Oktava MK-219 problem

[Di_Sousa]

Hello,
I modified my Oktava MK-219 with a PCb from Russel Technologies called RT-219.
Basically it's an higher quality pcb than the original and based on some small changes to values in the circuit as suggested by Scott Dorsey and a few others.

I built it and now I have very low volume, I had to bring the preamp gain all the way up.

I checked all cap values and Lytic cap orientation, resistor values, FET leg orientation - everything is fine.
Checked all connections to ground - fine
Checked all connection to Out transformer and from it - all fine
followed the schematic and checked all connections between components - all fine
changed the capsule for another working one - the same behavior
changed the FET - the same behavior

I'm out of options to solve the issue and I have no more ideas at the moment of what the problem might be.

In the RT-219 schematic there's references voltages for the circuit and my voltages are way off, I wrote my voltages in Blue and Red in the following schematic:

Any ideas of what the problem might be?
Any other troubleshoot I could do?

Thank you so much

 

[Khron]

Try using a trimmer (10-turn if possible), 20-50k, instead of R4.

Going by those voltage readings compared to what they "should" be, the JFET is passing way too much current at idle.

Even for a single part number, JFET specs can span orders of magnitude (even 1:10 between min and max for Idss or Vgs(off) ), so even just trying another J201 might get you closer to the goal. But a trimpot would offer the most flexibility, i'd say.

Given the voltage readings with the capsule "shorted out"... Are you 10000% sure that the resistor you used as R1 is indeed 500meg/1Gig? And/or that C2 isn't shorted internally or something?

 

[Gus]

Adding the capsule should not change the voltages
Open T4 and T5 and measure voltages without the capsule
Then lift one end of C2 and measure voltages again
report back

Remember the more - the gate to source voltage the JFET is more off. The closer the the gate the source voltages are the more on the JFET is. If C2 is conducing you might have the problem you are reporting Khron posted about C2 above

using a J201? Do you have a real J201? Did you measure it before you used it?

read the J201 spec sheets IDSS Vgs etc.

IIRC The numbers written on the transformer should be the two resistors selected for the stock JFET
Nothing wrong with the stock JFET.

Also it is a mixed bias circuit fixed and source bias. The two resistors in the source are are part of the bias and gain the voltage divider R3, R4 is the fixed part

 

[Di_Sousa]

Try using a trimmer (10-turn if possible), 20-50k, instead of R4.

Thank I would do that, I actually tried a Pot but it was a 22K pot, from 0r to 22K there was no difference in the voltages. Maybe need to go higher

Given the voltage readings with the capsule "shorted out"... Are you 10000% sure that the resistor you used as R1 is indeed 500meg/1Gig?

It's 1G, Completely sure.
Those were pretty expensive

And/or that C2 isn't shorted internally or something?

I can desolder it and measure

 

[Gus]

Did you fix it?

 

[Di_Sousa]

Hello Gus,
I still didnt had the chance to go back at the workshop, I'm mixing a really difficult record and it's taking all my time.
But next week I will go there and work on it, thank for asking.

If you have any suggestions please let me know

 

[Di_Sousa]

Hi friends,
life got complicated last year, and this project was put on hold.
Fortunately I got back to it.

I already replaced the capsule and transformer, just to exclude those possibilities, and with other capsule and transformer the problem is the same.

I really think its something to do with the FET,
I would like to ask you how can I bias the FET in this circuit?
Whats the method/process I could do to bias the FET?

Thank you so much

 

[Gus]

Did you change C2 or remove it and then install the capsule to check voltages again?

 

[Di_Sousa]

Did you change C2 or remove it and then install the capsule to check voltages again?

Hi Gus,
yes I removed it and tested it, it tested perfect. I replaced it anyway.
Voltages are the same.

Are R7 and R8 the Bias resistors?

Thanks

 

[Gus]

R3, R4, R7, R8 are part of the bias and gain

IMO a J201 is not a good JFET for this circuit. Measure your JFETs

According the RT marked up schematic you want 10VDC across R6
10VDC/15K =.67mA
Look up the IDSS range of a J201 and compare it
Then look up ID vs Vgs
Look for the "Vishay AN102 app note" for more information

 

[Di_Sousa]

IMO a J201 is not a good JFET for this circuit. Measure you JFETs

why is that Gus?

thank you

 

[Gus]

Look up and and measure the JFETs IDSS
compare IDSS to ID in circuit
Also look up gate capacitance

 

[Di_Sousa]

using a J201? Do you have a real J201? Did you measure it before you used it?

read the J201 spec sheets IDSS Vgs etc.

I both the J201 from a reputable shop, I don't have any reason to think they're fakes but I uderstand there's some fake FETs being sold so I will check that , thanks

It's branded Fairchild, I measured the J201 before using it with a Peak Atlas DCA75, it measured as an N-Channel JFET

I don't know how to interpret the measurment values but I compare it with the spec sheet.

 

[Di_Sousa]

Hi,
I measured the 2x J01 JFET units I have with both the Peak DCA75 and the MK-328 tester,
here are the results. I don't really know how to interpret this results so please help me out on this, I'm just learning on this subject, thanks

 

[Gus]

ID should be less than IDSS at the same D to S voltage

If the correct voltage drop across R5 is 10.5VD and R6 = 15K
10VDC/15K =.67mA

The peak might be useful what voltage(s) does the peak test IDSS at? EDIT it has that as 3VDC
The MK-328 is almost useless for microphone building

Note the range of IDSS
https://www.mouser.com/datasheet/2/308/J201-1306636.pdf
Do you have a breadboard to prototype circuits?
If so
build a fragment of the circuit on the breadboard this makes it simple to change resistor values and the JFET if leaded
You can use a 1meg for R2 this fragment is just for testing
you will need a power supply set to 29.5VDC or wire up the dropping resistor to phantom R10. I would start with 22K or higher
you will need R3 R4 R6 R7 R8 and a JFET(s)

You want to check the DC voltages at the drain, source and R3, R4 node

You are checking/setting up the DC operating points with the test circuit

R7, R8 could be a fixed resistor in series with a trim potentiometer wired as a variable resistor
Select a fixed value for R3 vary R4
The voltage difference between the gate(R3, R4 node voltage) and source voltage is what sets the bias

IIRC the transformer had two resistance values penciled on them that were selected resistor values for the JFET used.

 

[Di_Sousa]

The peak might be useful what voltage(s) does the peak test at?

Sorry Gus, there was part oh the photo missing, please chek the table again, I edited the post

 

[Di_Sousa]

IIRC the transformer had two resistance values penciled on them that were selected resistor values for the JFET used.

I know that the transformer have the R7 and R8 values written in it, although I bough my mic used and there's no values written then.
Anyway those values are for the original PCB with original FET.

I'm using a replacement PCB (Russel Technologies RT-219) and J201 FET, so those values would be of no use in this case.
I need those values but for the J201 JFET

Thank you

 

[Gus]

Look at the IDSS meaursed on the Peak and then work out the ID in the circuit using ohms law.
The ID looks to be higher than the IDSS the Peak measured
I would measure the IDSS of the JFETs with a 9VDC battery and a current meter. Look for DIY JFET testers

I would build a circuit fragment like I posted and test if you can get something to work

 

[Di_Sousa]

Do you have a breadboard to prototype circuits?

Yes sure I have it,
I will build part of the circuit in the breadboard ad you suggested no problems

you will need a power supply set to 29.5VDC

Fine, I can set my bench PSU to 29.5VDC



So just to be sure,
it's this the fragmented part of the circuit I should build in the Breadboard?
(with 1M fro R2 instead of 1G)

 

[Gus]

Yes
You don't need C3 you are looking for DC voltages

R7 and R8 can be something like 1.8K with a 5K trim in series for 1.8K to 6.8K. I would set it to the 4.4K to start if the values are the RT suggested values.

Set R3 at 200K and adjust R4

Now I don't think your J201s are going to work because .67mA is higher than the J201s IDSS readings from the Peak tester