(Topic ID: 238086)

Proto F-14 Tomcat - Blown fuse after Q33 issue / U20?

By baseraider

9 months ago

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  • 2 posts
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  • Latest reply 9 months ago by wayout440
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#1 9 months ago

Proto F14 Tomcat here.

Transistor at Q33 recently blew after coil locked on.

Now if power is turned on; the fuse highlighted in the reference picture shows continuity to ground at right side. I noticed continuity to ground on pins 1J11-4 and 1J11-5 also. These conditions are only with power on.

Replaced the coil associated, Q33 and its pre-driver.

Since these all tie back to U20...Im assuming the 7408 @ U20 is most likely bad.

Any thoughts? Thanks in advance

image-A29 (resized).jpg
#2 9 months ago

Don't assume anything - troubleshoot first. Put the game in coil test and track the trigger pulse through the circuit with a DMM, logic probe, or oscilloscope. If anything, you wlll be able to make a more informed guess.

Screenshot_20190310-164000 (resized).png

BTW If you actually have one of the 10 prototype F-14 Tomcats with an "X" in the serial number, electronically they are no different. Just the cabinet paint and graphics finish is different.

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