Re: First mixer diode on a 7L18 : how to test (and possibly replace) ?

Tom Lee

Hi Jose,

I recommend that you add "check the attenuator" to your to-do list. Attenuators often act as very expensive fuses.

The mixer diode, whatever it is, will be quite delicate. I've never owned a 7L18, so I have no idea whether the mixer diode is replaceable without special tools. If it is, a possible substitute would be something like a BAT14-104 (a Siemens/Infineon part, IIRC). I've used that diode as a Ka-band mixer, and it works well. They are not cheap though, and easy to damage or kill by casual handling (the capacitance is only around 100-150fF, so when charge transfers from your 100pF body, the voltage can multiply upward by orders of magnitude).

If you get a chance, please take pictures and upload them. I'm very curious to see what the internals look like in that unit.

Good luck!

-- Cheers,

Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070

On 1/14/2021 00:58, Miguel Work wrote:
Wow, a 7l18 I want one!

Can you please upload some photos?

I have some experience with single diode mixers, I have two tek491


-----Mensaje original-----
De: [] En nombre de Jose Luu
Enviado el: miƩrcoles, 13 de enero de 2021 23:46
Asunto: [TekScopes] First mixer diode on a 7L18 : how to test (and possibly replace) ?

Dear group,

I am fixing a deaf 7L18, it is responsive to a 510MHz signal injected into the second mixer, I still have to test the 510MHz IF, I am anticipating that the first mixer diode has failed (likely from a power overload).

Testing the diode: I suppose I should pay attention to the voltage applied and possibly limit the current, 1V and a few milliamps?
Replacement: the part number I find on the service manual is: 119-0972-02 , googling it does not fetch much, any ideas, is surgery oof the A45 unit possible ?

Thank you for any insight

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