An illustrated guide to building an F5

[IZHAKKATZ]

resistance in R4 is drifting and not stable

 

[AudioSan]

yes. all that is spot on.

 

[AudioSan]

hmmm. it drifts even when you dont turn the pot?

 

[IZHAKKATZ]

yes it does

 

[AudioSan]

thats not right.

 

[IZHAKKATZ]

I've checked with two meters- its keep going up (resistance of R4)

 

[AudioSan]

do you use the 4.7K thermistors?

 

[IZHAKKATZ]

yes I do

 

[AudioSan]

and they have good contact with the fets?

 

[IZHAKKATZ]

had a chance to check only now
one leg of Q2 does contact one leg of TH2

Anything else I can check?

 

[Itsmee]

resistance in R4 is drifting and not stable

Thats probably due to the thermistor being in parallel with P2//R4.

The bias problem is unusual, people usually have problems biasing the IRFP9240 (Q3).

If you can adjust P2 to maximum resistance and are not getting any voltage across R12, what is the gate-source voltage of Q4? If its more than the gate-source voltage of Q3, Q4 is probably dead.

 

[AndrewT]

.................. now one leg of Q2 does contact one leg of TH2

The legs/leads of the thermistor should be electrically insulated.
The body of TH should touch the output device.

 

[IZHAKKATZ]

Hey guys
Thank you so much for trying to help !

I followed your steps itsmee:
P2 to max, 5.3K
V on R12 is "0"
and like you said, gate to source V (between the outer legs) is higher on Q3 (3V compered to 0.006V on Q4)

I have no idea how Q4 could have died...

Thanks again, hopfully things will work after I'll get a new IRFP240

 

[bobodioulasso]

You need to check your input jfets prior to replace mosfet.

 

[bobodioulasso]

For instance:

 

[Itsmee]

You need to check your input jfets prior to replace mosfet.

That can be done using some rough and ready checks with the JFETS in circuit.

DC check.
If there is a voltage across R1 & R2, Q1 & Q2 are conducting current, the voltages should be approximately equal.

AC check.
Fit a shorting link across the gate-source of Q3 & Q4, set P1 & P2 to mid point resistance, apply a signal to the input and scope the drains of Q1 & Q2 (or measure with a multimeter set for AC volts).
Reset P1 & P2 to minimum resistance before removing the gate-source shorting links.

 

[bobodioulasso]

If you short g-s of Q3 and Q4 you also short the pots (across 47 ohms).
What's the point?
Better to unsolder the mosfets to check jfets in circuit.

 

[Itsmee]

Correct, you have ~47 Ohms to develop a signal across, the signal at the drains will be approximately 4.7 times larger than the input signal, this will show if the JFETS are working or not.

 

[a.wayne]

Hey do you guys remember who had built the 3 pr output F5 ..?

 

[Itsmee]

I belive it was Juma.http://www.diyaudio.com/forums/pass-labs/168040-f5-2sk2013-2sj313.html