Vintage Welder SCR Gate Pulse Waveform Diagnosis

[tsxi11]

Hello All,

Electronics newbie here. I'm rebuilding and restoring a vintage Miller Shopmaster 300 AC/DC welder and running into a brick wall trying to get the main output rectifier to work.

I've utilized the official Factory Service Manual (FSM), diagnostic procedures and as many OEM parts as possible during this process with a few exceptions made for availability, price, etc. One of those exceptions was using DCR604 SE1616 SCRs in the main output rectifier.

The welder control / SCR triggering board is a new old stock unit, and I'm using a RIGOL DS1054 scope for diagnostics. All control board test input and output voltages measure within spec...but the SCR pulse driver waveform is not correctly shaped when connected to the rectifier. The SCR gates are apparently not opening / staying open / playing nice, and of course that means no welder output.

Attached pics are of the FSM pulse driver waveform, and what I'm seeing on my scope. Attached pdf is the DCR 604 datasheet.

As noted, I also have the FSM which contains full schematics and could upload excerpts or in whole if necessary. Plenty of pictures / notes as well.

I'm reaching out to the masters and experts on the EEVBlog to look for any assistance on what could be causing the negative voltage spike on the driver waveform, or any other diagnostic / repair info on this problem.

Many thanks in advance for your replies.

Best regards,
Anthony



 

[Armadillo]

Apparently the time bases are not the same. One ~7ms duty with a period of 17ms and the other is 300us duty with a very long period. I would suspect timing and capacitor related problem.
You would need to provide the FSM for a collaborated fault findings, if you still can't locate the faults.

[oldway]

Hello and welcome to the forum

I do not see anything abnormal in your circuit.

The small negative pulse is probably due to the demagnetizing current of the pulse transformer and its amplitude does not seem to be sufficient to damage the SCR.

On the other hand, it is short positive pulses and it is therefore necessary that there is a current greater than the holding current to obtain the conduction of SCR's.

Without load, the output voltage remains equal to 0 even if the SCRs receive a correct pulse.

For testing, connect a 47R 100W power resistor at the output of your welding generator and everything will be fine.

NB: did you checked if the syncronising information of the pulse generator board is correctly phased ?
If it is 180° wrong, pulses occur during the reverse anode / Cathode polarity of the scr's and it will never conduct.
Try to invert the two wires coming from the synchronisation transformer.

NB2 : For diagnosis of a phase controlled circuit, you must always use two channels of your oscilloscope, one with the anode/cathode voltage and the other with the gate/cathode pulses.
Synchronisation of the oscilloscope on the anode/cathode voltage information or on line option if available.

[tsxi11]

Thanks very much for your replies, Armadillo and Oldway.

I should take the opportunity to clarify the capabilities / characteristics of this welding power supply. It's a 240 volt, single phase, 60 Hz, 300 amp output, AC / DC and CC / CV machine. 13 kW. Manufactured in mid 1995. It's a big, heavy transformer-based stationary shop welder designed to handle all metals and processes.

Oldway - there is a power resistor (R4 in FSM, 5 Ohms, 300 W) used for SCR holding current when the welder is in CV mode. Upon close inpection, I discovered a broken Nichrome leg on it, which I have patched up for testing here. However, it reads out of spec at 10 Ohms. I've hooked up the second channel to the rectifier output, but unfortunately I dont presently have an R47 100W resistor on hand to do as you suggest.

Armadillo - Whilst appreciating that the time bases differ widely, what's interesting to me is that subsequent to repairing the resistor the pulse waveform shape has now changed. The scope shows it has lost the negative voltage dip and assumed a form more in line with the FSM example.

Still no output from the rectifier, however.  But I'm hoping we are on the right track with this outcome. Could the 5 ohm discrepancy make the difference here?

I've attached the latest waveform pics for your kind review at your convenience. The entire FSM is too big at 1.95 Mb so I'll upload other separate relevant portions / sheets shortly in a separate post.

Also attached is the OEM parts list for the control board with component values. Hope that helps.

[tsxi11]

More FSM pages attached here:

[tautech]

I should take the opportunity to clarify the capabilities / characteristics of this welding power supply. It's a 240 volt, single phase, 60 Hz, 300 amp output, AC / DC and CC / CV machine. 13 kW.

Actually, looking at pdf 36, it a two phase welder !

Two line supply, not phase and neutral.

Edit.
Sorry I see there is provision to operate on single phase outlined in pdf 12.

[floobydust]

Are you using an isolation transformer for the scope?
I'm wondering how the DC secondary-side is referenced to earth ground, and how the scope is connected. It looks like the secondary floats but has EMI capacitors to chassis ground, so control board voltage-sense must be isolated as well.
I would check continuity to make sure nothing is shorting to ground, as far as SCR heatsinks or a ground fault elsewhere.

The output contactor is pulling in, and has good continuity?

For the scope, I'd set the trigger source to LINE for troubleshooting, to make sure AC sync is correct phase.
If you replaced the SCR's maybe the gate leads got mixed up.

[tautech]

R7 if fitted is across the output in all modes. Does it check OK ?
(pdf 36)

[tsxi11]

Hello All,

Tautech - Just looked at R7, and it checks ok at 27.5 ohms. It appears to be in decent shape.

floobydust - Over the last few months with this project, I've performed numerous ground fault / windings leakage tests with my trusty old Megger BM100. The power transformer megs fairly clean at around 10 to 30 megohms for all primary and secondary legs to ground.

I discovered a ground fault thru one of the "Z1 Stabilizer (inductor) windings to ground and ended up rewinding the unit. It was originally wound with 6mm x 6mm square aluminum magnet wire, but rewound with standard #6 AWG THWN copper wire exactly as possible to OEM specs with # of turns, winding direction, etc. Tight fit, but no problems so far. It looks clean at 50 megohms to ground.

SCR heatsinks are mounted on fibreglass isolation boards, they look good. No cracks, etc. Main selector lugs, switch legs and other bits meg clean as well.

The unit showed the power transformer and inductor sharing a common chassis connection, and the chassis is connected to the earth ground wire. The control board sits on plastic standoffs attached to the chassis.

There are 2 contactors. CR1 and CR2.

CR1 (24 & 120 VAC coil) "Provides means for gun trigger from wire feeder or remote contactor control to turn on and off weld output", switching between the main panel and remote device control by way of switch S3. CR1 makes or breaks the connection between wires #104 & #105 (thermostat TP1).

NB - CR1 did not close or energize the welder upon power up from either S3 Panel or Remote positions. Jumpering pins A & B (24VAC source) on the RC1 (14 pin remote receptacle) closes CR1, but still does not energize the welder in any mode. [ #104 & #105 were joined for all recent testing. ]

CR2 "Provides proper connections to SR2 for either CV or CC voltage feedback signal". Switches between #21, #22 & #52. No problems with closing. Solid 10 VDC signal from PC1.

Scope connections were Channel 1 backprobed on RC53 at PC1 (gate signal ref'd to cathode) and Channel 2 at positive & negative plates of rectifier.

Added the FSM Troubleshooting Guide and another welder schematic to this post as well.

[tsxi11]

Floobydust - forgot to mention no iso transformer was used.

I was also super careful to not cross gate / cathode leads or any SCR physical locations on the rectifier assembly, even going so far as to engrave SCR numbers on the heatsink.

Plates were tightened to 8.8 KN clamping force, as per SCR datasheet specs.

[floobydust]

You done some good work, this looks like quite a big beast to repair. I'll offer some ideas but not an expert with this.

The SCR drive waveform looks wrong, in that it should be a gated high-frequency burst. I think op-amp A4 (pg44) maybe is the SCR-drive oscillator and it will stay off without a permissive signal from: RC54-3 remote/on switch; CR1; limit switch LS2 then RC54-10 thermal circuit breaker? on the transformer secondary.

edit: op-amp A4 +ve feedback looks like a comparator with power-on reset. With SCR's this big, I don't expect a single gate-pulse.

It's a bit weird, the CV or CC (panel potentiometer) setpoint comes in on RC54-8 and then out on RC54-10 for the permissive. Below 0.7V on RC54-3 seems to enable everything, it pulls up to +15V otherwise. Can you check the voltage on those two terminals.


The SCR's may be turning on; just no continuity to the power flow, or the phase-angle is so small that output is zero, from the setpoint being seen as zero.

It's important to know the control board grounding, on similar gear I have seen an output inductor (i.e. Z1) ground fault melt a wire or PCB trace and everything was floating when it should not.

The voltage sense across R6 is an op-amp instrumentation amp. So it's not isolated from ground, it can take limited common-mode voltage and has resistors from RC55-9, -8 to ground. You should be able to read those with an ohmmeter (not megger kind of values).  The control board is grounded at RC51-4.
I'm saying it looks like the welder's output floats only a bit, within the limits of the voltage-sense circuit.

[tautech]

The SCR drive waveform looks wrong, in that it should be a gated high-frequency burst. I think op-amp A4 (pg44) maybe is the SCR-drive oscillator and it will stay off without a permissive signal from: RC54-3 remote/on switch; CR1; limit switch LS2 then RC54-10 thermal circuit breaker? on the transformer secondary.

edit: op-amp A4 +ve feedback looks like a comparator with power-on reset. With SCR's this big, I don't expect a single gate-pulse.

That's what I thought too and as you say that is the normal way to trigger power SCR's...with a burst.
But the first image in this thread apparently from the FSM shows the correct waveform ....for this application.

It's apparent R7, the output burden resistor is OK so there should be sufficient latching current as it's operative in all output modes whereas R4 is only connected the CV mode where it also acts to discharge the smoothing cap bank.

It seems to me that the OP's jumpering of TP1 or some other control mechanism is the fault here.
Like what is TP1, thermister or mechanical thermal switch ?

Rather than just bypass them, exactly what they are and how they interact with the control board needs to be ascertained.

[oldway]

Very interesting but unfortunately I have no time to analyze all this in detail now.

My impression is that everything is normal but that the pulses are generated at the minimum (conduction) angle while the anode cathode voltage of the thyristors is close to 0 .... That's why there is no output voltage and why the pulses are only 350µs long instead of 5ms.

It should be checked whether the current limitation is not adjusted to zero current, this could explain the problem.

EDIT: also possible : defective HDI Hall current sensor