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These parts are being operated in their linear region, so Rds(on) isn't relevant. As long as they can take the heat, what we need is agility, that is, minimal Ciss/Coss/Crss.
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From: [email protected] <[email protected]> on behalf of Chris Elmquist via groups.io <chrise@...>
Sent: Wednesday, April 2, 2025 11:18 AM
To: [email protected] <[email protected]>
Subject: Re: [TekScopes] horz output FETs in 2245?

On Wednesday (04/02/2025 at 05:47PM +0000), Dave Wise via groups.io wrote:

If I had a 2245 with your problem I'd be looking for... Hmm. Your parts are running naked, right - no heat sink? That's maybe 1W.

Yes-- they have no heatsinks and are spec'd at 3W max in the TO-39 can.

The left plate pair gets so hot I can't touch it but I realize that's not a
calibrated measurement.

An eBay seller has TO-92 VP0116 $15 for 10, glue on a copper fin. But nobody is selling VN1316 AFAICT.

There are also VP0120 and VN1320 which are similar devices with a higher
Vdgs (200V). These would likely work just fine too if they could be found.

Look at Mouser.com . They don't have any 160V P-channel RF MOSFETs so how about the regular ones? In 150V-300V select the lowest Ciss proxied as gate charge, that's Diodes Inc ZVP2545 in the SOT26-6 package. Put a copper fin on the drain pins and you should be okay.
For N-channel try Vishay SI3440 in TSOP-6. Second lowest gate charge and lowest Ciss.

Right. OK. Thanks. Ciss is the critical parameter then for performance
here? Was also wondering how critical the Rds (on resistance) would be
in this type of circuit? Modern FETs measure that in tenths of ohms
while these golden oldies measure it in 10s of ohms... eg, VN1316 is
at 40 ohm and VP0116 is at 25 ohm.

There's not a lot of clearance between the tops of these TO-39 and the
scope's enclosure though so some challenges with fitting new/different
stuff.

Could envision a little PCB that carries the SMT parts but has "leads"
coming out the bottom to mate up to the scope's PCB.

Chris

________________________________
From: [email protected] <[email protected]> on behalf of Chris Elmquist via groups.io <chrise@...>
Sent: Wednesday, April 2, 2025 9:28 AM
To: [email protected] <[email protected]>
Subject: [TekScopes] horz output FETs in 2245?


I was given a 2245 recently which of course doesn't work too well.

Investigation reveals that Q805 and Q806, in the horizontal right plate
output stage, are cold to the touch.

The service manual, section 6.2 (page 6-19) says,

The MOSFET output transistors (Q801 and Q802 left plate-- Q805 and
Q806 right plate) run hot. If either side is cold, it is defective.

So, that's me and now I need to find a couple of these VDMOS FETs, which
seem to have become quite rare.

They are Tek P/N,

151-1211 VN1316N2 N-Channel VDMOS FET, 160V, 250mA, 40 ohm
151-1212 VP0116N2 P-Channel VDMOS FET, 160V, 250mA, 25 ohm

Has anyone solved this problem with a modern replacement pair? and have
suggested substitutes? These are in TO-39 package which adds additional
challenge but I suppose TO-220 or equiv could be made to fit. Of course,
the real thing would be great too if there is a source.

Thanks for any leads!

Chris
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Chris Elmquist

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Chris Elmquist