Compaq CGA card, schematics?

A trick there is to solder short wires onto the points you want to monitor, plug the card in, and then probe the free ends of the wires. But this is only useful if you know what points are likely to be of interest, which you presumably don't

Are you sure it's the light pen header? The IBM CGA card has 2 header plugs on it, one fo the a light pen, the other for a modulator. The composite signal goes ot the latter.

Have you looked at the IBM CGA card scehamtic? Could any of it match up? The compostie video circuit for the IBM CGA card consists of : A shift register (1.5 'S74) to produce the various pahses of the subcarrier A 'LS151 mux ot select the corect phase based on the colour to display 4 buffers (half a 'LS244) to buffer the following 4 signals : Composite sync *which is not simply the sync signals from the 6845, there;s quite a bit of logic in the way, however this logic is also used for the outptus on the DE9 socket, the compoite sync signal is effectively the XNOR of those 2 outputs) ; blanking ; intensity (I bit); and the output of the mux The outputs of those buffers go to a resistor netowrk to produce the composite signal Which is then buffered by a 2N3904 emitter follower. There are no capacitors in the signal path on the IBM card. Assuming the Compaq card did work correctly at one time (this is not _certain_ of course), it would appear the fault must be around the reistor network/emitter follower stage (assuming that's what was used). Tracing that out shouln't be too hard. -tony