Oktava mkl2500 toroidal transformer

[Poc]

Hi!

I have a russian Oktava MKL2500 with the BP2 power supply. I really fancy this mic.

Some months ago due to a tension spike the toroidal power transformer went burnt.
I'm having a hard time finding a replacement.
Do you folks know were i could find one in order to replace it?
I'm from Europe, so if it's just 220v input would be fine.

Thanks!

 

[Whoops]

The drawing is a bit confusing to me...

in the 2 toroid drawing it shows 2 different taps for 220V, what is that?
Is that a 220V primary tap plus a 220V secondary?

I can see the primary has taps for 110V and 220V

For the secondaries it seems it has 2 or 3,
1x 7,5V
1x 75V
and maybe 1x 220V, or not, not sure on that one...

 

[Poc]

The tech guy who is going to repair it told me that according to the drawing I should find a 220v input and 2 output (7.5v and 75V) transformer.

Does it make any sense?

 

[Whoops]

The tech guy who is going to repair it told me that according to the drawing I should find a 220v input and 2 output (7.5v and 75V) transformer.

Does it make any sense?

Maybe...
But 75V seems tiny for a B+ voltage, but I don't know the circuit.

I don't think you will find an of the shelf Toroid with those 2 secondaries.

What I would do is one of 2 options:

1) contact Oktava and buy a replacement Toroid from them, they sell spare parts. This would be the best option

2) second option is to buy a transformer with 1x 75V secondary (probably 70V or 80V secondary works as good also) and then get another transformer a smaller one for with a 7,5V secondary and mount the 2 of them inside the PSU case

 

[Tubetec]

MKL 2500 is a cathode follower , HT supply is around 105 volts , so that seems correct ,

 

[Tubetec]

https://groupdiy.com/threads/oktava-mkl2500-mods.48934/
Some pics there , not all that much space inside the PSU enclosure ,
The PSU can do with some tidying up of the wiring inside , the grounding also can do with a tidy up.

Did you ever manage to get your MKL-5000 fixed Whoops ?
I dont have a schematic for the psu but its fairly conventional , regulated heater and passive HT ,

I moved the transformer in mine up into the top left corner and paid better attention to wire routing within the chassis . I also swapped the switch onto the HT line , so once you connect power the heaters are energised , that saved a few feet of mains wire making its way around inside the box .

The Modern Oktava MKL2500 PSU still uses a small toroid , so probably worth trying them for the spare part .
The MKL2500 is a great sounding mic , different to the typical modern condenser with pushed hi mids , its fairly flat and neutral , gives a great account of itself with acoustic instruments and voice , never sounds harsh ,brittle or sibilant .

 

[terry setter]

The secondary rectified HT voltage in my 2500 is 140V. That gets dropped through (as I recall...) a 4-stage R/C filter and hits the mic at just about 110V.

Mine is a plate-follower, not a cathode-follower. Both designs were marketed by the "real" Oktava over the years. I believe that most of the Chinese knock-offs were cathode followers. The Chinese supplies were mis-wired and every mic had audible hum. That's why Guitar Center blew them out at $99 ea. The hum problem can be corrected, but it involves a lot of soldering and connection changing.

As noted above, they sound very good when working properly.

 

[Whoops]

Did you ever manage to get your MKL-5000 fixed Whoops ?
I dont have a schematic for the psu but its fairly conventional , regulated heater and passive HT ,

The mic works, but I was never able to get rid of a Low End Hum.
I have no idea where that Hum comes from...

I will try to replace the Tube, as it's the only idea I have at the moment of trying to solve the low end hum problem. Don't really know if a bad tube could do that, but the other components seem to be all good

 

[Tubetec]

If it sounds ok appart from hum I'd suspect a wiring issue ,
They used quite poor quality connectors and cable on the MKL2500 and 5000
I used the Gotham GAC 7 cable to replace what came with it ,
check the shrink wrap on the pins inside the xlr at the mic end of the cable ,sometimes it can create a ground loop.
The BP-2 PSU has funky grounding , although that in itself only causes a small increase in noise level , mine had a bad connection in the xlr plug at the mic end until I rewired it ,sometimes it would hum loudly .

 

[Whoops]

I will check it again but already lost a lot of time with it, I didn't see any loose connections or wire.
Soldering at the connectors seemed good also.
In electronics when you're chasing a problem but you can't solve it then starts to be a ghost. And after wasting hours on it , you can get really upset and even depressed with the situation, it's a lot of frustration.

I think that's what happened. I have to get some energy to check it again

Thank you

 

[terry setter]

The 2500's were wired to accommodate the ground lift switch in the British-made power supply. If you trace the wires, you'll see that the grounding of the mic's frame and (other parts) is not done well. Re-wire your connectors so that you have excellent grounding (through more than one lead) and get rid of that ground lift switch. On the Chinese supplies, I had to make a direct ground connection from the post-regulator heater caps to the ground pin on the plug. Worked swell. Best of luck with it.

 

[Tubetec]

Ive been in the repair trade a long time , I know well that sense of hopelessness that can creep in ,
sometimes I spent way more time than it was worth or possible to charge the client for fixing something , in the long game it balances out with other jobs that go smoothly .

Its true that sometimes you need to step away from it for a while to see clearly again .

Now that Terry mentions it I think I may have made some small modifications to the grounding ,both in the psu and the cable/connectors which improved matters .
Much of my gear is in storage at the moment so I cant check the details .

The one thing about tough jobs is , if you persevere and finally get it fixed the feeling of frustration instantly turns to satisfaction .

 

[Poc]

But 75V seems tiny for a B+ voltage, but I don't know the circuit.

I don't think you will find an of the shelf Toroid with those 2 secondaries.

What I would do is one of 2 options:

1) contact Oktava and buy a replacement Toroid from them, they sell spare parts. This would be the best option

2) second option is to buy a transformer with 1x 75V secondary (probably 70V or 80V secondary works as good also) and then get another transformer a smaller one for with a 7,5V secondary and mount the 2 of them inside the PSU case

I wrote to Oktava and they told me they didn't have the transformer to sell.

I probably should go for the second option.

Thanks a lot for the advice guys