Minolta MOD III Lamp/Power Modification

Back in the late 70s the voltage regulated 19V power supply for my Minolta Mod III enlarger went bad. I converted it over to a 1/2 wave rectified non-regulated supply with a big diode. It has a 24V transformer, so half of that is around 13 measured volts.

I used the enlarger like that all these years. Now I letting my kids use that enlarger. The "Kodak Projection Print Scale" works great with kids, they can just pick out the pie that looks good. However, our printing times were not always under 60 seconds with Y/M multigrade filtration dialed in.

So, I decided to look into optimizing the light output.

The enlarger has used the EKG bulb all these years. That is the original type bulb and it is rated at 80W at 19 volts. Well, since I only have 13 volts, I thought a 12 or 13V lamp would get me back to optimum intensity.


A search of various bulbs led me to the DED bulb. It is 85W at 13.8V with the same GX5.3 2-PIN base. Cost about $5.00. It is on order, but I'll post my results when it arrives.

Mod III power supply options

Any suggestions of how to fix a power supply?
Blown the circuit board after my unit was in storage for many years.
Cannot find replacement boards from OEM.
Does not have schematic to fix circuit board.
Any advises??

kinghl said:

Any suggestions of how to fix a power supply?
Blown the circuit board after my unit was in storage for many years.
Cannot find replacement boards from OEM.
Does not have schematic to fix circuit board.
Any advises??

Click to expand...

You can easily half-wave rectify the voltage from the 24V transformer with a large diode. That is how my unit was repaired in the 1970s by Minota service. Subsequently they re-engineered the power supply that shipped with new enlargers.

I wound up buying a power supply with the newer circuit from an APUG user. How do you know if you are buying a newer circuit power supply? Easy, if it is still working it is probably the later version

Another thread here: (there was a url link here which no longer exists)

There is a minolta power supply on ebay now for $30 but the text reads " I am not a professional on how it really works." which usually means it is broken and does not work.

best small frame enlarger I ever used!! someone should start production of these again...
best, peter

Do you have the specifications of the 24V transformer and the diode?
Your description really does not tell me how to get component and how to put it together.
Like to get my unit working so I can sell it to someone who really can use it.

kinghl said:

Do you have the specifications of the 24V transformer and the diode?
Your description really does not tell me how to get component and how to put it together.
Like to get my unit working so I can sell it to someone who really can use it.

Click to expand...

If the term "half wave rectification of 24V to 12V with a diode" does not make sense to you then this procedure will be difficult and perhaps dangerous. The primaries of the transformer are 120V which is lethal voltage.

Having posted the above, lets start from the basics. Do you have a soldering gun? If so then maybe you have an extra diode laying around. Diodes are rated per reverse voltage. If you have a 1N4001 (35v) that should do but if you need to buy one you could get a 1N4002 (70V) to have more of margin against failure.

The principle of half-wave rectification is described here: http://web.physics.ucsb.edu/~lecturedemonstrations/Composer/Pages/64.57.html

You need to find the secondaries of the transformer and connect the diode between one wire from the transformer to the lamp.

After the procedure the device is no longer UL or CE approved. You may want to take that into consideration if you are going to sell it.

Are you referring to the original transformer in the power supply?
Use the diode to by-pass the circuit board straight 12 v to the lamp?

Yes, the transformer in the box should be a 120V to 24V transformer if I recall correctly. The circuit board electronics are not used after the procedure.

You could also look up the Beseler 23C III color head with "built-in solid state power supply" which I believe is just a rectifying diode (part #90).

I remember (over 30 years ago) when I bought this, the ads said the power supply has "stabilized" feature to minimize lamp variation.
Does the original circuit board suppose to do that? If the circuit board is by-passed, what effect will that have to the variation of the lamp?

Yes, converting it to run on half-wave rectified current makes the lamp just like any other lamp plugged straight into an outlet. If you don't notice a lot of dimming of your lights and brown-outs from your utility company, then it should not be a problem.

Any ideas of why Canon configure the original power output of the PS to be 19V with the circuit board instead of running full 24V from transformer? Is this to stabilize the output and make the lamp more consistent? Was the 80w/19V lamp only available lamp back then?
I was thinking either 12V or 24V would be more commonly used and available for current use also.
Lumen differences between the original 80w/19v lamp and the proposed 50w/24v lamp will probably affect exposure timing and/or aperture setting and will need trials to determine. However, this change appear to be more robust than the 1/2 wave rectified method.

Cruzingoose said:

Also, the OP did not state what diode was being used.

Click to expand...

Thanks for the comments Cruzingoose. My mistake on the rectifying diode, it is 1N5401 (3A), not 1N4001. Again, that is what Minolta service put in back in the 1970s, probably want to go with a little more forward amperage rating for a de novo conversion. However, going straight to 24V with a different bulb seems like an excellent idea for anyone out there with a non-working power supply like this.

Also, readers, realize this is the re-activation of very old thread about an experiment that has been subsequently aborted due to some of the issues pointed out by Cruzingoose above. (I got a new working 18V Minolta power supply for that enlarger a few years ago).