Soon after building the SC4 camera, I decided to work on an Amp-Off design for it. In the end I settled for a Burri- Behrens modification. I have used this with some success, but have never managed to rid the camera of a annoying levels of read-out noise and the dreaded diagonal banding. What was particularly annoying was the way the noise varied - sometimes it was acceptable, whist at other times it would ruin an whole imaging session. I tried to adjust the layout of the CCD PCB and cable routeing, but to no avail. With the amp-off circuit 'in circuit' there was noise.
I discovered the following modification during investigation of Vesta Pro EEPROM tweaks. The Vesta Pro and ToUCam EEPROMs are used to store settings for the cameras on-board power supplies. I was looking to see if these settings could be adjusted during image capture to reduce the voltages applied to the CCD. The idea was that this could give us a 'software only Amp-Off' modification controlled through the USB port. I guessed that the power supply circuits were similar to those shown in the Philips Application Note AN00065.
The power supply works as follows:
The CCD circuitry requires regulated +15V and -5.5V. These need to be generated from the +5V dc USB supply. They are produced using a single switching-mode power supply controlled by the SAA8112 (Vesta) or SAA8116 (ToUcam). The output from the switching transistor feeds a boost circuit to give a raw +18V unregulated supply and a buck circuit to produce the unregulated negative supply. The raw boost and buck supplies feed individual fixed voltage regulators that produce the steady +15V and -5.5V supplies needed by the CCD front end.
Unfortunately this means that adjusting the SMP output cannot readily be used as a way of independently reducing the +15V during exposure, to reduce ampglow. However, the investigation got me thinking about how the regulator could be modified to reduce the output voltage.
Both regulators use Zener diode references to produce the regulating drive. The modification works by lowering this reference voltage during the exposure to a level that prevents amplifier glow.
Q1 acts as an inverter, to ensure that the when the parallel port is disconnected the regulator output is kept at +15V. With the parallel port connected and in the initiation state Q1 will be on, causing its collector to be LOW. Q2 will be turned off. The voltage regulator will operate in its normal mode with ZR1 regulating the output voltage through Q4.
When the parallel port goes low as a result of an AMPOFF signal from the software, Q1 will switch off, causing its collector to be HIGH. Q2 will start to conduct, effectively applying VR1 across ZD1 and D1. This lowers the voltage reference causing the regu
lator output to be reduced.
At the end of the exposure cycle, the parallel port is restored to 5V causing the regulator output to return to +15V ready for the readout of the CCD.
I'm sure that it is possible to improve on the circuit, but I wanted to minimize the modifications to the circuit I had already build for the Burri-Behrens Mod.
A single connection needs to be made to the 15V Zener reference at the point shown below. The positioning of VR1 is not critical. I made the connection to the Zener with a 6-8mm length of wire-wrap without any additional noise burden.
The output voltage of the regulator should be adjusted as a compromise between glow reduction and sensitivity reduction. I suggest starting with a voltage of about 14V and monitoring the ampglow reduction in 60 second dark frames. When you are happy, check that sensitivity is not significantly reduced by comparing AMP ON and AMP OFF light frames at 60 second exposures. You will need to take account of the natural increase in levels on the AMP ON frames caused by the glow, so test the levels in the bottom centre of the frame. The approach is necessarily iterative as the adjustment is a trade-off. You can monitor the regulator output voltage to earth on the large circular gold pad below the first 'n' in 'Connect Here', shown in the photo above.
The benefit of this approach is that there are no additional components added into the power supply chain. This minimizes the risk of noise entering the system.
The main disadvantage compared to other modifications is that it is not possible to reduce just Vdd. The 15V supply to the NEC uPD16510 and the O/P preamplifier are also affected. In practice this is not an issue as long as the voltage is not reduced by more than about 5V. If the voltage is reduced much further, sensitivity is reduced. I found by experimentation that a voltage drop of between 3 and 5 volts gives acceptable reduction in ampglow in 60 second frames, without reducing sensitivity.
I have tested this modification on my Vesta Pro Sc4 (ICX414AL) camera. I would imagine that the modification could also be applied to the ToUcam, though I have not had an opportunity to investigate this yet.
I hope that you find these instructions of interest and use. If you do attempt this modification, please let me know how you get on. All I ask that you agree to the following terms and conditions.
- You will not distribute these instructions.
- You will not place information derived from these instructions into the public domain. If you have any improvements or tips you would like to give others I will be happy to add these to these instructions, and acknowledge your input.
- You will not sell a camera modified in the way described for any more than the value of the parts, unless all extra money is passed to MSF or Cancer Research UK
- You will not sell a camera modified as described as part of a larger deal.
- The instructions are provides 'as is' with no guarantees of being correct, accurate or even working. You agree to be responsible for any injury or damage that results.
- ry or damage that results.