Video AGC auto/manual modification to PC23C
(version 2)

The following modification has been "developed" (hacked is a better word) by me by trial and error on a borrowed camera, since both of mine are "version 1". Like the version 1 modification, it appears to prevent the video gain on the camera from rising as image intensity falls. By probing the circuit board, I have discovered that grounding a particular point on an unidentified component through a 330K ohm resistor in parallel with a 4.7 uFD capacitor clamps the video gain at essentially "unity". Since the imager itself still controls the shutter speed, the result is that images are much more correctly exposed, without the decrease in signal to noise ratio caused by boosted video gain on small bright objects. At prime focus on my 8" f10, I need no filters at all, and the resulting images of (so far) the moon and Mars are well-exposed.

The camera is in three parts (kinda reminds me of Latin 101.... sorry). The front and rear castings are attached to the center sheet metal with a couple screws at each end. Remove the ones from the front, and the camera will separate with the front casting loose and the enclosure still attached to the rear casting.

Make up a parallel combination of a 330K ohm resistor and a 4.7 uFD capacitor (electrolytic is OK, at least 20V recommended). Using the photos below, locate the yellow wire running from the rear circuit board to the front circuit board. Carefully un-solder the yellow wire from its current location on the front board, leaving it attached to the rear board. Solder the end of the resistor / capacitor combination to the upper end of the indicated component with the "+" lead of the capacitor towards the component. Solder the free end of the yellow wire to the other end of the resistor / capacitor combination and you're done. Note that this is a somewhat fragile arrangement, so be careful not to stress the resistor at the point where it attaches to the board. A piece of sleeving over the wire where it joins the resistor / capacitor would also be a good idea.

Following modification, the shutter switch becomes the video gain boost ON/OFF switch, while shutter speed control remains in AUTO exactly as if the modification had never been made and the shutter switch was left in the ON position. In other words, the camera will operate exactly as before if the shutter switch is in the ON position. However, with the shutter switch in the OFF position the shutter speed is still automatic, but the video gain is prevented from responding to scene illumination.

The capacitor prevents a small brightness gradient from top to bottom in the image. The modification works without it, but the image looks better (more even) with it. I did not do a lot of experimentation to see what the lowest usable value was, although I did try 0.1 uFD which was not enough. The next value I tried was 4.7 uFD and that worked so I stopped. The 330K value for the resistor was also determined experimentally, and does seem to matter within broad limits. Too high and it does nothing. Too low and the image quality deteriorates.

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