Minox Shutter and the Quest of Longevity of Minox 35 camera

MTC Photography · April 9, 2000

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<TD><FONT SIZE=+2>Analysis of Minox Shutter and the Quest of Longevity

of Minox Camera</FONT></TD>

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<H3>

How Minox Electromagnet Shutter Works</H3>

Minox 35mm cameras and several Minox 8x11mm cameras(C/LX/TLX/EC

etc.) have electromagnetic mechanic

combo shutter. The mechanical parts control the open/close

of the shutter leaves, the electromagnet and its associated

electronics circuit controls the timing of shutter.

<P> The details of each shutter is different, but

basically they follow the model described below

<P> The heart of Minox shutter is an electromagnet;

it's current flow is control by electronic timing circuit

inside the camera ( On top of Minox 35mm camera, just below the

top cover ). The output of the timing circuit

depends on output of exposure meter. For example

if the film setting, aperture setting and external light

<BR>condition calls for 2 second shutter, the the electronic

timing circuit will drive a current through the

<BR>electromagnet for 2 seconds, then cut off current.

<BR>

<OL>

<LI>

When you wind the film winder, it tension

up the open/shut main springs, and push a lever towards the electromagnet

( which has no current flow yet ). The lever holds a small iron strike

plate. When you tensioned the shutter , you hear a

'click' that is the sound of strike plate pushed against

the electromagnet. A mechanical latch holds the

lever prevent it from pulling back by the spring attached to the lever.</LI>

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<TR>

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<CENTER> electromagnet and iron strike plate</CENTER>

<BR>

<OL>

<LI>

You press the shutter release the

current flow to the electromagnet is turned on, the magnetic force

holds the shutter-close lever tight</LI>

</OL>

<OL>

<LI>

The electronic timing circuit start counting and the mechanism

open the shutter</LI>

</OL>

<BR>

<TR>

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<P>

<BR>

<OL>

<LI>

Electronic timing circuit times up, cut off current to magnet</LI>

<LI>

"Shutter close" lever springs back, close the shutter.</LI>

</OL>

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<P>

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<H3>

Causes of Shutter malfunction</H3>

<UL>

<LI>

No Battery or Battery Weak</LI>

</UL>

<UL> Minox camera always 'clicks' even without

battery. The clicks are the sound of the mechanical catch

<BR>latching on or off. If there is no battery, then there

is no force to hold the 'shutter close' lever back

<BR>the moment you press the shutter release, the 'open' and 'shut'

lever spring back at top speed

<BR>under the tension of the springs. In Minox 110S, which has a

lighter shutter blades, you may see the

<BR>shutter actually open up for probably 1/1000 sec; but in

Minox 35mm camera, which has bigger

<BR>and heavier shutter blades, the shutter has no time to open and

close that fast, and remain closed.

<BR>

<LI>

Electromagnet failure due to heat</LI>

<P>The shutter control electromagnet in Minox camera consists

of a metal core and a solenoid. The solenoid

<BR>is wound with thousands turns of very fine gauge lacquer wire.

The thinner the wire the higher the

<BR>resistance. The wire in Minox 110s is thicker than those

in Minox 35 and 8x11. 2.7 v is adequate

<BR>for 110S, but Minox 35 and 8x11 must use higher voltage

battery (5.6v) to drive sufficient current

<BR>through the electromagnet.

<BR> When current flows through the electromagnet, heat generated

is proportion to the time.

<P> One sec shutter

= the load of 6000 1/100 shutter

<BR> One minute shutter

= the load of 360,000 1/100 sec shutter

<P> Since the solenoid bobbin has about 20 to 30

layers, it is hard for the heat generated by overly long

<BR>exposure to dissipate, the temperature of the solenoid will inevitably

rise and shorten the life of the

<BR>solenoid. Rising heat may also reduce the permeability of the

core, render the electromagnet less

<BR>effective. These two effects will eventually burn out the solenoid.

Hence overuse of long exposure

<BR>reduces the life of solenoid.

Failure of Electromagnet due to Dust

<P> The second cause of shutter

magnet failure is dust.

<P> <FONT COLOR="#FF0000"><FONT SIZE=+1>Air

gap pull force of electromagnet is inversely proportion to

the square of air gap width.</FONT></FONT>

<P> I test the Minox 110s electromagnet

by insert a piece of 5 mil paper between the

magnet and the strike plate.

<BR>-- the shutter failed.

<BR> This indicates any foreign substance

such as dust, lint etc. accumulated between the air gap will

render

<BR>a Minox shutter inoperative

<P> When the electromagnet failed due to burnt out or

dust clogging, the effect is the same as

<BR>no battery -----when you wind up the shutter and press the shutter

release, you can hear click click

<BR>but the shutter is stuck.

<BR>

<H3>

How to Prevent shutter failure</H3>

Base upon the afore analysis, the key to longevity

of Minox 35/8x11 shutter

<BR>

<UL>

<LI>

Use long exposure sparingly. Particular in

Minox 35 cameras which allows for exposure as long as half an</LI>

</UL>

hour, the load on solenoid is huge, do it at your own risk.

<UL>

<LI>

Don't put Minox 35 camera in pocket.

As macrolon material which makes up the body of Minox 35 camera</LI>

</UL>

generates static electricity when rubbed with fabric, particular

nylon or wool, and thus attracts dust and lint

<BR>which will find its ways into the camera.

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MTC Photography · April 9, 2000

The issue of 'reliablity' of Minox 35mm camera has being debated for

long time on the net. In some people's hand it went to repair shop

almost every year, while other reported using a Minox camera for 18

years without fail.

<p> What is the cause of this disparity ?

<p> The most frequent failure is either shutter stuck or shutter fail.

<p> This prompted me to think that the longevity of Minox camrea is

very much dependent on the habit of its user. Thus I took a closer

look into the working and failure modes of Minox 35mm shutter

MTC Photography · April 9, 2000

Read the interesting thread about how a Minox MB shutter problem was

successfully fixed.

<a

href="http://www.greenspun.com/bboard/q-and-a-fetch-msg.tcl?msg_id=002

vl3">Fixing MB shutter </a>

<p> I have no direct experience in Minox 35mm repair. My reply and

suggested fix for Anders' Minox MB stems from my own analysis of

of Minox shutter, in particular dust severely weakend the air gap

pull force of electromagnet in Minox shutter

<p> I hope my analysis will contribute to the longevity of Minox 35mm

camera for every user

lars.hagglund · April 11, 2000

Thank you for a very intresting and informative article. Perhaps you

also can tell me why the shutter of my GL only works on a single (not

known) time, regardless of which aperture I use. The pointer of the

meter swings normally, but I understand that there is a separate

circuit for it.

MTC Photography · April 11, 2000

Lars, your Minox camera probably works only at 1/125; the cause of

this single shutter speed may well be that the Hotshoe Micro switch

is on--- ie, the metal contact is depressed down by dust or other

substance. Clean the contact, press it slight down (press hard ) and

use a Q tip to clean the gap then try again.

If this fails, try a new battery, if that fails, go to see Doc DAG

lars.hagglund · April 11, 2000

Thanks Martin.

I'm sorry to say that your advice did not work. I also removed the

top, and found the inside clean. The lever worked ok, so I suppose I

have to send the camera away

MTC Photography · April 29, 2000

<h3> Typical Wire Gage of Solenoid in Minox shutter </h3>

Minox 110s camera's shutter controlling solenoid is easy to

access,

it is right under the bottom plate.

<p> The wire bobbin is about 10mm long, with a diameter of 4mm, wound

with resin coated AWG 39 wire.

<p> Since I cannot measure the diameter of the wire directly, I use

a Seibert Emoskop to compare its thickness with the cut edge of a

piece of 4.5 mil paper-- the wire is thinner than the paper, about

3.5 mil-- about AWG #39

<P>

MTC Photography · January 11, 2001

Further analysis:<P>

<a href="http://www.greenspun.com/bboard/q-and-a-fetch-msg.tcl?msg_id=002yOM"><font size=5>The Lung of Minox 35</font></a>

leo__ · December 31, 2001

It is a great resource about Minox. However, I would like to correct

some points here.

<p>

Minox 35 has two solenoids. The first one (upper) is for the "open"

action of shutter. The other is for the "close" action. The winding

mechanic is to clock the shutter and the solenoids are used to

trigger the "open and close" mechanic only. Same amount of energy or

electricity current was applied to these solenoid for 1/500s or 20s

exposure.

<p>

One of my hobbies is to repair Minox 35s. The shutter of Minox 35 is

good and wisely designed. I encountered 20+ minox 35 has the shutter

problem. No incident was caused by solenoid failure...

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Minox Shutter and the Quest of Longevity of Minox 35 camera

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