Hi everyone,

I've been struggling with this Space Shuttle issue and I am hoping someone may know of a possible solution.

Problem: Lap Row 3 does not pulse correctly, some lamps never turn off but dim. Video of said issue can be seen here:

If you see in the video the "U", "50,000", and 4X do not follow the light show. If you look at the lamp matrix you will see they all fall upon the same lamp row:

lampmatrix (resized).JPG

I've tested all diodes on all sockets and they all tested well with no apparent shorting to the base of touching any other metal.

In the schematics, here's the piece of IJ6-3:

Lamps (resized).JPG

Above, I've replaced the following:

* Q33 - TIP122 (original tested fine but replaced anyway)
* D13 - 1N4004 (was 1N4001) (was testing .3 in-circuit, but was fine out of circuit. When replaced with the 1N4004, it tested the same in-circuit)
* S3 (original did not test fine, replaced but problem still occurring)

All other resistors tested fine. I did not test the cap (C45 but I've never seen one of those fail).

I worked my way back to U59 (7406). From my logic analyzer, everything "seem" to be OK. I compared/contrasted to other pins and seem to have a similar result.

Next, I worked my way back to the 6821. Here I did notice a difference. It seems that PA2 with a logic analyzer was pulsing, but the LOW indicator never completely turn off but dimmed (much like the lamps). When I compared this with PA1 and PA3, I noticed that the LOW indicator would turn off from a pulsing perspective.

I socket-ed and changed the 6821 with another 6821 that is confirmed working, but had the same result.

So, I'm totally out of ideas. Anything I am missing or should look out for?

I just rechecked all diodes on the entire row and also checked continuity and everything looks good. Any ideas?

Problem solved. Somebody had replaced a controlled lamp with a 2 lead socket and not a 3 lead socket! Therefore the diode wasn't installed correctly!