I bought a little Unitek Dental Spotwelder from the tip about eight years ago, and we used it for welding battery tabs, which it did beautifully. But most of its new life has been on the shelf gathering dust. The other day, my neighbour asked me to look at his electronic wall clock, which had stopped. It is a tuning fork type, and it didn't take long to ascertain that the battery contacts were the cause of the problem. The positive contact had a piece missing, right where the positive battery button has to connect. (A "C " cell ) After some scrupulous cleaning, and a trip to the SPRING box, I found a nice scrap of stainless steel, which had once been in the roof of a VHS video cassette. I cut from this, a neat piece of stainless, which I intended to spot-weld across the hole in the contact. But the spotwelder was mucking about.
The welder charges up an 800 uF 200v capacitor through a 27K resistor from a 200v supply. Then when the SPOT button is pressed, there is a mercury relay which dumps the charge into the primary winding of an Iron-cored transformer whose secondary (about two turns of flat shim copper) connects across the welding contact points..
I could measure 200 v across the relay contacts,with power disconnected , after the cap has charged, which disappeared quickly when the SPOT button was pressed, but no sign of a spark across the job. I checked the capacitor for capacitance and ESR, both good, and when a screwdriver blade was placed across the relay contacts, a healthy SPLAT was observed. So I reckoned i"d better check out the pulse transformer. I thought there might have been a moisture buildup in the winding, so for a few hours I fed some low voltage AC into the primary to warm it up a little. It made no difference. I thought I'd try to test the transformer, so I dug out my self-designed tranny tester ( which was made to check TV line output transformers using a single loop through the core) but the instrument wasn't up to the task. I compared with a known good garden-variety power transformer to be sure, But I found that a neon lamp across the primary gave a good light when the EXTREMELY low impedance secondary was momentarily touched onto the 4v winding of a Scope (in the UK please read SUPERSPEED) soldering iron transformer. So I have tenatively decided that the tranny is OK. I noticed a capacitor mounted below the relay, which is 3uF 660 volt AC. Where it's connected I know not. It is straight into the wiring harness. But it checks to be only 1 uF.

I have tried to find more info on this capacitor but with no success. I was thinking of our days experimenting with Capacitor Discharge Ignition systems, where we had to use special types of caps to achieve any longevity. I haven't checked the voltage across the cap, but I wondered if there was a good reason for its being 660 VAC rated. And I couldn't find anything small enough, unless we go to some paralleled polycarbonates or such. But it could be something to do with the speed of relay pull-in. The heiroglyphics on the capacitor: MALLORY OPN160 32NB6601 3MFD 660VAC. and its 65mm high including the terminals. If no new info comes to light, I'll need to pull the harness apart and see where its connected. Has anyone been down this path already?
The welder charges up an 800 uF 200v capacitor through a 27K resistor from a 200v supply. Then when the SPOT button is pressed, there is a mercury relay which dumps the charge into the primary winding of an Iron-cored transformer whose secondary (about two turns of flat shim copper) connects across the welding contact points..
I could measure 200 v across the relay contacts,with power disconnected , after the cap has charged, which disappeared quickly when the SPOT button was pressed, but no sign of a spark across the job. I checked the capacitor for capacitance and ESR, both good, and when a screwdriver blade was placed across the relay contacts, a healthy SPLAT was observed. So I reckoned i"d better check out the pulse transformer. I thought there might have been a moisture buildup in the winding, so for a few hours I fed some low voltage AC into the primary to warm it up a little. It made no difference. I thought I'd try to test the transformer, so I dug out my self-designed tranny tester ( which was made to check TV line output transformers using a single loop through the core) but the instrument wasn't up to the task. I compared with a known good garden-variety power transformer to be sure, But I found that a neon lamp across the primary gave a good light when the EXTREMELY low impedance secondary was momentarily touched onto the 4v winding of a Scope (in the UK please read SUPERSPEED) soldering iron transformer. So I have tenatively decided that the tranny is OK. I noticed a capacitor mounted below the relay, which is 3uF 660 volt AC. Where it's connected I know not. It is straight into the wiring harness. But it checks to be only 1 uF.


I have tried to find more info on this capacitor but with no success. I was thinking of our days experimenting with Capacitor Discharge Ignition systems, where we had to use special types of caps to achieve any longevity. I haven't checked the voltage across the cap, but I wondered if there was a good reason for its being 660 VAC rated. And I couldn't find anything small enough, unless we go to some paralleled polycarbonates or such. But it could be something to do with the speed of relay pull-in. The heiroglyphics on the capacitor: MALLORY OPN160 32NB6601 3MFD 660VAC. and its 65mm high including the terminals. If no new info comes to light, I'll need to pull the harness apart and see where its connected. Has anyone been down this path already?