Apex / D+L TIG 200 AC/DC - No control panel after IGBT fail - any ideas?

[wcloudy]

Hi,

Trying to diagnose my Apex TIG200 - Have run out of ideas hoping someone might be able to point in the right direction (Or if anyone is willing to have a shot at repairing)

IGBT failed, last time I just replaced and everything was OK - this time even after replacing the control panel does not light up. AC power is present in the green area through the inductors/filter caps. There appears to be a neutral and live track running to the left of the daughterboard in purple (it has LEDs for HV and LV and neither light up). There appears to be a DC power supply below the purple area (and also a -15V linear regulator) If I inject 12V to the cap in blue, the control panel receives 5V via a regulator on the horizontal logic/microcontroller area - control panel lights up and rotary controls work - however none of the press buttons work.

The relay on the left couples the high current AC to the the DC. It appears the white relay to the bridge rectifier and the two tiny connections to the purple board (via diodes) are the only connections from the green AC area. I guess I'm missing some DC voltages. Transformer under purple area doesn't appear to get power & I've tested the FET out of circuit behind heatsink to the left of it (which seems good).

Any good suggestions? Any idea what the small daughterboard pcb is doing? Lower picture is the reverse of this board....

 

[eddie49]

Hello, and welcome to the Forum.
On your photos, I'm trying to locate what I call the DC bus. From the On/Off switch ( on the left with blue and yellow/green ( ? ) wires ? ), and via the mains noise filter components, there is normally a direct connection to a bridge rectifier. There may be more than one rectifier, in parallel. A soft-start circuit, before or after the rectifier, uses a relay to short-out a pre-charge resistor or thermistor after a short delay. This is to allow the large electrolytic smoothing capacitors to charge. These are typically about 500uF and 400v each, and will provide the 325v DC bus that usually runs the whole machine. I think your second photo shows the pin connections for four capacitors, under the board. Have you confirmed that this voltage is present across the caps?
( As you know, that is a dangerously high voltage, and charge will be retained in the capacitors.)

 

[wcloudy]

Am aware of the dangerous/lasting voltages present - but always good to reiterate, thanks! There is a soft start varistor over the relay contacts (right of white relay) for precharge before the rectifier. However the relay doesn't energize. I will check whether HV DC is being pre charged. Maybe if not a check circuit prevents the relay closing onto discharged caps (and maybe the low voltage DC coming up)?

 

[eddie49]

The relay coil is probably 24v, and often the low voltage power supplies need to be enabled before it can operate. Those supplies should be coming up during the precharge period, if they are based on DC-DC converters, as the voltage on the smoothing capacitors rises.
( I think on a few machines the low voltage supplies are switch-mode PSUs tapped directly off the AC mains, before the bridge rectifier.)
( The soft-start timer may just be a capacitor charging through a resistor, and triggering the relay through a Zener diode.)

However, the relay is there to short the precharge resistor and take it out of the circuit. This allows the inverter power stage to run at full current capacity. Even if not shorted by the relay, the resistor ( typically 40 to 100 Ohms ) will not have much effect on the expected DC bus voltage.

This recent thread, and others linked in it, may be helpful:

Thermal arc 175te PTC/NTC

Hi new here i recently got hold of the thermal arc 175te for cheap as not working kept blowing fuses upon inspection i noticed a blown capacitor and a bad ibgt so i ordered new and replaced them. The machine powered up for a second a went bang and blew the same cap upon further research...

www.mig-welding.co.uk

 

[wcloudy]

Eddie - good call - there is no voltage at all on the main DC bus, and the PTC is up at 60-70c. PTC tests OK out of circuit. Suggests to me the DC bus is shorted? Perhaps some other IGBT's/diodes or main caps have blown?

EDIT: Another visibly popped IGBT underneath the heatsink - Machine needs a fair amount of disassembly to get to it though

 

[wcloudy]

Alas, swapped that one - no difference, will need to pull and test all I guess... (Unless there is a more likely candidate?)

 

[eddie49]

I doubt that a main cap has shorted. They tend to go off like shotgun cartridges, or at least bulge, if internally shorted.
As you suggested, it is more likely that one or more IGBT is shorted, or being held permanently on. This failure scenario is mentioned in post #37 of the "Thermal Arc" thread I linked above.

When you can reach them, resistance checks would confirm if there are shorted IGBTs.
As suggested in post #33 of that linked thread, a shorted IGBT can destroy it's gate driver circuitry, and then the absence of control on the gate can cause a new IGBT to self-destruct. Using a low-voltage current-limited DC supply instead of the normal high-power mains-driven 325v DC bus is one method of limiting the risks whilst the fault is investigated.

 

[wcloudy]

Thanks again - will go through that thread... Will certainly do further tests on bench supply to try and avoid further damage...

 

[wcloudy]

Pretty lucky - it was the 3rd IGBT I pulled (of the 8) - shorted across all 3 pins. DC bus now unshorted, welder starts up, control panel works and engages AC relay with 60V of bench supply across the DC. Have some more IGBTs on order, but obviously the concern now is the gate driver/zener.
Thanks Eddie, your insight led me to the fix