OKTAVA MKL5000 / Electro Harmonix EH-TM Schematic

[cmuller0420]

I tested the amp by tapping on the resistor...and got an audible noise as yo described (tapping 57 body) so it seems that the amplifier circuit is working, to test the 1 gig resistor I saw that it was over the range of the multimeter...soo that could be a problem, but which 1 gig resistor are we talking about, the one coming up from the wiper of the pot or the one after the cap going to common.

When you mention proper polarization voltage, what would that be? listed below is what i measured at the capsule

As for capsule polarization, the rear will get 0V, 59V, 118V depending on pattern selection, the front is only a few millivollts, and the center (this goes to the amplifier circuit) has about 1.25-1.5V on it. 

Oh i do have a quaestion on the schematic posted above.  At the potentiometer it says to set the voltage to B+/2 (approx 60V) is this indicating at the wiper of the pot, or between the pot and the 10K resistor?

 

[cmuller0420]

Oh one other question...is there a way to test out the capsule to confirm if it is the problem? I am not sure how you could do this.

In my previous post, I meant tapping on the 1000pF capacitor!! not resistor

 

[ArR]

At the potentiometer it says to set the voltage to B+/2 (approx 60V) is this indicating at the wiper of the pot, or between the pot and the 10K resistor?

At the wiper of the pot.

I realy should have said trim to match rear voltage when in omni position, so 59V in your mic.

 

[mad.ax]

So we know that the amp is working. In other word everything right of the 1000pF is OK.

The most important 1Gig resistor is indeed the one between the wiper of the pot and the front of the capsule (the one left of the 1000pF on the schematic)
You first need to check what voltage is there on the wiper of the pot. Should be around 59V as ArR said.

You cannot check polarization voltage with your multimeter. That's because impedance there is 1GigaOhms, so when you touch those points with your multimeter, you load the ckt with your multimeter's impedance which is probably 20M, or 50 times less!

If you do have 59 or 60V at the wiper, then it's either the 1G, either the capsule. There's no easy way to check the capsule, except wiring it to a known working microphone. So the easiest method is to try another 1Gig (order two, just in case).

Can you post a close up pic of the resistor? Or tell us what the color bands are? I'd like to be sure of the value...

Axel

 

[mad.ax]

Looking at the pics previously posted, it seems that the 1Gig resistors are the blue one (one on the tube socket, and the other one near the pot)
The value should be  written on them... Can you look and tell us?

Axel

 

[cmuller0420]

The color bands are not existant and there is no visible label on the resistor, so I will grab a couple 1gig resistors and replace that one to rule it out.  If that is replaced and still not working I guess I will have to try anothr capsule.

 

[mad.ax]

The thing is we are not sure that those resistor value is 1Gig. It could as well be 100M for all we know... And putting a 1Gig in a circuit that calls for a 100M would not help you with the noise!

Can you see any difference between the two blue resistors?

1Gig or 100M resistors are expensive animals. If you don't mind a 10 minutes experiment, you could measure it... What is the highest resistor value that you have at hand?

Axel

 

[cmuller0420]

Both resistors look the same...here is one thing im thinking...the mic worked before the fall, perfectly(as in full freq response and low noise) after the fall there are these problems.  No resistors were changed at any point and the mic probably fell 5-6ft.  With that in mind and the amplifier circuit functioning...im leaning towards capsule damage. 

The largest resistor value I have right now is 10 meg.

Is there other capsule options to consider? since I think that it needs to be replaced or where to source an original replacement? oktava-usa only seems to list the mics themselves.  Tomorrow I will track down a 1 gig resistor or two and check that...and then will know for sure that it is the capsule.

Thanks to everyone for your contributions on helping solve this problem.  Very helpful!

 

[mad.ax]

Chances are that capsule damage from a shock would be visible...
You may try to reverse front and back and see (hear) if it work in cardioid mode.

High value resistors can be damaged from a shock.
In order to measure them (if you've been lucky enough to have only one broken), you can make a simple voltage divider with your 10M and 120V mic PSU. Your standard multimeter will be accurate enough.
Go there:
http://www.raltron.com/cust/tools/voltage_divider.asp

and solder the 10M and the blue resistor in serie, connected to the 120V PSU. According to the small linked schemo, Vin is the PSU, R1 is the resistor to test (the blue one), R2 is the 10M, and Vout is where you connect your voltmeter. Respect safety rules as there is 120V around!
Enter the known values, Vin, R2, and Vout that you read on the voltmeter, and click 'compute'. You now know the exact value of your blue resistor.

Repeat the operation with the other blue resistor. 10 minute of work at worst, and you'll have a definitive answer...
Plus we finally get to know the correct value of those mystery resistors to include in the schematic

Axel

 

[ArR]

Ah dropped ......

The capsule orientation locating pin is at the back when you unscrew the capsule.
Once you have taken the top off the mic body it's easy to re-assemble with the whole thing back to front.
this will give the symptoms you describe.
(I have done this more than once when working too quickly, a real doh! moment)

One of the spring pins may be jammed and not pushing up correctly on the contact post above.

lastly, the capsule is held in the grille assembly by 3 metal saddles tensioned by grub screws on the out side.
The capsule is insulated from these saddles by some folded layers of plastic. (looks like plastic shopping bag material)
there are only a couple of layers.
Dropping the mic could cause the sharp corner of the capsule to cut through the plastic and short the capsule to ground.
This is more likely if the screws were tightened to hold the capsule very firmly.
I have replaced the thin plastic film with kapton tape for now but intend to replace the saddles with plastic blocks (PTFE or PEEK if you can get it)

if this sounds like guesswork ...... it is

 

[mad.ax]

ArR, those are very good points! You obviously have gone deep into this mic to make it work as it should...
Hopefully our friend will not end up buying a new capsule...

Looking one more time at the schematic you posted, I thought of an easier trick to measure the grid high value resistor without (de)soldering anything.
Unplug the tube.
Insert a 10M (or whatever highest value resistor you happen to have at hand) between grid and plate (pins 1 & 5 of the tube socket).
Measure voltage between ground and grid.
In the voltage divider calculator linked above, add 200000 to the value of R2.
Done!

Axel

 

[ArR]

Good measurement tip Axel, thanks, I will try that.

I should get a chance to do some work on the mic over the next couple of evenings.
(it was my sons birthday over the weekend, so other priorities)

I will confirm circuit diagram, value of high value resistors and get some pics of capsule.

Simon

 

[cmuller0420]

Over the weekend I will check that resistor....I didnt know that they could fail from a fall.  Good to know.  Thats the stuff we didnt cover in school.

I will also check that insulating layer as well as the pins that connect the capsule...and its orientation.

I would prefer not to buy a capsule if it can be avoided!

Thanks again

 

[ArR]

Capsule take apart.... for the records 

the take apart one with the damage is not the one in the mic btw
All brass construction.

 

[zebra50]

Hi ArR,

Nice capsule pictures - thanks for sharing that!

Stewart

 

[Whoops]

Hello everyone,
working on an MKL-5000 at the moment.
Tube is 6AK5

I measured the PSU voltages and like it's been said the Heater voltage is 4,5V.

I would like to ask you is there any reason why Oktava choose 4,5V for the heater instead of 6,3V?

Why do you increase the Heater voltage to 5V but not to 6,3?

Thank you so much

 

[Whoops]

Hello everyone,
working on an MKL-5000 at the moment.
Tube is 6AK5

I'm measure the PSU voltages and like you said the Heater voltage is 4,5V.

I would like to ask you is there any reason why Oktava choose 4,5V for the heater instead of 6,3V?

Why do you increase the Heater voltage to 5V but not to 6,3?

Thank you so much

Hi friends,
anyone could provide some help to the questions above?

Thank you

 

[Whoops]

Also for 1Gig read 'some very high value resistor, too lazy to get my good meter'

The 1G attached to the trimmer is on the board at the bottom and a looooonnnng wire runs all the way to the capsule.

About the filter cap at the trimmer/1G junction...

How did you checked 1Gig resistors? A standard multimeter will indicate the same "over-range" whether it's >20M, or 1G, or totally open.

to test the 1 gig resistor I saw that it was over the range of the multimeter...soo that could be a problem, but which 1 gig resistor are we talking about, the one coming up from the wiper of the pot or the one after the cap going to common.

The most important 1Gig resistor is indeed the one between the wiper of the pot and the front of the capsule

You cannot check polarization voltage with your multimeter. That's because impedance there is 1GigaOhms, so when you touch those points with your multimeter, you load the ckt with your multimeter's impedance which is probably 20M, or 50 times less!

If you do have 59 or 60V at the wiper, then it's either the 1G, either the capsule. There's no easy way to check the capsule, except wiring it to a known working microphone. So the easiest method is to try another 1Gig (order two, just in case).

Looking at the pics previously posted, it seems that the 1Gig resistors are the blue one (one on the tube socket, and the other one near the pot)

The thing is we are not sure that those resistor value is 1Gig. It could as well be 100M for all we know... And putting a 1Gig in a circuit that calls for a 100M would not help you with the noise!

Can you see any difference between the two blue resistors?

1Gig or 100M resistors are expensive animals. If you don't mind a 10 minutes experiment, you could measure it...

Hello,
this is al old thread, but still very helpful at the present for anyone repairing or servicing an Oktava MKL-5000.
A lot of good information around here although there's some mistakes also.

The resistors ArR said he didn't measure correctly and that he said was a very "High Value Resistor", and wrote in the schematic "1G" for it's value it's not correct. I just measured those resistors and they are 430 Mega.
So if reading this thread at this point, every time anyone mentioned "1G" replace that for 430M, also when checking the schematic.

I measured it using the method in this website, which even has it's own calculator:
https://www.giangrandi.org/electronics/gigaohm/gigaohm.shtml

 

[Khron]

Then again, is the difference between 1G and 430meg, in this circuit, really that massive? Technically or audibly?

 

[Whoops]

Then again, is the difference between 1G and 430meg, in this circuit, really that massive? Technically or audibly?

Did anyone say that?