modulating the B+ voltage or piggybacking some other frequency?

Maybe I'm just uneducated, but it seems like BS to me. :(
 
Good question. I didn't have time to verify that claim when I last had one on the bench. But I do see something that might be at work.

There is a small DC voltage across the primary winding of the output transformer which is in the cathode-to-ground leg of the tube circuit. This DC might bias the transformer so that some core saturation occurs and distortion results. Just a theory, haven't check it out yet, but there are no other intentional distortion topoligies that are obvious to me here.
 
Wish we could get someone with a Russian power supply to do an A/B comparison with the same mic and a good chinese PS.

Josh
 
Slowjett said:
Wish we could get someone with a Russian power supply to do an A/B comparison with the same mic and a good chinese PS.

Josh
Click to expand...

I got mine the other way around. I have the Russian power supply with a chinese mic. At least I think its chinese. There's no serial number on the mic. The power supply is definatly russian, though. The guys at Guitar Center in Torrance peiced it together from remaining packages.
 
PhilGood said:
I got mine the other way around. I have the Russian power supply with a chinese mic.
Click to expand...

Well send me the PS so i can compare!

=]
 
awsome post slowjet.

i got lucky, bought mine used for 150, all russian just before the GC blowout.
i was kinda mad about the price untill i realised that i had gotten a russian one.


michael,
are there any easy mods to the 2500? foam removal? hf discs? (not that i'd want this mic any darker) stuff even i can do?
thanks.
 
giraffe said:
are there any easy mods to the 2500? foam removal? hf discs? (not that i'd want this mic any darker) stuff even i can do?
thanks.
Click to expand...

Take the mic apart, remove the inner mesh from the grill and the disc in front of the diaphram. This will give you a nice top end (from what I hear).

Josh
 
Slowjett said:
Well send me the PS so i can compare!

=]
Click to expand...


Where do you live? I gotta make sure you're like a hot chick and "geographicaly desirable". (I hope my wife isn't going through these posts or she's gonna accuse me of being a gay mic slut.) :rolleyes:

I won't be using it for a while, so maybe we can work something out, as long as I can get it back. I live in the Los Angeles area. Redondo Beach to be precise.

Phil
 
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