Lead-free solder is such a PITA (rant/whinge)

I should add that coil failure mode is open cct when hot - as in, half-way through cooking a meal - and continuity returns when cooled to near room temperature. Also that replacing the wall oven comes with a penalty of having to carry out significant kitchen mods. Grrrr!

Grid independence, I'm sure.

We went with gas because the power system here was flakey and could go out for hours at a time. Our gas hotplates have one 'D' cell providing ignition, while the wall oven does require AC for the igniter/flame-monitor/gas-control (BUT that's all able to be jury rigged off a PC UPS in about a minute flat) so cooking during a blackout is a non-event.

I think you can still buy Goyen valves, solenoids, and rebuild kits:

Can you buy the same coil type but in a 24 Volts AC version? If so, that plus a transformer may be more reliable than the 240VAC version.

BHA didn't seem to have the coils. The other two are Pentair outlets (Pentair is slightly majority Tyco owned) and will be selling the same parts I currently repeat purchase (*) from Pentair/Tyco here in Oz. These are surprisingly made locally (Sydney) as they were in the "old" Goyen days. It's fair bet that any I source nowadays will have the ROHS curse on them.

I did some Dremel surgery today on one of the dead coils. Despite having differing copper resistivity figures at hand (no consequence) and the wire diameter testing my micrometer, from known ohms of a good coil and some spreadsheeted maths I guesstimate it is 33 or 34AWG.

Maybe Chris' suggestion is worth investigating. The surgery wasn't enough to find the dodgy termination, but thicker winding wire *may* get a better joint.

(*) I date-mark the parts as they go in. The one that just failed was one day short of 18 months, on an oven that gets used about once a week on average.

Well the winding wire will be heavier gauge at 24VAC. Whether that will achieve a more reliable ROHS solder bond is unsure, but it probably couldn't be any worse.

Have you checked the voltage at the coils ? No more than you are using that oven it seems there may be something more than just the coils going on.

I have had very little experiance with the lead free solder, but as it melts at a higher temperature it would seem to me that it should be beter for the simple joints as far as the thermal cycling.

It is possible that the solder has dissolved too much of the fine wire, making it too thin. I have had a lot of trouble soldering very fine wire, especially with lead-free solder - the wire will get thinner as it dissolves in the molten solder, leaving it even less robust than its original fragile state. Leaded solder that was deliberately pre-saturated with copper ("Savbit") was supposed to be good for preventing that, but I found it generally unpleasant to use.

As your coils develop their faults over time, I also wonder if the manufaturer left some fairly active flux inside the encapsulation that might be slowly eating the wire near the solder joint when it is warm.

Perhaps you can tell whether the break is at the solder joint, by measuring the low-frequency capacitance of each terminal of the damaged coil with respect to everything else. I doubt that knowing where the break happens would be much use to you, but it is something the manufacturer should be looking into.

Can the valves be relocated to a cooler location? Or can you substitute more robust valves made for hot location use? Eric

Ok, I see you've done your homework. Yes, ROHS is epidemic everywhere.

That's rather thin and could easily fuse. According to the wire table: #34 will fuse at 5A. What do you measure for coil DC resistance?

I agree. Heavier wire may solve or delay the problem. Methinks it would be interesting to know what temperature the solder joint is experiencing. A thermistor or thermocouple glued to as close to the solder connection as possible might provide some interesting numbers. If the position of the solenoid above the oven is the problem, that will show it. Perhaps adding a metal heat shield between the coil and oven?

For fusing, perhaps a small value resistor or "surgistor" in series with the coil might reduce the peak current enough to let the thin wire survive. With an inductor, there should NOT be an inrush peak, but I'm thinking some kind of glitch, spike, or peak might be arriving via the 240VAC line.

You seem to have the fault identified, but just to add another perspective - since you're getting multiple failures of the solenoid coils, and you're /assuming/ RoHS lead-free soldering is to blame without having (yet) found evidence, perhaps you could look at other common factors. Is the supply voltage to the coils stable, for instance? Loose connection causing unstable voltage/surges/sags at the coil? Voltage too high? etc. etc.

The line voltage is in the range 240-242, the coils are embossed 240V.

In my relatively limited lead-free solder experience (just the service bench - where I always use leaded solder for rework)> the joints seem to develop a dry crystalline characteristic over time when in situations subjected to thermal cycling. IMHO it isn't a melting point issue but an alloy characteristic.

Indeed. It would be handy to have a thermoprobe with logging on it. Note to self: see where I can borrow one.

The dual valve mechanism is bolted to the oven. The coils (see ebay item# 322017672259 for the ones we "consume" - but at a much better price than he's asking ...) sit over the metal housing which encloses the valve plunger (armature) and has a metal spacer each end for location and ?thermal separation?. So it would be necessary to elevate the entire incoming gas pipe/valve syhstem to relieve conducted heat to any significant degree. And if it IS the cycling - rather than the actual temperature reached - which causes the failures then reducing the latter may achieve nothing.

Dunno. Could do (but it'd be 18 months before I'd know if it made a difference).

Fair comment.

Without logging it (and I don't have access to a Dranetz any more) I would venture that it is. Reading taken at random times show good regulation, and nothing else in the place which is surge-prone (electronics and incandescents) is dying at all.

Nope, snug as.

Really don't know. The Pentair/Tyco/Goyen engineer that I discussed this with asked for a failed one to be returned for evaluation - so I sent two (as I have a pile of them otherwise just gathering dust). When I followed up a few weeks later, he had departed and his replacement was unaware of the whole matter, and could find no record of their return despite supplying him the RMA number.

He also declined my offer to send more, really being disinterested in finding out the reason for the failure.

The failure mode is going open cct when hot. As they cool continuity returns.

I have made up a test lead with pea lamps that I connect up so there is a (6V) lamp in series with each coil. When there is a failure I connect this into circuit and re-energise the oven. (Doesn't take long, I'm getting pretty polished at that now!) It typically takes about ten minutes to get back up to failure temp. Then I can see which coil is going O/C and replace it from my stock of new spares.

To be able to test it would require a fair bit of setting up.

See above re manufacturer "interest". They sell quite a number of these during our winters as they are used in gas wall furnaces. I guess they are figuring that as long as they keep selling them as replacements, why investigate.

See reply to JL for what would be involved.

The Pentair people (valve manufacturer) said they have no such solution.

Missed that bit. Around 1600 ohms.

Have you looked into another valve company ?

I retired a few years ago from a large company and we had hundreds of valves similar to that and very few of them failed, especially the coils. Most did operate on 120 volts.

Not sure how the valves are in your oven, but maybe you could try 2 of the 120 volt coils in series.

Assuming 240VAC, that's: P = E^2 / R = 240Vrms^2 / 1600 = 36 watts That's way too high dissipation for a solenoid valve. Are you sure those are 240VAC solenoids and some other AC voltage? Are the solenoids run by 240VAC or some other voltage?

Also, could you check the eBay listing number? I want to see the coil specs. Nothing found: