SIP Wire Feed Motors

[Richard Pearson]

I will check the readings again tomorrow but I am pretty certain I did switch my meter to dc!
Whilst I got an ac reading on the low side of the transformer I did not check the ac terminals on the bridge rectifier which I will do tomorrow.
If there is no dc output from the rectifier surely the relay would not operate and no mains power to the welding transformer?
Where does the 24vdc come from to operate the wire feed motor if not from the small transformer and rectifier?
When testing the transistor how will I know if it is bad?
Thanks again for your help

 

[eddie49]

Yes, you're right, it is illogical that the relay would click if there was no DC output from the rectifier.

The small transformer on the PCB can only supply a couple of Watts, which is only used for the relay coil. The wirefeed motor needs 10 Watts; it gets this power from the main welding transformer ( after the relay has turned it on ).

The first component to test would be the 1K potentiometer. This can be done in-situ, no need to unsolder it. There are three terminals. Across the outer two you should read 1,000 Ohms, or less due to other components in parallel across it. The centre connection of the three is the moving contact or "wiper", which moves along the circular carbon track when the control shaft is rotated. From the centre contact to either of the two outer contacts you should see 1K ohms, or less, and this should increase to a maximum or decrease to zero as you rotate the control left or right. If a pot fails, it is usual that the centre ( wiper ) completely looses contact with the track.

The blue 100 Ohm trimpot should read about 100 Ohms, or less, across the outer two of the three contacts. No need to unsolder it, and don't rotate it. If it has failed, it will have gone open-circuit, or possibly short-circuit - zero Ohms.

There are only two fixed resistors on the board, marked with coloured bands. If they fail, they would burn or crack. It's enough to check across them in-situ, to confirm neither open- nor short-circuit.

The diode, the black component with a white band, needs to be unsoldered at one end to test it. It lets current pass in one direction only. Your meter should have a "Diode Test" function, marked with the symbol for a diode - like an arrow. Connecting the meter leads across the diode one way will read "OL" or "1." meaning open circuit, infinite resistance. The other way should show a low number such as ".600" or ".700", or maybe a number between 500 and 700, which is the forward voltage across the diode. These two readings mean the diode is good, it is letting current pass one way only. A faulty diode will either be open-circuit or short-circuit in both directions.

The diode is actually a 1N5367B 43v 5W Zener diode, available here:
https://www.ebay.co.uk/itm/Zener-Diode-5W-Voltage-Regulator-50-Values-3-3V-200V-5pcs-/121919177687

 

[eddie49]

The pinout of the TIP35C power transistor is as follows:

For the purpose of testing, an NPN transistor can be thought of as two diodes connected back-to-back, with the two anodes ( the arrow-head part of the diode symbol ) joined to become the Base of the "transistor".
There are 6 steps to test a transistor:
Set the meter to the Diode Test range.
1. Clip the positive ( red ) lead of the meter to the Base ( Pin 1, leftmost pin of the TIP35C ). Put the negative ( black ) lead on the Collector ( Pin 2 ). Expect a low resistance, meter should read about 0.600 or 600.
2. Move the negative lead to the Emitter ( Pin 3, rightmost pin ). Expect a low resistance again, as in step 1 above.
3. Remove the red lead from the Base ( Pin 1 ) and replace it with the black lead. Put the red lead on the Collector ( Pin 2 ). The resistance should now show as high, reading "OL" or "1."
4. Move the red lead to the Emitter ( Pin 3 ). The resistance should still be high - "OL", as in step 3 above.
5. Using the two meter leads, check from Collector to Emitter ( Pin 2 to Pin 3 ). Resistance should be high.
6. Reverse the leads and check again, Collector to Emitter, 2 to 3. Expect high resistance again.

If this transistor is causing the fault, it is probably short-circuit Collector to Emitter.
[ Note, if you use an old-fashioned analog meter to test diodes or transistors, the red lead actually supplies a negative voltage.]

 

[Richard Pearson]

Thanks for your very helpful reply.

I am really confused now!!

The PCB transformer receives 240vac and outputs 19vac.
The bridge rectifier registers nothing either vac or anything dc
But
When I press the torch button there are plenty of clicks:
One is the gas solenoid
The remote PCB relay doesn't click but you can feel it vibrate
And the main rectifier which clicks louder the higher the power setting.

So power is obviously getting to the main transformer unless I am being really dim!

I tested some of the components on the PCB.
1k pot showed 26.3ohms across the ends, 15.8 middle and left and 1.2 middle and right.
When turned up to max I got 1.3 and 20.3
My meter was set on 200ohm range as nothing registered on the 2k setting

The 100 pot read 20.5 across the ends

The resistors which I believe are red, red,black i.e 22 ohms were showing 21 for one and 1.3 for the other.

I haven't tested the transistor or diode yet as I need to remove from the board.

Are the readings low because the components are still on the board or are they faulty? Assuming of course that I am doing everything correctly!!

Any thoughts?

 

[Seadog]

If you're getting nothing on the bridge rectifier it sounds like there is a broken track or a very dry joint, probably on the transformer. A photo of the underside of the board would be nice. The pot may be OK. It's impossible to work out how the resistors are connected without an underside view, but may be paralleled with the wiper.

 

[Richard Pearson]

Here is a photo of the back side of the PCB.

I still don't understand why or how the relay is turning on the main transformer if there is no dc coming out of the bridge rectifier. The small transformer on the board is producing 19vac at the low end.
What am I missing?

 

[eddie49]

The original fault was that the wire feed motor fell apart. It was replaced with a new wirefeed assembly, with a 10 Watt motor. It was then found that the speed control potentiometer did not have any effect on the wire speed ( but stepping welding power ranges up did increase the speed ).
For the new motor to turn at all, the main welding transformer must be on. For it to be on, the relay ( off-board version ) must be closed. For the relay to be closed, the coil must be getting power. That power must be coming from the small transformer on the PCB. That transformer outputs 19v AC. The only way that power can reach the relay coil is via the bridge rectifier on the PCB.

Maybe the board was coated with varnish after it was soldered, and you need to pierce that with your meter probes to get correct readings at the soldered joints.

The off-board relay should not vibrate, it is a DC relay. Click on, click off. Perhaps the bridge has gone short, and the relay is seeing AC ? That would be strange, because it would require two of the four diodes in the bridge to have gone short, and the other two to have gone open....

( Hmmm, well, if two diodes went open, and the correct other two are still diodes, that would give you half-wave DC, which probably would make the relay vibrate ... )

Regarding the gas solenoid, is it 230v AC mains or DC? The coil voltage may be marked.

The main rectifier shouldn't click, but the adjacent main transformer may hum.

With the resistance values that you have been able to check, it now seems more likely that the TIP35C transistor is shorted. That could have happened whilst the commutator of the original motor was self-destructing. I would suggest removing the transistor from the heatsink and the board. Drill out the Pop rivet, and cut the pins if you have to. Measure the resistance values on the board again, and then test the transistor.

 

[Richard Pearson]

Hi Eddie (?)

I really appreciate the time you are spending helping me with this.

You could be right re varnish on the board - I will have another go tomorrow.

But as you say the relay must be getting power as it switches on the main transformer. I may have slightly mislead you re the vibration. The relay doesn't click - well I can't hear it but that may be because the fan is running, but if I touch it when I pull the trigger I definitely feel a click. At the same time there is a bigger click (on low power setting barely audible but on high power setting quite loud - my SIP 140 does this also). Sounds like an electro-magnet! After the click it does hum.

The click is coming from the rectifier pic below - assume this is a rectifier?

The gas solenoid I assume is 24vdc as it takes power from the rectifier above - the 2 black wires which I had disconnected from the solenoid.

I'll get the transistor out tomorrow and test again.

Many thanks
Richard

 

[eddie49]

Hello Richard,
You're welcome - I'm sure we will solve this problem soon!
Really odd that the rectifier seems to make a noise. There are four diodes with their bases pressed into each aluminium plate. The other end of each diode is a thin solid wire, soldered to the power connection tags so that the diodes share the load in parallel. Sometimes these wires break off or corrode. It may be worth checking for that ( with power off ) using a torch and probing the wires gently to confirm that they are still attached.
Regards, Eddie

 

[Richard Pearson]

Hi Eddie,

Ok an update. I tried again but this time using my analogue mete

I used another meter and am prett certain that the 2 pots are ok. On the 1k pot I am getting readings across the 2 outer tabs and variables between the middle and outer as I rotate the knob.
The 100 pot reads 60 across the outer.
I snipped one of the diode wires and get a reading of 30 one way and nothing the other.

I then removed the transistor and think we have found the culprit! Your first guess was right! I was getting a short circuit across all pins.

I will order a new transistor and hope that sorts it out.
Would it be advisable to change any of the other components as they are cheap enough or leave as is?

I was just thinking about the clicking I mentioned when I pull the trigger. Would you expect the click to be more pronounced as the power was increased? Perhaps it is just the relay which is mounted to the case amplifying the noise. After the click I get the low him.
The fan is a little noisy so I may change that. Assume it is 220vac? Any idea where I might get one?
Thank you for all your help - I’m learning!!
Regards
Richard

 

[eddie49]

Generally only one component fails. Sometimes a high-current device can take something out. In this case, I think the motor blew up, began to short internally, pulled a lot of current, and blew the power transistor. From your measurements, I think the other components are OK, so I would not replace them.
The trigger-on noise could be coming from the main transformer, rather than the rectifier. Something metallic might be loose and moving when the power-on current surge magnetizes the core. Probably nothing to worry about.
Yes, the fan runs on AC mains, but again, I'd live with it, until the bearings starts squealing or it grinds to a halt.

 

[a111r]

Yes, externally the same layout but you can see the feed drive is different. Hopefully no flex and the motor beneath is larger - we'll see...

It's interesting that one hardly ever sees a post about Clarkes with this poor wire feed issue.
The motors are same size and are still driven by the welding voltage. The drive mechanism may flex less, but it's hardly a 'step change' in quality...

I'm keen to get this old SIP back because it was mine until 5 years ago. I sold it to a mate who it appears has kept it outside, so it likely looks the same state as that ebay example!

Ye olde red / silver Migmate 100 has now come home to roost.
Superficially rusty but electrically it's actually still working, after 5 years in damp / outside storage.
In the four years that I first owned it, I did nothing to it at all. I feel a resto. is now afoot, having seen inside it.

Note the nice motor, internal gas valve, tiny wire feed PCB!

 

[Richard Pearson]

I ordered a replacement transistor that arrived this morning and fit it.

All is now well again with the wire feed.

Many thanks for all the help, especially eddie49 for his expert guidance.

 

[eddie49]

Richard, thank you for the update. I'm glad you have been able to fix it, that is a good result. It's always nice to know that a decent old machine has been brought back to life. Regards, Eddie