Durst M707 color - What electric circuitry should be inside ?

Hello,

About 1,5 years ago, I purchased a second-hand Durst M707 color enlarger.
After a (more or less) monthly printing session, I had a 'meltdown' last
weekend ...

When opening the head of the enlarge, I found a) the wires connected to the
lamp were brazed to some kind of connecting plate and b) the wires from the
cable entering the enlarger were bolted to that same plate.
Apparently, the tin connecting the wires coming from the lamp had melted,
and the wires had come loose.

This made me start considering wether there should be some kind of
overheating-protection in the enlarger-head, as the temperature can become
very high (as my old chemistry books tell me tin melts @ a temperature of
231.9°C).

Since there's nothing to be found except for a few wires connected to each
other I wonder :
1. What was originally in there.
2. If the state of my enlarger more or less ressembles the original state,
can I make an improvement so that the enlarger only needs some cooling down
instead of opening and fixing it when the temperature gets too high.
3. Would it be a good idea to build in a small fan in the enlarger-head (as
I've heard when using a laborator @ school). Since the enlarger came with a
12 V transfo, I could maybe use a fan similar to those used in computers
(which would be my first step into the digital era ;-) ) ...

Thx,

Philippe

 

Durst designs there enlargers rather well, so it is hard to imagine the
enlarger getting as hot as you describe. Be sure, though, that the right
lamp is being used in the enlarger. It may be made for a 50W and have a
100W installed, which would lead to big time overheating.

Or, there may have been a short in the lamp wiring and the 10-20 amps the
transformer can supply (for a short time) was enough to melt the solder
joints.

But "brazed" -- or do you mean soldered? Brazing is half way between a weld
and solder and I doubt you melted a brazed joint - the aluminum housing
would have to melt first (IIRC).

Solder is the stuff radios and circuit boards are put together with.
Around the house it is a soft wire made of 60% lead and 40% tin.

That _is_ pretty hot, in any case.

Besides the solder melting, you mean...

There are thermal cut-off switches:

http://www.thermtrol.com/products.htm

I am sure you have something just like them in the eu. The usual application
is to have a self-resetting disc thermostat in series with one or two thermal
cutoffs. An old coffee maker will yield both the thermostat and the cut-off(s).

If it really runs that hot, yes. Most small computer fans run on 5VDC, the
enlarger transformer provides 12VAC so you would need to build in a power supply
circuit. OTOH, you may want to have the fan run all the time, this is the
normal way air-cooled equipment works. A small 'wall wart' 5VDC power
supply would run the fan and most folks have a box full of the things from
old electronic gadgets and the like.

 

Typically, in appliances where heat / current is an issue, crimped
connectors (and brazing) are used to prevent "de-soldering" although as
another poster indicate, its hard to imagine that an enlarger draw enough
current to cause this but maybe in combination with the heat from the bulb.

 

I meant soldered. Life isn't always easy without a dictionnary @ hand ;-)

 

Have Nick run down to Electronic Surplus on Broadway and 55th and get you
some inexpensive Teflon wire.

 

Or a miscontact in some of the wires, and due to the high current (12V
bulb), lots of heath.

Jorge