Yashica Electro 35 electrical noise induced by shutter - strange problem

[monopix]

I was sent a Yashica Electro 35 for repair with the report the shutter was failing to stay open. Did all the usual but found no issues. The magnet that holds the shutter open was letting go prematurely. I changed out all the electronics and the shutter magnet and proved the magnet was able to hold the shutter open but still it wouldn't. Eventually looked at the voltage across the magnet and found huge interference. Also found it throughout the electronics. I disabled everything that might cause interference like chattering switches but still the noise is there. It happens as the shutter opens. The shutter is tripped mechanically so there should be nothing electronically happening at that point. I couldn't find the root cause so added a suppression capacitor across the power rails - problem gone and camera works fine.

Has anyone ever seen electrical noise induced by a mechanical shutter before? Or have any other ideas?

 

[koraks]

I'd check for the usual suspects such as weak battery and bad/corroded contacts. I put my money on the latter, especially the contacts around the battery compartment.

One more thing that might give insight is measure current draw from the battery. If you see the same pattern, but inverted, it's a sign that something associated with the shutter mechanism shorts the battery intermittently.

 

[monopix]

I'd check for the usual suspects such as weak battery and bad/corroded contacts. I put my money on the latter, especially the contacts around the battery compartment.

One more thing that might give insight is measure current draw from the battery. If you see the same pattern, but inverted, it's a sign that something associated with the shutter mechanism shorts the battery intermittently.

Thanks for the thoughts but when the above picture was taken, the camera was running from a bench power supply wired (soldered) directly onto the PCB so definitely no problems with batteries or battery contacts. Good idea about checking for shorts - there is one place that might happen. Though I doubt a cap across the supply rails would eliminate the noise, as it did, if the supply rail was getting shorted. But I'll check.

 

[koraks]

Ah, gotcha on the bench-top PSU. What kind of supply are we talking about?

Yeah, I see what you mean about the cap. What's the amplitude on that waveform and is it possible to pinpoint where it occurs on a schematic of the electronics?

 

[monopix]

Ah, gotcha on the bench-top PSU. What kind of supply are we talking about?

Yeah, I see what you mean about the cap. What's the amplitude on that waveform and is it possible to pinpoint where it occurs on a schematic of the electronics?

Supply is 6V (or thereabouts) but the noise is much larger, which is also confusing me. The scope picture has zero in the middle, the top of the waveform is 6V and the noise extends off the bottom of the screen so +/- 6V at least. How does that happen? The noise was on the supply so could be seen everywhere on the circuit.

I won't be able to look at this further. The camera is working with my cap mod and I need to get it back to the owner so I've buttoned it up. May never get to the bottom of this.

 

[koraks]

Sounds like back-EMF from the shutter coil. That it happens once on every shutter cycle is to be expected if no blocking diode is mounted across the coil. That it happens so extensively in a single cycle suggests an oscillation of some sort. If it's problematic, I don't know.

 

[monopix]

Sounds like back-EMF from the shutter coil. That it happens once on every shutter cycle is to be expected if no blocking diode is mounted across the coil. That it happens so extensively in a single cycle suggests an oscillation of some sort. If it's problematic, I don't know.

Strange thing is the coil gets energised before the shutter is triggered. The only electrical changes that happen when the shutter is triggered is the timing cap, which has a shorting switch across it, has the short removed so the timing can start. But I tried keeping that shorted and also tried pre opening it and it made no difference either way. I think you're idea of a short somewhere is the likely answer but it will go as it is. Thanks for your thoughts. Here's the circuit for interest.