Simplistic NJFET RIAA

[merlin el mago]

Wrong channel
C5 not connected to GND & R5 connected to R7
R7/GND 0.68V
Drain Q5/GND 10.50V
If I have to change Q6 I only have one SK170BL max. 10.02 Idss, can I use it?

Ok channel
R7/GND 10.14V
Drain Q5/GND 21.4V

 

[Salas]

OK channel perfect. You can use your spare as Q6 no problem. But double check and fix everything for connections first. Use the OK channel as total prototype to review and correct the wrong channel.

 

[merlin el mago]

Ok tomorrow I will check twice the connections & after I will change Q6.

Changing Q6 will increase the voltage between Drain Q5/GND to 21V?

 

[Salas]

No, Q6 has to do with feeding the LEDs only. When you will fix ''Wrong channel
C5 not connected to GND & R5 connected to R7'' there is chance 2nd and 3rd stage will work if nothing had burned. Leave Q6 change last after you review every tiny connection from in to out and voltages will be measured once more.

P.S. Did it ever get more than 45V during your shunts problems?

Anyway, Merlin The Wizard in the end can lift spell.

 

[merlin el mago]

Seems that the dual njfet lsk389 is bourned, due to the fact after connecting C5 to GND & disconnect R5 from R7 I measured R7 to GND 0.68V & Drain Q5 to GND 10.50V. To change lsk389 I have two options: one lsk with 11.23 Idss + 11.29 Idss or other lsk with 9.74 Idss + 9.72 Idss, wich one could I use it?

Can't remember during shunt process if voltage exceeded 45V.

Yes, always I have the last chance in the end to lift spell.

 

[Salas]

Changing Q6 will increase the voltage between Drain Q5/GND to 21V?

No, Q6 has to do with feeding the LEDs only.

 

[Salas]

Seems that the two njfets lsk389 is bourned, due to the fact after connecting C5 to GND & disconnect R5 from R7 I measured R7 to GND 0.68V & Drain Q5 to GND 10.50V. To change lsk389 I have two options: one lsk with 11.23 Idss + 11.29 Idss or other lsk with 9.74 Idss + 9.72 Idss, wich one could I use it?

First option.

 

[merlin el mago]

Thanks & good night.

 

[merlin el mago]

After checked 4 times the connections I replaced the dual LSK389 (Q2 & Q5) for the new LSK389 & the voltages follows the same Drain Q5/GND 10,50V & R7/GND 0.68V.

I suspected about the resistors values for R9, R10 & R11 so I checked:

R9 100R4 instead 100R
R10 2K37 instead 2K4
R11 6K12 instead 6K2

 

[Salas]

Is your trimmer under Q2 connected and set as in the working channel?

 

[merlin el mago]

Yes

 

[Salas]

What is the voltage on Q2's drain now?

 

[merlin el mago]

0.2v

The input voltage at the phono RIAA is between 12.1-12.3V instead 45V

 

[Salas]

So you got some problem with its individual channel shunt? Try the working's one.

 

[merlin el mago]

I do the measurements with the same regulator

 

[Salas]

So something is overloading it or upsets it. What is different to the other good channel where the regulator touches the audio perfboard? Some connection, some small capacitor is there or not, the wires are mixed, something must be different to the other channel.

 

[merlin el mago]

I have extracted the small 0.1uF cap but still continues the same voltage. I follows looking for bad connections.

Is possible wrong Q6 do that?

 

[Salas]

Haven't experienced such a failure but Q6 is right at B+ so I would check that out too.

 

[merlin el mago]

I AM VERY VERY HAPPY.

Only was necessary to change Q6 now I have:

LEDs 7.33V (return original colour: after orange, now very nice yellow)
R17 347mV
R10/Q2 10.06V
R1/Q3 11.5V
R7/GND 10.21V
Drain Q5/GND 21.1V

Now I have only to wait a couple o day the new Tx from Mouser so finally I will hear the heaven

 

[Salas]

Congratulations. Pictures alight please.

Why don't you just use a single reg for both till then? Most phonos made are not double mono for PSU. Will be near best and you could run it in a bit until you got the extra Tx.