Morelius21

problem whit ELA M 251E
« on: June 12, 2019, 06:22:36 AM »
Hi

i have a this mic and there are a component that i don't know what is it.

the first picture you can see the position in the mic section. The part is the first, begin in the left section


 

then the part



the colors are:

Green  Black  Violet         Yellow Black

A resistor can't be because there arnt tolerance black

A inductor whit five bands , the first must be silver.

A Capacitor maybe?

Any help please.

thanks for advance


Khron

Re: problem whit ELA M 251E
« Reply #1 on: June 12, 2019, 06:38:45 AM »
Can you measure it with an ohmmeter?

At least comparing the first photo with the schematic, and if my circuit node guesses are correct, that might(?) be (at least originally) the 30Meg resistor, between the triode gate and ground.

I *think* that white wire, from the lowest "pin" in the first photo, should go to the triode gate.  The bottom end of that fat silver 10nF cap should be connected to ground, and through the jumper-wire next to it, so should the "top" end of that "mystery" resistor.

More importantly though, what exactly are the symptoms?
« Last Edit: June 12, 2019, 06:47:09 AM by Khron »
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Morelius21

Re: problem whit ELA M 251E
« Reply #2 on: June 12, 2019, 09:29:05 AM »
Hi

The problem is that the sound is smaller than other mic the same model. 

When i try to mesure with my fluke meter in ohm and capacitance position or my cheap inductance meter the result is nothing or damage. 

this part go to pin 7 of the tube 12ay7 and the capsule. Whit the schema must be a resistor 30Megh o 8 Megh but the problem is that i don't understand the color code.

Thanks

Khron

Re: problem whit ELA M 251E
« Reply #3 on: June 12, 2019, 10:51:57 AM »
Pin 7 is indeed one of the triode grids.

http://www.mif.pg.gda.pl/homepages/frank/sheets/137/1/12AX7.pdf

Depending on what model your multimeter is, it might not be able to measure values that high - often the handheld ones top out around 2Meg or 10Meg.

Judging by the connections, that should be indeed the 30Meg one.

Either way, unless that resistor somehow lost value (ie. its value went down, by a LOT), that shouldn't really affect anything. Plus most often, when resistors fail, they fail open-circuit.

What's the "B+" voltage, coming from the power supply? You should be able to measure that between the common connection of the 100K resistor and one of the 499K ones (2nd and 3rd black resistors, counting from the left), and ground.
Around 110V DC is what we're expecting.

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Re: problem whit ELA M 251E
« Reply #4 on: June 12, 2019, 11:17:11 AM »
If you read as 4-band it's 50MΩ 5%.

Looks like 3rd band is blue. Violet would be 500MΩ.

In this position you can use anything from 30M to 250M with good results. Use higher values for more lowend.

musix2000

Re: problem whit ELA M 251E New
« Reply #5 on: July 20, 2019, 01:58:59 AM »
Telefunken have changed the values of some components in the ELA M251e (C12 also), for these you can not use the schematics of the old M251e from the sixties. The two components on the left and the right side of the ROHM capacitor are 100% resistors. About the codes I`d already wondered years ago and I still don`t have an explanation. But what I know: The resistor on the right side of the 10nf (10000pf) styroflex should have 10M (can you measure it?). The one on the left, I`m 99% sure, 50M. This resistor is there to keep the voltage on this place at 0V!! To measure the 50M with your multimeter you take a resistor with a value you can measure with your multimeter and then measure it parallel with the  50M (or whatever it is, you will find out). Then you can calculate the value of the unknown resistor. There is a lot of information in the web how to calculate resistors in parallel.
If the sound of your mic  is "smaller" as in other microphones of the same serie then these resistors are not the cause for it. Same I´ve seen in several M251e. Maybe your mic get`s to low voltage, did you measure the outputs of your powersupply?? Should be around 110VDC and 6,3VDC for the heater (measured with load!!).  You can do this in the microphone or/and the powersupply. Be very careful, there`s high voltage in the psu that could kill you!!
The components in the mic are much stronger as needed, I can`t believe that there`s a damage.
One question: the 10nf capacitor (ROHM) seems not to be the original by telefunken. The red ring on one side means 160V (not means  +!!), that`s still ok, but usualy 400V (or higher) are Telefunken standart. The red ring also shows the side with the outer foil and should be connected to ground. In your mic it`s connected to +.  Did you do this? If you or a friend did it: have you tested the capacitor for ESR, Vlost etc. before using it? Maybe this capacitor has a leakage (if not the original by Telefunken, as it seems). To measure this capacitor you must desolder it on one side. Cheap LCR-meters often can`t measure capacitors under 100 or 200nf. So don`t wonder if your inductance meter doesn`t show a result.
At last: do you use the original JAN GE 6072a or is there another tube inside the mic? There are only very few tubes that can be used in the M251e to get a good quality of sound. EH-tubes etc. will not bring the sound you expect from this mic and shouldn`t be used in such a high end microphone.
« Last Edit: July 20, 2019, 08:43:37 PM by musix2000 »


 

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