OKTAVA MKL5000 / Electro Harmonix EH-TM Schematic

[cmuller0420]

I am trying to locate a schematic for these mics (they seem to be virtually the same)....The mic took a fall and had some wires loose and can find nothing online about these mics other than purchase prices which is useless.  If anyone can help it would be greatly appreciated.

Thanks

Chris

 

[zebra50]

I haven't been inside one, but I think that's a multi pattern version of the MKL2500.

I've looked at two of those and they had different circuits. If I recall right, one was a fairly standard plate follower, and the other one was a bit unusual. I traced the other one out and that drawing is here:

http://www.xaudia.com/xaudia/Schematics/Pages/Oktava.html

Maybe you could take some photos - someone here might be able to spot where the wires should go?

Stewart

 

[ArR]

I am working on a MKL5000 right now.

have worked on another too, different board versions but same circuit.
I have attached a first version of a schematic (may be a dumb error)

Can post photos when I get home.

Lucky, I didn't mod it yet  

Both the ones I have seen, the PSU says heater is 4V (way underheated) but can be set with a trimmer in the PSU.
They were set for 6.3V (can't say for sure if this was the factory set or otherwise)

 

[zebra50]

Thanks for that - looks like it should work.

I'm surprised to see the 2.2uF electrolytic cap at the rear capsule pattern selector. Might be a good candidate for an upgrade.

The trimmer for the backplate is an interesting idea too. But why not just match resistors? Isn't there a filter/stabilisation cap at the trimmer & 1 gig junction?

The MKL2500 was also underheated - set to 4.5V, fwiw. Sadly that mic got stolen. I remember it sounded quite good.

 

[rodabod]

I run my MKL2500 at 5V (I was lazy and sub'ed the 7806 plus a diode for a 7805).

Sounds great. Big bottom (ooh err).

 

[ArR]

I'm surprised to see the 2.2uF electrolytic cap at the rear capsule pattern selector. Might be a good candidate for an upgrade.

Surprised to see the cap there ? (could be my mistake ) or surprised it is electrolytic ?
The body is so huge, there is even room to replace the cathode bypass with a film cap!

The trimmer for the backplate is an interesting idea too. But why not just match resistors? Isn't there a filter/stabilisation cap at the trimmer & 1 gig junction?

I assumed it was to get a good match with the voltage from the pattern switch to get the omni spot on.
"Isn't there a filter/stabilisation cap" I made some assumptions while doing the quick sketch, I'll look again.
Also for 1Gig read 'some very high value resistor, too lazy to get my good meter'

The MKL2500 was also underheated - set to 4.5V, fwiw. Sadly that mic got stolen. I remember it sounded quite good.

They do sound good, I have a damaged capsule from a previous one, any one want to see it in bits ?

 

[zebra50]

Surprised to see the cap there ? (could be my mistake ) or surprised it is electrolytic ?

Surprised it's an electro, that's all.

The body is so huge, there is even room to replace the cathode bypass with a film cap!

Now that would be fun!

I assumed it was to get a good match with the voltage from the pattern switch to get the omni spot on.

That's right. I guess the extra part (trimmer) saves assembly time over matching the resistors and testing.

... any one want to see it in bits ?

Go for it!

 

[ArR]

The body is so huge, there is even room to replace the cathode bypass with a film cap!

Now that would be fun!

Fun indeed, been there already  see fuzzy phone photo of all film one from before.
This is the one that had the damaged capsule, so I went to town on it.

The 1G attached to the trimmer is on the board at the bottom and a looooonnnng wire runs all the way to the capsule.
Picks up noise and vibration very well indeed.
Moving it up to the other end improved things no end.

... any one want to see it in bits ?

Go for it!

Coming soon......  ;D

 

[cmuller0420]

Hey

Thanks for the schematic...It was the red (120V) wire that was not connected properly.  After reconnecting and checking all the voltages I could (ie voltages on rear for pattern selection)

Figure 8 - 118V
Cardiod  - 60V
Omin - 0V

The Plate has 60V at it, and the transformer has OV drop across it and approx 60V across the cap below it.  The Cathode has only about 1.5V across it

The Heater is set to 5.0V

I tested it without the main body (metal chassis) on and no metal cover over the tube.  There was some audio signal coming through but it was cutting in and out, and loud...very loud noisefloor that was crackling and at different levels (louder than audio level).  I lost the original tube and replaced with a 6ak5 which i believe is an acceptable replacement.  This is a 7 pin tube with no middle pin.  On the tube chassis there is a center pin, but looks like it has been used to act as a intermediate point to join the circuit common connections for the grid resistor and the cathode resistor and capacitor. 

On the tube I have:

Pin 1 - Grid
Pin 2,7 - Cathode (pin 2 has resistor going to pin 4 and pin 7 has 22uF cap going to pin 4 and pin 4 goes to common)
Pin 3,4 Heater  (3 has 5.0V, while 4 - has 0V and is connected to circuit common)
Pin 5 - Plate - connected to 200k resistor that goes to 120V
Pin 6 - Gs (suppresion grid?) is hard wired to pin 5


About a year ago I was out of town and someone who was using my studio knocked it over.  Before it worked great and since then it hasn't....go figure.  The diaphragms seem to be okay..no cracks or visible defects from the fall, but not sure if they could still be the problem or not? or something in the circuit....any suggestions would be greatly appreciated

Thanks for all the info so far

 

[mad.ax]

In regards to ArR schematic,

I'm surprised to see the 2.2uF electrolytic cap at the rear capsule pattern selector. Might be a good candidate for an upgrade.

I'm not that much surprised that it's an electro, since even the output coupling cap is an electro! I agree that it could be upgraded...
But I wonder how long does it takes to reach the desired voltage when changing pattern?

About the filter cap at the trimmer/1G junction... Well, the 2.2µF is only about 500K from there.

The trimmer saves on matching resistor indeed, but it may also makes up for capsule tolerance, or allows to dial in different polarization voltage in order to 'tweak' the sound...

@Cmuller0420
Don't worry about the middle pin, the original tube didn't have one, it's just there for convenient wiring...
I suggest to start with a good cleaning of the components and wires... High impedance circuits do not like dirt... Wait for all the voltages to fall down to zero, and use a qtip with isopropylic alcohol. Do NOT touch anything with your fingers once it's done!
Also, some tubes do not like underheating. that may be the case for your 6AK5. You could try to set the heater to 6V and see if that improves noise.

Axel

(edited a spelling mistake)

 

[zebra50]

But I wonder how long does it takes to reach the desired voltage when changing pattern?

Yes, it seems a bit too big and a bit too electrolytic, doesn't it? Conventional thinking would have something like a 33M or 100 Meg polarising resistor and then a 10n or 100n polystyrene or other good film cap there. Not 470K and 2u2.

Chris, do check what Axel said, and also have a look at all the grounding, including in the cable. Try touching the mic body to the PSU body whilst it's running and see if things get better (or worse).

 

[Gus]

Work out what happens with the trim pot when switch to cardiod.


Next pattern switch change
5 RC
470k 2.2uf.  5 x (4.7^5 x 2.2 ^-6)about 5 sec then sometimes add another 470k so looks like about 5sec and 10 sec a change in pattern control depending on the what contacts are being switched

 

[mad.ax]

Gus, that's what I thought. But we can assume that in the PSU, there must be another electrolytic cap, much bigger than 2.2µF, can't we?

Axel

 

[ArR]

It was the red (120V) wire that was not connected properly.

A similar problem with one or more other connections to the XLR may be causing the trouble.

I don't know if it is the choice of wire or bad soldering but have found this to be a weak point of the mkl-5000.
Worth inspecting all the wires there, I found 50% of them hanging on by just a couple of strands or broken but just held in place by the heat shrink.
"wire just fell off" seems to be a feature.

 

[cmuller0420]

new update...I found that chassis ground and common were disconnected from each other, so connected, checked other connections and re-soldered some iffy connections. 

On figure 8 there is decently audible audio coming through but still alot of crackline/popping noise, and not a whole lot of audio coming through on cardiod or omni.  Some noise would be from handling, but not at that level, and it seems as the tube warms up the noise level is increasing and crackling more.

Short of replaceing all components and wires, im not sure what to do...would that be a good idea?

Oh when touching the mic to the PSU nothing changes

 

[Sonic]

I do not advise voltage of a lamp less than 5.7 volts.

 

[cmuller0420]

well reachedroad block....replaced all the wiring...double checked all connections...getting proper voltages across the plates, and tube has heater of 6V and plate has 58V

checked the cable and is good....all biasing seems to be in order

pulled out caps and resistors and measured, all checked out fine...but still buzzing and no real audible audio...seems like its a project for another day...such a simple circuit and so mayny problems

 

[zebra50]

...I tested it without the main body (metal chassis) on....

I just spotted this. With high impedance circuits like a tube mic you do need the metal body in place to shield from hum. Hopefully you've found that out through your won experiments. If not put the body back on and test again.

What about the capsule? Is that connected?

Maybe post some photos?

 

[mad.ax]

pulled out caps and resistors and measured, all checked out fine...

How did you checked 1Gig resistors? A standard multimeter will indicate the same "over-range" whether it's >20M, or 1G, or totally open. But if it's totally open, then you'd have no polarization voltage on the capsule, therefore 'no real audible audio'...

If all your other checks are good, you should have a working amp. A quick dirty test confirm this is to (gently) tap the 1000pF capacitor's body (use a plastic or wood stick) while listening the audio output. You should clearly hear it if the amp is working well (a bit like knocking with your finger on a SM57 body).
If you hear it, then your amp is working. That leaves either the capsule, or the 1Gig resistor (assuming you have proper polarization voltage before that resistor)

Axel

 

[ArR]

A compilation of pictures.
Note bottom view, another "wire fell off xlr" incident during photo.