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  Sweep Trigger TD Substitution
   
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 4.7mA Tunnel Diode in place of the impossible-to-find 10mA original.
 
Our thanks to Morris Odell, VK3DOC, who devised these modifications and has allowed us to share them with you here.
 
The original 547 A Sweep Trigger circuit diagram is available here.
 
Step by Step Modification Instructions:
  1. Replace the bad 10mA Tunnel Diode (D245) with a new 4.7mA Tunnel Diode (Tektronix part number 152-0125-00) (available from Stan Griffiths).
     
  2. Replace transistors Q234 and Q244 with silicon transistors having Ft around 200MHz and Beta around 100, such as 2N4248.
     
  3. Replace resistor R236 (30.1K) with a 47K 1Watt resistor.
     
  4. Add new 30K 1 watt resistors in series with R247 and R248, which are each 15K 2 watts. (Total resistance is now 45K each).
     
  5. Because the Tunnel Diode must be biased below its trigger point, there is less adjustment margin with a 4.7mA Diode than with the original 10mA 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.

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Last modified on  9/11/04 7:18:10 PM