Author Topic: Tunnel Diode is it dead???  (Read 798 times)

0 Members and 1 Guest are viewing this topic.

Offline Zenwizard

  • Regular Contributor
  • *
  • Posts: 71
  • Country: us
Tunnel Diode is it dead???
« on: March 01, 2019, 02:47:36 am »
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?

Offline Zenwizard

  • Regular Contributor
  • *
  • Posts: 71
  • Country: us
Re: Tunnel Diode is it dead???
« Reply #1 on: March 01, 2019, 03:56:17 am »
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.

Offline NWerner

  • Contributor
  • Posts: 13
  • Country: de
Re: Tunnel Diode is it dead???
« Reply #2 on: August 14, 2020, 08:17:29 pm »
Just my 2ct:

- those biasing resistors are very drifty.  Yours look perfectly fine.
- check whether the termination resistor module is fine (see
- 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.

Offline srb1954

  • Frequent Contributor
  • **
  • Posts: 267
  • Country: nz
  • Retired Electronics Design Engineer
Re: Tunnel Diode is it dead???
« Reply #3 on: August 15, 2020, 01:14:27 am »
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

Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo