Miller 300DX - Help 6 message diagnosis

[JOEPRO]

The Owon HDS272S oscilloscope will be a useful tool for you, and being battery-powered and in a plastic case, there are no concerns about accidentally shorting mains voltages on inverters to ground with the probe "earth" clip!

If the precharge timer control circuit on Page 92 of the Service Manual does apply to your machine, only transistor Q28 is involved.
Transistor Q27 is in a Darlington Pair with Q29 on Page 94.

However: diode D44, just above Q27, does not look too good? It is a Zener diode, part of the Error and Compensation Amplifier on Page 95. It should test as a normal diode, conducting one way and blocking the other way, until a specific ( but unknown... ) Zener reverse voltage is reached, when it will break down and conduct in reverse.

Checking further the 2nd one looks OK visually at least.

They are all the same values accross this bpard, this photo is just confirmimg the spec of these zenor diodes- 48 PH



The other old one I blew was the same zenode diode



So I have ordered new replacements. I am getting readings both ways on the one in the board. I presume to test it id have to lift one leg and at that point I may as well renew it too as they cost pennies.

Now that we know the total DC bus voltage across both capacitors is 323v, which is close to the expected value, I think we should step back a moment and confirm that the precharge bypass thyristor has not, or has, been triggered. If it is not conducting, the bus voltage is being reduced by the voltage drop across the 200 Ohm precharge resistor. When not welding, we don't know how much current the idle inverter is pulling, but if it is, for example, 0.1 A, then from V=IR, there should be 20 volts across that 200 Ohm resistor, and if it was shorted by the thyristor, that would drop to nearly zero and the bus voltage across C3 and C4 would rise to 343 volts.
Could you please check the voltage across the precharge resistor, as in your Post #40, but measuring DC volts and with the machine powered up? The voltage should jump to a high value with the initial current surge into the empty capacitors, then drop exponentially, then - hopefully - fall to zero when ( if ) the thyristor shorts it. An old-fashioned analogue voltmeter would probably show this better than a digital meter.

Please see the readings in this video

Those are DC volt readings between the terminals as picture in post #40. C3+ and R2+.

I think the Thyristor could have got damaged too by my reverse polariting the relay. So I think my next steps are renew the Thyristor and both zenor diodes. I will test the old ones when off the board to know the results. Then power up again and see if any behaviour changes. Ideally the relay will be able to pull in fully. By then the scope should arrive too so I can return to test the gate signal properly.

(I sort of sound like I know what I'm talking about here. Rest assured I have no idea!! Eddie your input is fantastic, thank you!)

 

[eddie49]

The diode that looks a bit odd - D44 - is shown as a Zener on the circuit diagram, but a flyback diode across a relay would not be a Zener, and a 1N4148 is not a Zener, so they should not be the same type number. Maybe the circuit diagram does not apply correctly to your model?
Anyway, the D44 diode can be tested by unsoldering one leg and then using the Diode Test range on a meter. We do not know the Zener voltage, but it should show the usual "OL" or "1." one way and "700" or less the other way.

The voltage test across the precharge resistor seems to show about 2 volts after the initial surge, which is possibly correct across a thyristor that is fully-conducting.
Another way to confirm this would be to isolate the thyristor output pin ( the second from the top ) of the mains input bridge rectifier SR1.
At the moment that pin joins the capacitor bank via the small screw-on PCB connector, so measuring it will show the capacitor voltage regardless of whether the thyristor is being triggered on or not.

To isolate the thyristor pin, remove the PCB connector board, put a couple of layers of tape on the underside of the board at the pad that surrounds the screw for the second-from-top terminal, and put the board back on but leave that screw out. Then isolate your meter probe with sleeving or tape, leaving just a millimetre at the very tip bare. Push that probe through the tape so it connects to the thyristor screw hole, and put the other meter probe on the negative ( top terminal ). If the thyristor works and is being triggered, a second or two after power-on you should see about 320v DC on the meter.

 

[JOEPRO]

The diode that looks a bit odd - D44 - is shown as a Zener on the circuit diagram, but a flyback diode across a relay would not be a Zener, and a 1N4148 is not a Zener, so they should not be the same type number. Maybe the circuit diagram does not apply correctly to your model?

Yes I dont think the wiring diagram matches my machine exactly.

Anyway, the D44 diode can be tested by unsoldering one leg and then using the Diode Test range on a meter. We do not know the Zener voltage, but it should show the usual "OL" or "1." one way and "700" or less the other way.

I Removed the old transistor and it was definitely bad. 1.4+ volts and where it should have been open line it was 0.005V. I tested a new matching 2N3904 transistor which read as expected and fitted it to the board.

Confriming the old one type -



I also reversed polarity using my bench power supply on a spare 2N3904 to see if it got damaged, retested and it did not affect it at all.

So I think my original transistor was indeed bad before I worked at it which is good news.

The voltage test across the precharge resistor seems to show about 2 volts after the initial surge, which is possibly correct across a thyristor that is fully-conducting.
Another way to confirm this would be to isolate the thyristor output pin ( the second from the top ) of the mains input bridge rectifier SR1.
At the moment that pin joins the capacitor bank via the small screw-on PCB connector, so measuring it will show the capacitor voltage regardless of whether the thyristor is being triggered on or not.

To isolate the thyristor pin, remove the PCB connector board, put a couple of layers of tape on the underside of the board at the pad that surrounds the screw for the second-from-top terminal, and put the board back on but leave that screw out. Then isolate your meter probe with sleeving or tape, leaving just a millimetre at the very tip bare. Push that probe through the tape so it connects to the thyristor screw hole, and put the other meter probe on the negative ( top terminal ). If the thyristor works and is being triggered, a second or two after power-on you should see about 320v DC on the meter.

At this point I have the new working transistor fitted. I will await the 2x new correct zenor diodes and fit them then re fit the board and turn on the welder to see if the relay now closes. It'll be a few days until the zenor diodes arrive so I will come back to update in a few days