Drying Oven - Preset Temp Fail

[tmcw]

Here's an unusual one...

This is a Gallenkamp oven, used in a laboratory space, goes up to 220 Celcius, but is usually maintained at about 115 Celcius. Used for drying glassware, molecular sieves, and so on. Oven is in excess of 20 years old.

It heats up fine, there is an orange light on the front that indicates that the unit is heating, and when it reaches the preset temp, the orange light goes out. All good so far.

However, after the preset temp is reached, the temperature continues to rise, power to the heating elements isn't cut off.

Temp continues to increase until the over-temp temp setting is reached, in which case the red light comes on, and the power to the elements is shut off, the over-temp switch is tripped, and the unit cools down.

Pressing in the reset switch allows the unit to heat up the chamber again, and it will again over-temp and switch off, etc.

I've attached a couple of photos of the front panel and electronics inside.

Fuses are ok. Any area in particular to home in on for testing?

[redwire]

Check if the triac on the big heatsink is shorted between MT1 and MT2. Those are the fat wires on it. That would leave heat on all the time until the safety cutouts trip.
Triac is TAG semiconductor T1513MJ 15A 600V TO-220 isolated, non-sensitive 50mA gate.

Is the temperature readout correct? If it's stuck then I would look at the sensor for water ingress or corrosion etc.

[tmcw]

Thanks for the tests...

With the multimeter on continuity/diode mode, I got a readout of 0.127 either way on the fat wires.

When I disconnected one of the fat wires from the circuit, testing the triac, the meter read open.

I'm pretty sure that the temperature readout is correct, at least a couple of degrees Celcius either way.

We actually have 2 other ovens like this in the building, but I'm not sure if they work. I turned one of them on earlier, but the temperature display was a bit wonky, and the temp seemed to be fluctuating, so it may have different issues. I'll test the other one tomorrow.

[R_J]

What is the model number? maybe there is a manual/schematic available. That white/green wire connected to G2/G1 controls the triac, If this wire is disconnected does the oven heat up? if not the fault is likely on the board, maybe the optocoupler is bad.
Is the heating element connected to triac? or is the heating element controlled by a relay somewhere?

[tmcw]

The model number is OVE.100.110S. A search throws up nothing. When it was supplied 20-odd years ago, it probably had all the schematics included, along with a hardware list.

I'll try disconnecting the white/green wire tomorrow and see what happens.

Yes, the heating element is connected directly to the triac, on one side of the element to the triac, the other side of the element to the door switch. When the door is opened, I would presume the heating element would normally switch off, and the circulation fan also switches off (which it does).

[tmcw]

When I disconnected the white/green wire, the oven doesn't heat up at all. Turns on, and orange light comes on for heating, but no heat.

On a more serious note, I just noticed that the door seal is completely perished away, probably like that for months/years. I don't think we'll try saving this one, maybe take some parts out of it for the others.

[stj]

try replacing the 2 chips,

if you fix it first, you will know all the parts are good.

[R_J]

If disconnecting the triac gate does not turn on the oven, then it could be a bad optocoupler (IC4), can you work on the board when the unit is operating? if so check the voltage across pins 1 & 2 of the optocoupler when it is heating and again when it should be at temp and the heat element off.
With no power applied, check the resistance between pins 4 & 6, maybe the internal triac is shorted
Here is the datasheet for the ic.

[redwire]

Mechanical failure - temp sensor beatup up, broke, got water in it, broken wires, corrosion I would also rule out. It might be a thermistor but the resistance could be compared between ovens.

The PC board has a +5V, +9V and +18V testpoints so I would check those.

The opto-coupler should have high resistance pin 4-6. Possible replacement MOC3063.

[kby202]

Hi, hope someone can offer some advice. We have the same Gallenkamp drying oven. I tried to perform a Portable Appliance Test; it failed on the insulation test. I read if it's been unused in a long time there may be a build of moisture or if its dirty. It got cleaned and switched on to dry out any moisture. Initially it did pass the insulation test, but once the oven cooled it failed the insulation test. After some troubleshooting, the only way to pass the insulation test was to disconnect the element. As this oven is no longer supported does anyone have a pdf of the manual and recommend a generic replacement heating element?
Thank you in advance, kby

[stj]

you dont insulation test devices that contain mains filters or switching psu's
not unless you want to repair them afterwards.
your putting 500-2000v into parts rated for under 400.

PAT testing is only required for PORTABLE equipment with fixed cables anyway - dont fall for the scammers who say you need to test everything.
https://www.hse.gov.uk/electricity/f...ce-testing.htm

[sam_sam_sam]

Can you please post some pictures of the heating elements

To add to what STJ said you want to ONLY test the heater element with a megohm meter no other part of the device or will probably be repairing more because of he said earlier

[kby202]

Thank you for the information I've been reading a lot of conflicting information.

[stj]

the whole idea behind pat testing is to check for cables that are damaged or coming loose inside a device.
thats why the "P" is for portable and you dont test stuff with removable cables - although you can inspect the cables and plugs.
the main things that actually should be inspected are power tools like drills and vac-cleaners/floor polishers etc.

[redwire]

I think there's a myth out there that the heating elements absorb moisture and develop high ground-leakage currents. As if it's raining inside the oven, the heating element ends are outside the oven anyway and have silicone end seals. "Drying it out" there's really no way for moisture to get out of the heating element tube.
The MgO powder inside I think gets a thin spot or the element expands and just curls and shorts to the tube through it. So anything to shake it up a bit can temporarily make it work (no ground fault) sometimes, for a little while.

The PAT test looks like only 500V, the triac's and opto's insulation should take it.

[CapLeaker]

Food for thought. About 20 years ago I had the top element in the oven fail like every 6 months or so. After a while it started popping the fuse too. Moisture one the element, because the climate around here?
Well, back then I had the old fuse panel which had clip in fuse blocks that contained two screw in fuses. Guess what?! Yep, the metal clips that connect the fuse block to the power bars in the fuse box were corroded. Cleaned the fuse block up with a wire wheel, put new fuses in and never had a problem with the stove again.

[stj]

Originally Posted by redwire

The PAT test looks like only 500V, the triac's and opto's insulation should take it.

that depends on the device used,
some can run upto a couple of KV and the really high end ones can dump several amps into a big crock clip to check for a solid earth connection between cable & chassis!

[redwire]

It makes no sense to do that PAT test in an earth-grounded appliance, a heating element short to chassis is a common fault and not a shock hazard. There's no cords to fail if it's mounted permanently.

I think this thread found the moisture bake is a myth: https://www.badcaps.net/forum/showthread.php?t=84293
Glowing red at 600°C/1,100°F expels moisture lol. I can see a GFCI/RCD tripping due to storage rust/corrosion and/or the heating wire coil expanding and touching the tube when it gets hot.

[CapLeaker]

I live in a high moisture area. I never had any success repairing or rejuvenating heating elements by driving the moisture out of them. Maybe it works for a very brief period, but to me a spent heating element is a spent heating element. Like I said before in my other post, the cause of my problem was corrosion in the fuse box destroying the heating element multiple times until I figured it out. A lot of older fuse boxes around here have this inside corrosion problem. Took me a few months and trips down the basement.

[redwire]

I don't get the fuse box affecting the heating element. Usually there is high temperature wire/spade connectors from the heating element to the control circuit, and then that goes to the fuse.
Corrosion I find causes hot spots whichs melt things such as PVC wire or switches etc.