A 40 year old cruise control unit (sample and hold circuitry) has failed on me and I'm looking to replace a BSV81 transistor to see if it fixes my problem. This is an old n-channel depletion mosfet. A rundown of the cruise control unit it will be used in can be found here: https://jenniskens.livedsl.nl/Technical/Tips/A/MyTip1098.htm (The transistor I'm looking to replace is referred to as Q9, can be seen in Figure 2). In this blogpost it is also stressed that this transistor is very sensitive to static.

Unfortunately, I can not get my hands on an exact replacement from local suppliers and I am not too familiar with transistor characteristics to select a suitable replacement.

Does anyone know a replacement part for this transistor? The transistor package is irrelevant to me.

  • \$\begingroup\$ Depletion mode FETs tend to be rarer than hen's teeth. Without trying to fully understand cct operation, as Q9 is AC coupled, adding some bias to the gate of an enhancement mode FET may achieve the same result. || Q9 appears to be a source follower in a feedback loop (closed by K2). Purpose is perhaps to buffer loading on opamp output from R76 and R56 (fig 3). SO a bipolar NPN with some playing may work. \$\endgroup\$ – Russell McMahon Mar 25 at 12:28

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