Le Maitre Neutron XS fault

[davepick]

Hello all.I'm trying to fix this hazer, it goes through the cleaning cycle during which it makes some smoke, then the fan starts and no smoke. The LED flashes one time indicating NTC failure. So I obtained a new element from the customer who had one in stock and fitted that, then I got four flashes (indicating over-temperature) but I guess it could also be caused by thermocouple problems?

There is hardly any Voltage on the test point at R5.

I just want to know what the thermocouple resistance should be when cold? Both of the ones I have read very low, about 1 Ohm, surely this can't be right? Heating the element up with the soldering iron produced no change in resistance.. suspicious! Physically they look OK no breaks or twists in the fine wires.

The elements are quite expensive so I don't want to order another one unnecessarily!Dave.

[gareth]

Call Le Maitre and speak to Steve Lloyd in the service department - he's one of the most helpful service engineers I've had the pleasure to deal with, and he'll almost certainly be able to help you to diagnose the issue over the phone.

[richardash1981]

I just want to know what the thermocouple resistance should be when cold? Both of the ones I have read very low, about 1 Ohm, surely this can't be right? Heating the element up with the soldering iron produced no change in resistance.. suspicious! Physically they look OK no breaks or twists in the fine wires.

 

If it is a Thermocouple (not a Thermistor) then it should have very little resistance.

A thermocouple consists of two different metal wires joined (twisted / soldered / welded) together. So the resistance just that of the wires. It senses temperature because the different metals produce a voltage across the terminals, but this voltage is only a few mV at most (so you can't measure it with most general purpose meters). It is vital to get the two wires of the thermocouple the correct way round for this reason!

The normal failure mode is open circuit (which is interpreted as very high temperatures in most systems), or a short from the thermocouple terminals to earth.

 

[AndrewE]

I've had the 4 flash issue a couple of times with just a poor connection. Try unplugging and reseating the DIN connector as a first step, and then checking the connections inside the DIN plug too to make sure the contacts are still good.

[davepick]

Yes, of course a thermocouple will have a low resistance... it was the mention of NTC in the service guide that set me on the wrong track.I've seen no Voltage from it but I'll look more carefully with sensitive equipment, as I said there is no voltage on the op-amp output and I've changed it in case it had failed.I wondered if leakage from the element tube to the thermocouple (it measures low) could have blown the op-amp up.

I'll try to talk to the LeMaitre man on Tuesday.Thanks for the suggestions.

 

[davepick]

I've tried a few more tests (it's cool in the workshop). The thermocouple was short on both elements and produced no volts when heated. I found a similar thermocouple in the junk box and tried that and it worked when touching the element (Voltage appeared on R5 test-point) so both elements are faulty.Oddly when I ripped the thermocouple off the old element it started working! Around 10 Ohms resistance and a few mV output when heated with the soldering iron. Trouble is, there's no way of fixing it back on...

New element time.

 

[KevinE]

if it works when you detach the spot-welded thermocouple from the heater tube, it suggests there is leakage somewhere. A tiny electrical leakage from the tube heating circuit to the temperature control circuit (they're normally mutually isolated) will cause the temperature reading to be misread by the controller and trip to fail. The heater needs several amps and the thermocouple several millivolts..the two don't mingle well.

 

Typically this is caused by either a breakdown in the triac insulating pad (the triac switches the heater on the secondary side) or a breakdown in the insulation between windings in the mains toroidal (it sits low down and can fill with haze fluid on a badly maintained machine).

 

For the record the heater/sensor assembly is plug and play, you can't get the polarity wrong as you already know.

 

Le maitre do a comprehensive service manual for the Neutron Star XS and the diagnostic checks are all in there.

 

PS..the controller also measures rate-of-change of temperature on the heater, and it knows what it should be. If it's outside that window, it will also trip with an led blink code. B-)

Edited August 27, 2019 by KevinE

[davepick]

Thanks Kevin. I've sorted it now, the new element's thermocouple was shorted by the cable-clamp in the DIN plug, slackening that off produced the expected reading of about 10 Ohms cold and around 10mV when heated with a soldering iron. The voltage on the test point only got up to about 2.5V - it should be more than 4V - so I suspected loose connection of the high current heating circuit. It turned out to be a crack in the Schottky rectifier, one leg was a bad joint. Replacing the rectifier fixed that fault and it all works a treat.

[KevinE]

so faulty element from new? What was wrong with the original one, did you find out?

[davepick]

so faulty element from new? What was wrong with the original one, did you find out?

 

The original was also shorted (but not in the plug!) I actually pulled the thermocouple off the element without breaking it, and then it measured OK. Anyone know of a high temperature adhesive?!

[Hilary Watts]

Anyone know of a high temperature adhesive?!

On the advice of Steve Lloyd at LeMaitre I silver-soldered the thermocouple back on to the element of a Neutron hazer and it works fine.

Hilary

[davepick]

I'll give it a go...

Thanks, Dave.