Oktava MK-219 problem

[Whoops]

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

But the Peak IDSS readings were with a VDs voltage of 3v, could the value increase if more voltage is used?
The Peak DCA75 has also a curve tracing function, I can connect it to my computer and maybe get IDSS reading at other voltages, I will try that.

I used J201 because that was what was recommended by RT in the project's documentation and page, but I also have plenty of other JFETs in my stash that I could use

Thanks Gus

 

[Gus]

What other JFETs do you have?

 

[Whoops]

I used the curve tracing feature of the peak and I measure one of the J201,
see if this readings are more useful than the Peak quick test.
I will check which other JFETs I have and let you know

 

[Gus]

Lets do some math

Assuming the values you used are not in ( ) and the voltages below are what you want. I am not sure what voltages you want to achieve and resistor values you used.

Using the last graph it looks like you will need about .5VDC difference between the source and gate for about .7mA Id

29.5VDC - 19VDC =10.5VDC
10.5VDC /15k = .7mA (drain)
.7mA x 4.4K = 3.08VDC (source) assuming no gate current

29.5VDC / (200k + 22k) = .132mA
.132mA x 22K = 2.90VDC (200k 22k node)

3.08VDC - 2.90VDC = .18VDC

3.08VDC - .5VDC = 2.58VDC

so you will need to adjust the 22K resistor value so the 200k 22k node voltage = about 2.58VDC

Khron did post to use a trimmer at the 22K position

I just added the math as to why

 

[Whoops]

Lets do some math

Assuming the values you used are not in ( ) and the voltages below are what you want. I am not sure what voltages you want to achieve and resistor values you used.

Hi Gus,
I used the values that are not in ( )
The voltages on the schematic are there for reference and troubleshooting

the values in ( ) are the ones used in the original Oktava MK-219 circuit, with the original FET ( R7 and R8 are tuned and written on the transformer)

So I would like to make it work with the J201 and values recommended by RT

thank you so much for all your help and patience

 

[Whoops]

Khron did post to use a trimmer at the 22K position

I already installed a 22K trimmer there, and it made no difference in the voltages measured at any position of the trimmer...

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

 

[Gus]

If you built it on the PCB there might be something wrong with the board or it has a solder splash causing an issue.

Did you build the fragment?
Did the 200k 22k node voltage move?

 

[Whoops]

If you built it on the PCB there might be something wrong with the board or it has a solder splash causing an issue.

I also though so, so I measured all components, cleaned everything, looked for solder joints, checked continuity in all connections and traces.
And nothing, everything was fine

Did you build the fragment?
Did the 200k 22k node voltage move?

not yet, because I haven’t been in the workshop since, but I will do it when I’m there

thanks

 

[Whoops]

Hi Gus I did a schematic drawing of the fragmented circuit to help me out



Let me know if this is fine, I will do this and test it next time I go to the workshop
Thank you so much for your patience and help

 

[Gus]

That looks good to find the DC voltages