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Hi all,

I managed to shot one channel of my SC502 when repairing an old TV set. It seems the victim is a dual FET in the input differential amplifier with TEK part number 151-1031-00.

To confirm its indeed the dual FET I paired two FETs out of a bunch of N4091 and managed to get them connected to the socket. A heavy challenge to my eyesight... see the pics here:
/g/TekScopes/album?id=261709
The scope works well with these FETs, but is very sensitive to hum and my approaching hand. So I guess a more compact solution is required.

Anyone who can tell how obtainium these dual FETs are, or if there are pin-compatible replacements?
I wonder if its feasable to pair two general purpose FETs in a TO92 casing and glue them against each other?

Thanks for any help, cheers
Martin

Hi Martin,

You can look at Tektronix parts cross-reference:
It's on page 7-7. Looks like it's a dual "same package" but not matched as there is no mention of that in the comment section.
Note that sphere.bc.ca also sells replacement for $10.

Best regards,

Martin,

You may see here:
(2 items)
Note that for pictures you may add comment for each photo.
(possible after with the edit function)
I think this is better than only for the album.

Regards

Alain
France

Hi Martin, would you think that the dual FETs Teledyne SU2369A would be
useful for your application I can send you a couple. Contact me off list.
G?ran

Hi all,

What is curious is that I tested the FET on my curve tracer, and it looked OK - both FETs display near identical curves (Id over Uds as well as Id over Ugs) and the curves do look normal and very close to spec. The only moment they differ is when I switch to "base open" on the tracer. The grid voltage of the good FET starts to float a bit towards positive with very high impedance, whereas the bad FET goes much more to positive and with much lower impedance... clearly shot!

See the photos section, I added some screenshots there.

cheers
Martin

That sounds like a pretty classic failure mode of a dual FET. I had one in
a 7A22 do just the same thing. It worked fine, but there was an
uncorrectable offset which made the more sensitive ranges unusable.

I had good success replacing it with a matched pair of ordinary J113 FETs
stuck together under the little plastic hat that Tek put on the socket. All
I did was look up the circuit's DC operating conditions of the dual FET
(what Id for what Vgs - I think it was 1mA and -2.5V but don't quote me)
and find a couple of single FETs that met it. I used a quick test rig with
an op-amp to measure a dozen or so devices. Then I had to add gate
resistors for stability, but that might not be such a problem for an SC502
which probably has a wider bandwidth than the 7A22. Set up offset and gain,
check risetime and noise, job done.

Chris