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Electronics => Repair => Topic started by: lowtension on March 30, 2017, 09:11:17 am

Title: Replacing N-Channel MOSFET 2SK972
Post by: lowtension on March 30, 2017, 09:11:17 am

Hello,

I am trying to look for a replacement of 2SK972, found a match  in a shop near me but a stp16nf06l. The 2SK972 hag diods between G and S but the stp16nf06l dosn't, does this matter ?
Attached specifications of each one.

Thanks

Title: Re: Replacing N-Channel MOSFET 2SK972
Post by: Ian.M on March 30, 2017, 11:02:38 am

The STP16NF06L has a lower gate oxide breakdown voltage (Vgs_max), very different S.O.A, and significantly different characteristics.  What's the application, what's the max drain current, drain voltage, and gate drive voltage? A partial schematic would help.

Title: Re: Replacing N-Channel MOSFET 2SK972
Post by: lowtension on March 30, 2017, 01:25:40 pm

Hi,

This MOSFET drives a Yig filter,  it acts as a current driver to the coil in the yig filter, Supply to the MOSFET comes from the Power Supply with spec:24V/2A.  Coil R=48 Ohm  Inductance=1.2mH.  For each sweep the current Changes dependent on the specific frequency, sweep can de down to 1us or up to 500sec. Hope this helps.
Thanks

Title: Re: Replacing N-Channel MOSFET 2SK972
Post by: Ian.M on March 30, 2017, 01:31:10 pm

So what's the schematic of the coil driver stage?  What drives the gate?

Title: Re: Replacing N-Channel MOSFET 2SK972
Post by: lowtension on March 30, 2017, 01:46:56 pm

Hello,

it is driven by an op amplifier, please look at Q25 for a similar schematics, output Point is FT-. FT+ to the coil is from 24V supply
Thanks

Title: Re: Replacing N-Channel MOSFET 2SK972
Post by: Ian.M on March 30, 2017, 01:57:54 pm

It looks like there are clamping diodes for the gate to a +15V raiil and to the drain.  The STP16NF06L is only rated for +/- 16V Vgs_max, so cannot be safely used in a circuit that clamps the gate at +15V + 1 diode drop.   Find a +/- 20V Vgs_max (or greater) MOSFET . . . .

Title: Re: Replacing N-Channel MOSFET 2SK972
Post by: lowtension on March 30, 2017, 02:20:40 pm

Thanks!!!!!

Title: Re: Replacing N-Channel MOSFET 2SK972
Post by: Bjoern on July 26, 2018, 05:46:37 am

Does it make sense to measure voltages at the FM+ and FM- against Ground or would I need to measure the current ?

I have also a problem with my YIG. I measure the following on the connector:
FM-  = -3.8mV
FM+ = -1.2mV the resistance between FM- and FM+ is 0.48 Ohm !
When I disconnect the connector FM+ measures +7.5 V (with an 139khz signal on top of the DC), this is with a span of >10Mhz. If the span is <=10Mhz the signal jumps hence the DC is unstable (coming from the analogue board)
Main Coil has 13 Ohm resistance and 13V as well as 14.95 V respectively
I cant get a signal on FM- with or without connection.

Do you see any indication that the YIG is definitely damaged (too low resistance on FM?)?
It seems I might have two issues, one on the YIG with FM and another with the <>10Mhz span fed from the analogue board ?

Would be great to get your insides.  BTW my HP is the 8590L, I attach the relevant schematics.