Mystery switch. - UK Vintage Radio Repair and Restoration Discussion Forum

slacker · 19th Sep 2021, 8:58 am

I have a Pye Spot Glavonometer to repair. The mechanism is a coil suspended on thin wires in a relatively powerful magnetic field such that the coil can rotate when current passes through it. Light is reflected off a mirror attached to the coil structure onto a semi-transparent scale as a measure of the current flowing through the coil.

The first photo shows the unit, the second shows the insides, the third is a circuit of the measurement portion of the galvo, and the fourth is switch S2.

The problem with the unit is S2 which, as shown in the circuit, shorts out the meter mechanism when closed. S2 is permanently closed. When held open the galvonometer works as it should and all ranges are fine. There is no mechanism in the vicinity of S2 to actuate it and it doesn't look like anything has broken off - there was nothing rattling around when I opened the unit up. I'm told that the unit was working one day and the next time it was used it wouldn't work.

Don't get side-tracked by the bolt that looks like it's beneath S2 in the photo as it just holds a rubber foot in place on the underside of the case.

I can't think what S2 might be other than a bimetallic switch? The bottom contact is quite thick and solid, the top strip much thinner and more flexible. I don't know what materials the strips that comprise it are made from. The unit does contain a filament lamp and a transformer to step down the mains and power the lamp so I surmise it could get quite hot inside when left on for a while. However, this begs the question of why one might want to short out the meter mechanism when it gets hot? It's not as if this switch turns off the power to the transformer hence removing the source of heat.

I have repaired a number of similar devices from a couple of other manufacturers and none have anything like this inside them.

If anyone has any ideas or knowledge of these units I would be grateful for any insight.

regards
roman

Dickie · 19th Sep 2021, 10:14 am

Working from a position of total ignorance (!), the foot mounting bolt appears to have a hole through it. Is it possible there is a push rod missing that opens S2 when the instrument is placed on a flat surface, but allows S2 to close when it is in transit, hence damping the meter movement?

And welcome to the forum, by the way!

Terry_VK5TM · 19th Sep 2021, 11:28 am

And definitely not a bi-metallic switch - it is a leaf switch - used to be very common, especially in pinball machines.

I also suspect the activating rod is missing as per Richards post

wave solder · 19th Sep 2021, 11:45 am

In some information I have just read it says that the scale is shorted when the instrument is lifted off the bench which would imply some sort of mechanical plunger mechanism to operate the leaf switch.

slacker · 19th Sep 2021, 11:59 am

Thank you for your responses, very pleased to have a solution. I will make a rod and re-instate the activator for the cut-out.

lesmw0sec · 20th Sep 2021, 9:39 am

Quote:

Originally Posted by Dickie

Working from a position of total ignorance (!), the foot mounting bolt appears to have a hole through it. Is it possible there is a push rod missing that opens S2 when the instrument is placed on a flat surface, but allows S2 to close when it is in transit, hence damping the meter movement?

And welcome to the forum, by the way!

Good thinking that man!

19th Sep 2021, 8:58 am	#1
slacker Diode Join Date: Aug 2017 Location: Great Malvern, Worcestershire, UK. Posts: 2	Mystery switch. I have a Pye Spot Glavonometer to repair. The mechanism is a coil suspended on thin wires in a relatively powerful magnetic field such that the coil can rotate when current passes through it. Light is reflected off a mirror attached to the coil structure onto a semi-transparent scale as a measure of the current flowing through the coil. The first photo shows the unit, the second shows the insides, the third is a circuit of the measurement portion of the galvo, and the fourth is switch S2. The problem with the unit is S2 which, as shown in the circuit, shorts out the meter mechanism when closed. S2 is permanently closed. When held open the galvonometer works as it should and all ranges are fine. There is no mechanism in the vicinity of S2 to actuate it and it doesn't look like anything has broken off - there was nothing rattling around when I opened the unit up. I'm told that the unit was working one day and the next time it was used it wouldn't work. Don't get side-tracked by the bolt that looks like it's beneath S2 in the photo as it just holds a rubber foot in place on the underside of the case. I can't think what S2 might be other than a bimetallic switch? The bottom contact is quite thick and solid, the top strip much thinner and more flexible. I don't know what materials the strips that comprise it are made from. The unit does contain a filament lamp and a transformer to step down the mains and power the lamp so I surmise it could get quite hot inside when left on for a while. However, this begs the question of why one might want to short out the meter mechanism when it gets hot? It's not as if this switch turns off the power to the transformer hence removing the source of heat. I have repaired a number of similar devices from a couple of other manufacturers and none have anything like this inside them. If anyone has any ideas or knowledge of these units I would be grateful for any insight. regards roman Attached Thumbnails

19th Sep 2021, 10:14 am	#2
Dickie Octode Join Date: Sep 2004 Location: St. Albans, Hertfordshire, UK. Posts: 1,478	Re: Mystery switch. Working from a position of total ignorance (!), the foot mounting bolt appears to have a hole through it. Is it possible there is a push rod missing that opens S2 when the instrument is placed on a flat surface, but allows S2 to close when it is in transit, hence damping the meter movement? And welcome to the forum, by the way! __________________ Regards, Richard, BVWS member Last edited by Dickie; 19th Sep 2021 at 10:27 am.

19th Sep 2021, 11:28 am	#3
Terry_VK5TM Nonode Join Date: Oct 2010 Location: Tintinara, South Australia, Australia Posts: 2,339	Re: Mystery switch. And definitely not a bi-metallic switch - it is a leaf switch - used to be very common, especially in pinball machines. I also suspect the activating rod is missing as per Richards post __________________ Terry VK5TM https://www.vk5tm.com/

19th Sep 2021, 11:59 am	#5
slacker Diode Join Date: Aug 2017 Location: Great Malvern, Worcestershire, UK. Posts: 2	Re: Mystery switch. Thank you for your responses, very pleased to have a solution. I will make a rod and re-instate the activator for the cut-out.