Whoops

Oktava MK-219 problem
« on: November 26, 2020, 02:20:00 PM »
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

Re: Oktava MK-219 problem
« Reply #1 on: November 26, 2020, 02:31:19 PM »
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?
Khron's Cave - Electronics - Audio - Teardowns - Mods - Repairs - Projects - Music - Rants - Shenanigans

Gus

Re: Oktava MK-219 problem
« Reply #2 on: November 26, 2020, 02:53:09 PM »
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

« Last Edit: November 26, 2020, 03:39:35 PM by Gus »

Whoops

Re: Oktava MK-219 problem
« Reply #3 on: November 26, 2020, 02:59:06 PM »
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

Re: Oktava MK-219 problem
« Reply #4 on: December 05, 2020, 07:03:32 AM »
Did you fix it?

Whoops

Re: Oktava MK-219 problem
« Reply #5 on: December 05, 2020, 07:50:03 AM »
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


 

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