Minolta SR-T converting to use silver oxide cells

12/5/2018 Rokkor Files - Converting your SRT

This article has been reconstructed from the excellent Minolta SR-T site created by Dick Sullivan, which is
unfortunately now no longer on the web. Our thanks and respects go to Dick Sullivan for his committment to
other Minolta manual focus users.

As described in the page about the Mercury Battery, many early cameras including the SRT were designed to
operate on the now banned 1.35v mercury cell. In order to have the camera meter correctly with modern 1.5v
cells the voltage of the cell must be reduced to 1.35v. Dick proposed that the easiest way to do this was to
convert the SRT through the use of a Schottky diode. The conversion has been conducted and does work very
well, reducing the voltage to the camera meter circuit to 1.35v. The conversion requires simple tools, some
soldering skills, and the removal of the camera's bottom cover. Overall it takes about one hour.

A No. "0" Phillips Jewelers Screwdriver.

A Small Soldering Iron with a pointed tip, 25 Watts or so.

A 1N5711 Schottky Diode.

A short (about 2") piece of insulating plastic tubing, with an 1/8" inside diameter.

A short length of small gauge 60/40 solder.

A Fresh '76 1.5 volt Silver Oxide Cell:

Duracell® MS76H
Kodak® KS76
Varta® V76PX
Panasonic® SP76
Rayovac® RS76
Energizer® EPX76 or 357

Either a Plastic Adapter Ring, or a # 111 Rubber "O" Ring measuring approximately 5/8" outside dia. and
7/16" inside dia.

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12/5/2018 Rokkor Files - Converting your SRT

Technical Notes: The 1N5711 Schottky Diode is a Metal to Silicon Junction Diode primarily intended for high
level UHF/VHF detection and pulse applications. We of course, do not care about its frequency characteristics or
its intended applications, just its forward voltage drop.

Any forward biased diode will drop a certain amount of voltage depending on its type, construction, the current
flowing through it and to some extent, its temperature. The type and construction of a diode used is the major
factor in the amount of forward voltage drop it has. For instance, a Germanium diode will typically drop .4 volts or
more and a Silicon diode will drop about .6 Volts, neither of these types will suit our needs. Since a Silver Oxide
Cell delivers about 1.6 volts and a Mercury Cell reads about 1.35 volts we need a voltage drop of about .25 volts.
It turns out that a forward biased 1N5711 Silicon Schottky Diode will drop the required .25 volts at the current
range (10 ~ 200ua.) of the SRT metering circuit.

While temperature will affect this voltage drop a little, the effect will be extremely small at the temperature range
of the camera in normal use. Photography in Antarctica or the Sahara Desert might be another story, but the
mechanically operated SRT will likely have other problems long before those extreme temperature can cause
any appreciable exposure errors. Temperatures of about 32º to 95º F, should be just fine. From my
measurements at room temperature, the 1N5711 Schottky Diode was "spot on", reading a .25 volts drop, thus
delivering 1.35 volts the SRT's metering circuit, perfect for our application. Other Schottky Diodes (1N6263,
BAT41, BAT81, BAT82, BAT83) have a similar type and construction but I have not tested them.

Caution: The handling of the diode, as with any semiconductor requires certain precautions to prevent damaging
it. You should protect it from static discharges and excessive heat. Before touching the diode leads be sure to
ground yourself to a grounded metal surface to discharge any build up of static on your body. When soldering the
diode, try to limit the time of heating to about 2 seconds.

1. Assure that the SRT was or is accurate with the Original Mercury Cell and is functioning properly, Film
Transport, Shutter curtains, etc.

2. Unscrew the battery cover and remove the existing PX13 or PX625 Mercury Cell from the SRT camera, if
still in place.

3. Clean the battery area and cover, paying particular attention to the battery contacts. You can use a Q-Tip®
moistened with Rubbing alcohol. For corroded contacts, a pencil eraser works fine. Dirty or corroded
contacts can cause erratic readings.

4. If you are using a Plastic Adapter Ring, install it into the battery compartment and then place the battery
inside the adapter (+ side towards the cap). Alternately, install the "O" Ring around a fresh 1.5 volt Silver
Oxide "76" Cell, and place it in the battery compartment. Observe the correct polarity (+ side towards the
cap). The adapter or "O" ring is required since the Silver Oxide Cell is smaller in diameter than the Mercury
Cell. The Adapter or "O" Ring will center the cell in the battery compartment and can be re-used. The Silver
Oxide Cell is also slightly thinner (by about .030 inch), but should make no real difference as to fit. Should

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