Unitek Dental Spotwelder lost its spark

[Technomaniac]

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?

 

[73's de Edd]

The last part where the 850/ufd/200v E-caps stored up voltage is dumped into the prrimary of an iron core transformer, with its uber high current low voltage secondary.
Sounds like a Weller D550 soldering pistol /gun concept.
So have you tried a low voltage incandescent lamp . . .#122 . . .across the weld output cables to see if the lamp flashes on a spot weld attempt ?
If so, sounds like a connectivity problem to the weld electrodes. ( Whch you wouldn't want to touch during this test, if they are being of hand pressure application. )

 

[Technomaniac]

Hi. A vintage radio dial lamp across the transformer secondary does not flash even the tiniest red momentary glow.. When the relay connects the weld tranny's primary to the charge in the 800uF cap, the charge disappears, so the tranny is drawing a good current and putting nothing out. Of course I have the spotwelding points held apart. The tranny looks like it doesn't have many primary turns, so not many turns/volt. I'd already checked for leakage between points, relay contacts, windings and ground.
That 3uF cap that has degraded to 1uF, and we don't know where connected, has 285v AC across it, and the voltage persists when welding, so I think we can assume that it's not important. but at that applied voltage the 660v rating is probably necessary. The 110v mains must be stepped up in the mains transformer, as it must be producing the 200 VDC for the supply to the mercury relay contacts and the 285v AC that is across the Mallory 3uF.. I'm thinking at this stage that it must be the transformer that's the problem, but not likely to find one and not easy to rewind, with the heavy copper secondary, though only one or two turns, its pretty stiff. And not a lot of spare room for the slight increase in size that you get when you wind a coil by hand without the most appropriate equipment.
I haven't mentioned that I checked the tranny for a magnetised core. Since it's a unidirectional pulse that's applied, I thought that, although lamiinated, and it DOES appear to have a miniscule air gap, it might have acquired a residual polarisation. Nothing apparent there. I DID see a similar welder for sale somewhere, I suppose if I bought another one, with my current luck, it'd have the same problem by the time I received it, even if the seller claimed it to be operational.
Perhaps I need to build a shorted turns checker for low frequency low impedance trannies......

 

[73's de Edd]

With your confirmation of the "welding current " step down transformer as being conventional low frequency E-I iron lamination construction and not ferrite cored.
Why not use a series inserted 100W incandescent lamp in series with the 230VAC line input primary to see what comes out the LOW Z secondary, . . . E and I wise ?
OR . . . . if you have a Weller design based soldering gun, pull out the tip amd use two hefty gauge jumper wires to connect the Weller tip output level into the "welding current " step down transformer secondary, to see what comes out the primary . . . or also connect the 100W incandescent lamp to the primary to see if it lights when the trigger of the soldering gun is pressed.

 

[Technomaniac]

The weld transformer probably doesn't have a 230v primary, it doesn't appear to have enough turns, remember it is fed with a DC 200v pulse from the big cap. So would maybe have to be a bit careful, I haven't fed as much as that, AC wise, to date. I have tried a lower voltage which produced no output, so I'll try the 100w lamp test on a higher supply.

 

[Technomaniac]

OK. 240VAC through 100W 240V Lamp to Primary of Weld Transformer. AVO8 measured the secondary voltage as 2.5v. The voltage across the primary was approx 12V. (The lamp was BRIGHT) Could mean the tranny is OK, OR it could have one shorted primary turn.... Wonder if I could rig up a ringing test on the CRO......

 

[73's de Edd]

I think that all is well with that t-former and you move on to the full charge up of that large E-cap and its geting "banged " into the primary of THE ransformer forr its full milliseconds of glory and the resultant high weld current out of THE transformer.
Might you leave a "sacrificial " Red led connected across the welding terminals and fullly charge the E cap and then use a clip lead to discharge it into THE transformer primary.

I can't see their use of a manual mercury switch, except for its longevity to continual contact abuse that the charged cap inflicts.

If peak output V meets / exceeds LED vf and if pulse output is being of > 50 milliseconds duration you should see the flash.

Of course, in your case of having both been around and therewith, fully around the blocks, I expect you should have a triggered O-scope along with proficiency in same for conducting a more in depth analysis.
Since, on this forum, I am SO used to having to come up with techo work arounds for those who have NUTTIN' . . . test equipment wise . . . to work with.

 

[Technomaniac]

An idea came to me - Suppose the weld supply was a DOUBLER - of the single cap type like in Micro Ovens, putting the charged cap in series with the tranny output - And it isn't doubling. Mebbe should have a more detailed look at that possibility.

Regarding bypassing the relay contacts with a clip lead, When I bypassed the relay with my pocket neon screwdriver, was lucky the tip was still there after THAT test.
I thought of the doubler possibility a couple of days ago but discounted it because the only diodes in the whole thing are only 1A like 1N4007 etc. I'll try to trace out a bit more of that part of the circuit.

 

[73's de Edd]

What ever happened on this situation ? . . . . . . it was just getting interesting.

 

[Technomaniac]

Shelved owing to pressure of other repair work that hasn't abated. I regularly look for service data and even another unit of the same model, it's surprising how nothing has turned up. I found a unit somewhere but it was already marked SOLD. But its really a schematic that is needed. Thank you for being still interested in the project.