AVO* adjustment: Setting the maximum lens aperture on the T90
In Av (aperture priority) mode, the lens aperture is set electronically using the control wheel. The lens aperture ring is set to „A“ for automatic.
In order to calculate the exposure, the T90 needs to know what the maximum aperture is.
In the FDn system, this is transmitted from the lens to the camera via the
full aperture signal pin on the lens bayonet.
As a counterpart the
maximum aperture control pin is located in the mirror box of the T90 (green arrow).
This pin is pressed in by the signal pin on the lens more or less depending on the light intensity of the lens.
The different high of the signal pin stands for the corresponding maximum aperture value.
Flush plate
On the flush plate on top of the mirror box there are contact tracks (switches) and wipers for transmitting aperture settings and maximum lens aperture to the electronic control system of the T90.
The flush plate with contact brush on contact track, which transmits aperture values to the electronic camera control with its respective position.
Flush plate removed.
AVO contacts (switches) on a second board located behind the flush plate with wipers for transmitting maximum lens aperture to the camera.
The AVO system can be adjusted
using a screw in the maximum aperture control pin.
The Canon service manual provides height information for the pin, which is measured with a depth gauge to an accuracy of 1/100 mm. Alternatively, „known good“ 1.8 and a 2.0 FDn lenses can be used.
The screw is then adjusted until the correct maximum aperture (light intensity) of the lens is displayed on the LCD of the T90.
On my T90 featured in this thread, the small apertures are not closed far enough.
When checking the AVO setting, I saw that in Av mode my FDn 50/1.8 can only be set up to f = 2.0.
I corrected this by adjusting the maximum aperture control pin.
To do this, a small drop of acetone has to be applied to the screw to loosen a seal.
After a few minutes, the screw can be turned using a 0.9 mm flathead screwdriver. Be careful, it is made of soft brass.
After the correction, the maximum apertures on my FDn 50/1.8 and 35/2.0 are displayed correctly.
This has also improved the formation of the small apertures in Av and Tv modes. When set manually, the openings are still too large.
To do
Larry Lyells in the SPT journal:
For erratic exposures, you can
check the light transmission in AV
mode. If the diaphragm opening is
inconsistent, the problem is
probably a dirty aperture magnet or
flush plate. If the diaphragm
opening is consistent, check the
shutter speeds in TV mode.
Inconsistent shutter speeds may indi-
cate dirty shutter magnets.
I will therefore check the AVO switches with a multimeter and clean them, as well as the switches on the flush plate, the associated wipers and the aperture magnet.
I will also look at the function of the aperture control mechanism; I have gained experience with this in the last few days.
The binary values of the AVO switches can be measured at their cable connections (red, brown, grey, yellow) on the flexible circuit board. For this the mirror box does not have to be removed.
There should only be two clearly separated voltage ranges for high and low voltage to measure.
A table with the corresponding binary values for the connections can be found in the Canon Service Manual.
Additionally, the T90 switches to EEE/Help when setting smaller apertures in AV. There may be a connection, and/or the shutter may be stuck.
I'll check this when I disassemble the camera.
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*AVO stands for „Maximum aperture value correction sensor“, see
+++
All information provided without guarantee and use at your own risk.
