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I recenlty bought and restored a 7CT1N and have just made a simple adaptor for transistor testing.

Whilst everything works fine with AF devices, the set-up seeems unstable when testing higher-frequency devices (Ft around 300 to 400MHz).

1. Is this a common issue, and
2. do Tek place suppression networks in their adaptors?

John

Hi, if it is possible you can put 47 ohm resistors in series with
Base and collector and the instability will disappear.

骋枚谤补苍


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Hello John,

nice to hear about your successful repair.

I can confirm that the well-known use of ferrite beads is an effective method to prevent oscillations.
This is why I have used plenty of them in my own adapter, see


I might have gone over the top with it, but the little "puppy" I have built can test 2N5160 transistors (with an Ft of 500 MHz or higher) without any oscillations.

Cheers,

Magnus

On 11/28/2014 05:53 AM, magnustoelle@... [TekScopes] wrote:

Hello John,

nice to hear about your successful repair.

I can confirm that the well-known use of ferrite beads is an effective method to prevent oscillations.
This is why I have used plenty of them in my own adapter, see


I might have gone over the top with it, but the little "puppy" I have built can test 2N5160 transistors (with an Ft of 500 MHz or higher) without any oscillations.

Cheers,

Magnus

Magnus,

That's a heck of a nice adapter you put together there! Nice write-up too, and some very good pics to go with. I know what my next bench project is going to be. ;-)

Mark

On 11/28/2014 5:42 AM, 骋鰎补苍 Krusell goran.krusell@...
[TekScopes] wrote:

Hi, if it is possible you can put 47 ohm resistors in series with
Base and collector and the instability will disappear.

骋鰎补苍

A small ferrite bead also seems to work well, just slip it
over the base lead before inserting the part.
In a few cases you will need a bead on all leads.

Hi I agree with the advice about slowing feedback to the base circuit. Another thing is that I was concerned with the emitter connection on the 7CT1N. I never did find a solution but improving the emitter connection helps too.

Jerry Massengale

Thanks for all the feedback. As it transpires, I seem to have re-invented a couple of wheels! My adaptor is very similar to that described by Magnus.

Having spent part of my career taming class C rf devices, I'd already tried adding ferrite to the base drive, but this alone was not a complete solution. I've ended up with the following:

1. 22nF on the rear of the 7CT1N Emitter socket to the adjacent chassis ground.
2. Three beads on the Base lead of the adaptor
3. A multi-turn broad-band bead on the Collector lead in the adaptor.
(4. in case of RF FET testing, a switchable series R-C snubber Gate-Source) .

It's generallly bad practice to add reactive impedance to the emitter of a common-emitter RF stage due to posative feedback via the collector-emitter capacitance.

I've placed details in the files section. I'd still like to know if the Tek adaptors contain "special measures" !

John

On 29 Nov 2014 14:32:45 -0800, you wrote:

It's generallly bad practice to add reactive impedance to the emitter of a common-emitter RF stage due to posative feedback via the collector-emitter capacitance.

Ferrite beads intended for use as snubbers are lossy having low Q at high
frequencies.

I've placed details in the files section. I'd still like to know if the Tek adaptors contain "special measures" !

They did. I have uploaded a photo showing the three ferrite beads included in
the 013-0128-00 adapter for the 7CT1N.

Thanks David, confirms our hypothesis.

John