Tunnel Diode is it dead???

[Zenwizard]

I am working on a Tektronix 284 pulser and the tunnel diode is acting strange. There was a very intermittent short in it and I have seen it pulsing a little. In digging further ALL the biasing resisters are out of spec. The 2 series resisters in the back of the air line are 739.9 Ohms they should be 660 Ohms 5% the front resister is 797.0 Ohms it should be 750 5% closer but still out from the 5% spec. The air line termination resister is spot on at 54 Ohms. With the bias controls the wave shape will change but the square wave is messy not a clean step I would expect. My question, is the tunnel diode is still good? I did a diode check on it and I got .6 V in both directions so it is not dead shorted. But this is to be expected from a TD. This is the infumas D180 from Tektronix. I can source a new part if it need to but it is $250.00 Each Yikes. Is this worth fixing or should I put it up to a learning experance?

[Zenwizard]

Attached is the drive voltages for the diode. From the service manual this looks a little light especially on the top side. The second attachment is the scope with measure turned on. From the service manual it looks like there should be a + 13 V and a - 13 V square wave that are in time in a push pull config in to the diode.

[NWerner]

Just my 2ct:

- those biasing resistors are very drifty.  Yours look perfectly fine.
- check whether the termination resistor module is fine (see https://www.amplifier.cd/Test_Equipment/Tektronix/Tektronix_other/284.html)
- to test your tunnel diode :
    - mount it in some kind of spring loaded test fixture
    - apply a 1KHz square wave with eg. 10% duty cycle and cyrying amplitude. You should experience a Ip of approx. 20mA.
    - a diode tester is useless as a tunnel diode conducts in both direction.


If youre tunnel diode is fine you should be able to resurrect your pulser back to life by fiddling with drive voltage amplitude.

[srb1954]

Check the internal connections within the air-line. These are prone to tarnishing and that may be affecting the resistance values or tunnel diode currents