Leitz V35 bulb/transformer failure

[btaylor]

Looking for some repair advice here from people that know more about transformer/rectifier stuff than I do to repair my V35 enlarger.
I turned it on yesterday and got no lamp illumination. Bulb is blown. Checked further, voltage at the 12v dc bulb socket, zero. Followed wires and confirmed continuity from the transformer to the head, no output from transformer, 117v measured at transformer input.
So a replacement bulb and power supply are needed.
I am planning to wire in an external power supply and I need to determine how much output I need to run that 12v dc 75 watt halogen bulb. From my background in automotive electrical I can figure my way around circuits, but when I was looking for conversion formulas I came upon “power factor” and that stumped me. I guess my question is how many amps @12v dc do I need to run the 12v 75w bulb? I assume there is a surge of current needed to initially light the filament- but how much of a safety factor do I need? Thanks in advance.

 

[ic-racer]

6.25A, though frequently transformers are graded by Watts, in which case you would maybe want 75W.

Don't know about the DC thing you measured, but usually one would replace the AC transformer in the base with a similar value.

You might want to poke around (with your multimeter) to make sure the transformer is really fired before proceeding with replacement.

(Image Reference: https://www.bonavolta.ch/hobby/en/photo/v35.htm#Adjusting)

 

[btaylor]

Thanks ic-racer, on the page you referenced the second illustration of the transformer below:

Where the writer has done what I am trying to do- fit a 12v replacement power supply for the bulb.
Concerning the transformer, 117.8 v AC going on, -0- volts of any kind coming out.
The easiest thing will probably be to get a power supply with about 12v 8-10amps DC output, that should cover any surge to get the lamp lit I think.
Thank for your input.

 

[F4U]

I've seen open transformers and 1 open power choke in my radio days. Exceedingly rare. Replaement power transformers are easy to source, although they may not necessarily have the same mounting arrangements. Never stopped me. That part is minor. That said, I still don't go along with that as the diagnosis, having not seen the unit personally. Maybe so, though.

 

[koraks]

I guess my question is how many amps @12v dc do I need to run the 12v 75w bulb?

I=P/V=75/12=6.25A as @ic-racer also indicated.

To verify it's the transformer that blew, measure DC resistance of the primary and secondary coils. I expect you'll measure a few ohms on the secondary coil and infinite resistance on the primary. If you measure a few hundred to a few kOhms of resistance on the primary and also a few Ohms on the secondary, it's not the transformer that's the cause of your problem.

Low-frequency power transformers are going out of fashion, but many are still around. Using a parametric search on Mouser something like this comes up:

https://www2.mouser.com/ProductDetail/Triad-Magnetics/F-108U?qs=sGAEpiMZZMuKmRn7rpQYPV9VbLh9ENfU8xE408zDtEI%3D

Or this: https://www2.mouser.com/ProductDeta...=sGAEpiMZZMuKmRn7rpQYPRSGU26rTs7QcpbEpocYRUw=
They may not neatly fit in the original box. Sorry.

The bulb doesn't care whether it's AC or DC so you could replace the whole unit with a DC power supply which may be easier to find these days than a 117-12V transformer that will fit the box. Consider something like this: https://www2.mouser.com/ProductDeta...VrH5rwl1iobsNTqGe7rwARgZCkTcZaK%2BY4iuAO6ZQ==
Note that it's often (as is the case here) cheaper to get a brand new switch-mode power supply with all manner of protections etc. instead of a bare transformer. This is simply due to material cost; an old-fashioned transformer is heavy and contains a lot of copper (and iron). So material and logistics costs are relatively high.

If you include low-cost options like eBay and AliExpress in your search you can probably find something for around $10 ex. shipping that will fit the bill one way or another.

Check if it's really a dead transformer first. If so, ask yourself why it died. They sometimes blow spontaneously, but there may be an external cause that needs to be resolved.

 

[Sanug]

Is there no fuse in the transformer unit? Usually the primary side is fused and such fuses may get blown when the lamp fails.

Anyway, it is a good idea to replace the transformer with an ac/dc converter. They are stabilized and not expensive. I use a meanwell brand 12 V 10 A for my Durst M605 with a 100 W lamp.

RS-150-12 should fit.

RS-150-MEAN WELL Switching Power Supply Manufacturer

MEAN WELL is one of the world's few standard power supply mainly professional manufacturers, covering 0.5 to 25,600W products are widely used in industrial control, medical and other fields, in line with international safety certification, short delivery of spot inventory.

www.meanwell.com

 

[ic-racer]

If you search around transformers are available. Here is one: