Thoughts on amp switching by Matthias Meijer

(m.m.j.meijer@zonnet.nl)

Today I finished the SC3 B/W mod to a Vesta 680. As the amp glow was pretty obvious, even 30 second images, I decided to tackle this problem. After messing around switching every GND pin but one (pin 7) with an NPN transistor (with no results), I found that you posted this message some weeks ago, which I had forgotten.

Anyway, I had no small relay and wanted to try the last pin to test the standard SC1/2 amp-off mod. When attaching the NPN transistor as a switch between pin 7 of the ICX424AL and pad 6 of the Vesta it worked.  See the attached image for the schematic, and two darkframes (60 seconds, 100% gain, other settings at 50% and whitebalance: outdoor)

The parts I used were a BC548 transistor, a small 100nF capacitor (from an old computer board, where it was soldered between VDD and VCC of an IC) and a resistor of about 500 Ohm (may be less for a ToUcam, as it is less in the SC1/2 mod).  

Circuit here

As far as noise goes: I have not noticed any extra noise with this amp-off mod. The camera is a bit more sensitive to noise than a normal Vesta, but that is because the CCD is placed 2 cm from the Vesta CCD board, to allow for cooling. And this noise is mostly visible at 15-30 frames per second, and who needs this when doing long exposures?

Regards,

Matthias Meijer

Amp On

Amp off

 

Hi Steve,

Today I did some measurements on the amp-off mod.

I used the following setup:

·        BC548 NPN with the collector connected to pin 7 of the ICX424AL and the emitter connected to pad 6 of the Vesta CCD board.

·        a 560 Ohm resistor between the collector and emitter

·        a 100nF capacitor between the collector and emitter

·        the base of the transistor was connected to the parallel port in the same way as for the SC1/2 mod (with the 220 Ohm resistor between the parallel port and the base, and the 10k Ohm resistor from the base to 5V)

I measured a voltage drop of 2.75V over the resistor when operating the camera in the amp-off mode in K3CCDTools. There was no noise visible at 5, 10 and 15 fps in the normal ‘webcam’ mode. Only at 30 fps a little horizontal noise appeared, but I think that noise is mainly caused by the extension wires (normal IDE flatcable, about 15mm long) to mount the ICX424AL on a separate board in a socket. This noise was also present before adding the amp-off part.

See also the attached photo’s for more detail on how I made the SC3 mod.

Or should we now call it the SC3.5 mod?? (err... how about SC3.2a - Steve) The darkframes are a stack of 4 frames, each 60 seconds with full gain, and the white balance set to outdoor.   Tonight, I will try to make some images of a far away building. No chance for an astronomical image, as it is cloudy.

I hope that this is enough information for you at this moment.

Regards,

Matthias Meijer