Tunnel Diode Replacement and Modification

Sweep Trigger TD Substitution

(This article originally found on Bill & Stan's Tektronix Resource Site by Stan Griffiths, http://www.reprise.com/host/tektronix/reference/tunnel_diode.asp, retrieved on April 11, 2021 from the Wayback Machine's March 13, 2018 snapshot. The article itself was last modified on 9/11/04 7:18:10 PM)

Tunnel diodes are getting hard to find, although some types are more rare than others. Many Tektronix oscilloscopes use tunnel diodes in their sweep trigger circuits. When these diodes fail, the entire instrument is useless until you find a replacement.

If you cannot find the Tunnel Diode you need, it it sometimes possible to use an alternate type. This page describes modifications that were performed on a Tektronix 547 sweep trigger circuit to use an available 4.7 mA Tunnel Diode in place of the impossible-to-find 10 mA original.

Our thanks to Morris Odell, VK3DOC, who devised these modifications and has allowed us to share them with you here.

Refer to the S/N 100-6739 547 A Sweep Trigger circuit diagram in the manual.

Step by Step Modification Instructions:

1. Replace the bad 10 mA Tunnel Diode (D245) with a new 4.7 mA Tunnel Diode (e.g. 1N3717 or Russian tunnel diodes).
2. Replace transistors Q234 and Q244 with silicon transistors having f_t around 200 MHz and Beta around 100, such as 2N4248.
3. Replace resistor R236 (30.1 kΩ) with a 47 kΩ, 1 W resistor.
4. Add new 30K 1 watt resistors in series with R247 and R248, which are each 15 kΩ, 2 W. (Total resistance is now 45 kΩ each).
5. Because the tunnel diode must be biased below its trigger point, there is less adjustment margin with a 4.7 mA Diode than with the original 10 mA Diode. This means that the values of R247 and R248 can be quite picky. Because of the variations between individual tunnel diodes, it may be necessary to use slightly smaller or larger resistances.

Morris points out that the B Sweep Trigger circuit is identical to the A Sweep Trigger. This modification should therefore be equally applicable for that circuit. Dave Wise believes that for S/N 6740-up, you would modify R236, R233, and R246. The goal is same node voltages with <4.7 mA through the diode instead of <10 mA.