Hello,

I have an old scientific instrument that uses a photocell to detect light and this cell is connected to a galvanometer with a needle. When the instrument is turned on, all other electronics work, but the galvanometer needle will not respond to light. The photocell is ok since it's resistance seems to change drastically when I keep it in shade, or when I have it in the sunlight. The "zero" function of the galvanometer works when the instrument is turned on. But there is no response to light signals whatsoever. Any tips perhaps on how to start troubleshooting it.

thanks

 

Find or make a schematic of the parts.

 

Find or make a schematic of the parts.

I will try to take out the pcb out as I have very bad access to it.

thank you

 

First: what style of Galvo..........Taut-band, or does it have coiled ribbon hairsprings.

No need to remind you just how delicate these things are, I have repaired hundreds of them as related to light meters in classic film cameras.

A common problem, and easiest to correct in any galvo, is tiny debris collected on the magnet over time, and dragging on the coil frame, which is possible to clean out, tho' you need a remarkably delicate touch, excellent glare-free lighting, comfortable magnification and some contrived special tools. Both springs must be very close - within milligrams -- in at-rest or initial torsion.

Secondly shock, which may have dismounted the coil pivots, or caused tangling or distortion of the hairsprings - A real hair-tearing challenge to restore and still have a working unit, though it can be done.

Electrical problems are far the most difficult...........usually the hairsprings have broken, or the tiny solder joints have failed, or the coil itself is open. I have replaced a matched sets of hairsprings - on [ memory ] less than ten out of hundreds of units repaired..... it is a real challenge that one seldom makes money on..

For the challenge, and also because other technicians I worked along side said it " couldn't be done " I have replaced the cloth shutter curtains in Leica M3 bodies with handmade parts. My own Minolta SRT-102 has new curtains and drive springs in the takeup drums.

 

Monitor the voltage across the coil or put a sensitive current meter in parallel to make sure that the galvo is being driven before taking it apart!.
Frank

 

Too late. It has already been established that the meter responds to the electrical zeroing circuit but is not being driven in response to the photocell.

 

Too late. It has already been established that the meter responds to the electrical zeroing circuit but is not being driven in response to the photocell.

Exactly. The meter responds to the zero knob. So what could that mean? Would it mean that the galvanometer is fine?

By the way the galvanometer is nothing like ones found on cameras like PRK mentioned. This one is very large in size (2 inch needle). So it should be easier to work with if needed.

Chucky, when you say a sensitive current meter, how sensitive should it be? My current multimeter measures down to around 10 microamperes.

 

10 MicroAmps sounds like a decent level -- some of the ones in cameras are that sensitive.
With Galvos -- bigger is definitely better........I have rebuilt any number of older Simpson Panel meters...... some have considerably larger coils to safely measure 50 + Amps...........

Glad to hear it is at least active..............

 

When the meter movement responds to the electrical zeroing current, that means the meter movement is working. You have also established that the photocell is working. The broken part is in between those two devices. Referring to post #2, find out what is between the photocell and the meter movement. Find or make a schematic and let us see it.

 

This is the circuit. I am not sure if I am able to make a schematic out of this though. Does it look like anything standard?

By the way the pot on the bottom left is used for zeroing the instrument. There is another one on the other side which is for standardizing the instrument.