Very old Migmaster help

[johnrev]

Just had a mail from Dave who quite rightly said he thought the machine looked like an Oxford and not a BOC. No idea why I've been calling it a BOC as although it has no data plates or info on it anywhere it was sold to me as an Oxford and was just my fading brain that turned it into a BOC.
Glad I gave up the shotgun license many years ago- I'd be a danger to myself and others these days!! I'd better go have a lie down :-)

 

[a111r]

a111r I tried doing that with 4 of those cheap rectifiers a while back and got a nice pop as soon as I switched on. Not sure quality of those on ebay is very good and certainly not matched too well.

Maybe I was just lucky? Luck is what one needs with these low priced parts.
I bought 5 for £10, and then 10 for £5! Used them to make 250 Amp and 100 Amp rectifiers.

They do make a nice pop when they go. I destroyed one previously. I thought I could remember the orientation without checking (wrong!)

 

[johnrev]

First thing I thought when mine went pop but checked the wiring and it was all ok. I put tails on all connections all same wire of equal length about 6inches then soldered all together so each had same current path length and equal resistance in the hope none would be taking more power than another. I fixed them all equal spaced to a square ally heatsink and thought what a clever dicky I was. Then switched on and nearly wet myself!!
Probably just a bad unit and unlucky but the whole thing looked such a mass of wires and ended up way bigger overall that I wanted that I decided to try a Chinese rectifier instead.

 

[johnrev]

Well now had a bit of time playing and have had some fun.
Dave's wiring diagram shows my machine perfectly which has made it easier to follow the wiring of the machine but no circuit diagram I'm afraid.

Dave said the wire feed system is a bit crude and the resulting 'jerkiness' is typical of these machines. It's apparently a bit of a compromise penny pinching design and sends a chopped DC to the motor. At low speed the result is a horrible stuttering type of drive which only sounds smooth at higher speeds. I don't know how the welding would be affected (proabaly would be fine) but it sounds awful so I've done away with it.

I've gone for another PWM circuit for the drive motor as I have for the last three machines I've fettled. I was going to use the auxilary transformer to power this unit but it has two outputs of 12volts and 33volts. The 33volts once rectified and smoothed gave 44volts. I know that would have dropped considerably under load but it still seemed a bit high for a 24volt nom motor so I did the full conversion using a 22volt pwm unit permanently powered feeding the PWM circuit and then via a 24volt relay to the motor giving a brake using the NC contacts.

The relay is operated by the original control board motor output. This chopped output wouldn't pull the relay in at any setting until I added a 47uF capacitor to smooth out the 'choppiness'. With this cap it pulls in reliably and still seems to drop out instantly.

The pot control for the feed was, as usual, not sensible with the first 1/4 of it's travel doing nothing then half travel giving a usable speed range and the last 1/4 just way too fast to be useful. A resistor at each end of the pot track bought the control range nicely spread over the pot range.

Ebay informs me the diodes are on the way so will just have to wait now to see how the beast performs. I have noticed that the diodes are shown with no nuts in the picture. I'd like to think they'll be included but if not, I'll have to try and work out the size and thread and where to get hold of some I guess.

I will report back when I have things sorted with some pics of my shakey welding.

John

 

[eddie49]

Thanks for the progress update.
Regarding the original speed controller: when an SCR is triggered on, if running with a constant DC source it will stay fully on until the supply is interrupted. That is why an unsmoothed rectified DC supply is required when an SCR is to be used as a speed controller. As each half-wave drops to zero, it gives a chance for the SCR to turn off. Apparently back-emf from the motor helps to reverse-bias the SCR and force the turn-off. This type of circuit is described in this thread:
https://www.mig-welding.co.uk/forum/threads/modifications-to-change-polarity-on-cebora-883.20655/
I think that the new diodes are in the DO-9 ( DO-205AB ) outline, and the mounting stud will be M20 fine ( 1.5mm pitch ) :
https://www.ebay.co.uk/itm/M8-M10-M...ITCH-METRIC-THREAD-HEX-FULL-NUTS/201508158295

 

[johnrev]

Sounds like I'm a bit lucky my SCR is turning off then but maybe the cap is helping slightly. I haven't tried it in action obviously but testing seemed to show a clean action and fairly instant relay dropout. I'll keep in mind if I find any glitches in use.

Thank you for the diode info. For some reason I pictured the new ones as being physically smaller than the originals but they are obviously even larger?? I've never seen diodes this big before. Unfortunately the originals seem to be 18mm @ 1.5mm pitch so will need new nuts. I'll order a couple of those you pointed to now as I don't want to be all set up and waiting for them before I can try out my new baby.

Thanks eddie

John

 

[johnrev]

The diodes arrived today :-) They are actually just about the same size as the originals and also have 18mm threads so the nuts went straight on- excellent. They are indeed opposite configuration to the originals so I've had to reconnect the ground and power leads to opposite places on the rectifier/transformer to maintain polarity of the wiring.

Everything is now mounted and connected and ready to go but I have to wait till tomorrow to try it out as it was dark when I finished sorting it and didn't want to trip the fuse and have to stumble around in the dark back to the house to reset it then back outside to lock up the garage.
I did switch power on and take a quick reading of the DC voltage between ground and torch power lead. I will need to verify tomorrow but it seemed to be less than 30volts (not sure exactly what it was) at power setting 5 (out of 6). It didn't really register at the time as I was simply checking there was a DC voltage but that seems less than the AC readings suggested I'd be getting and much closer to what eddie had said would be normal for a machine like this.

Got to go get my Covid test done tomorrow morning at work but I'll get back to the Oxford and do some proper readings and put some wire and gas on it for a first weld. I'll do some runs and check that nothing feels hot then I should be able to fit the casing back on.

.............or maybe I could try fitting a capacitor bank in there. There's enough room in the case to park a small car so space is not a problem......mmmm!

...........and maybe I could look at altering it for a gasless option.........mmmmm!

 

[johnrev]

These three pics are of welds made by the Oxford. It was only a quick test in my cramped workspace so no finesse involved at all!
If they are in the correct order they show various tests at different power and feed speed settings each tried with the inductance settings at low, mid and high and this is what I am most surprised by.
I've never had a machine with more than it's supplied set inductance so have never had experience of what difference the settings make. It really is a massive difference to both the sound and feel of the weld.

The first pic was on a piece of 1mm sheet steel. I ran a quick bead and it sounded very nice. Penetration didn't look impressive but was actually good (top run of the four).
Next down I turned up power and feed speed and did a short stitch weld run as if welding some car body/chassis etc. (mainly to test out the new wire feed setup and make sure it started and stopped as it should) It was gentle and controllable with what felt like good penetration.
I then realised I had the ground in the highest inductance socket so reduced power and feed again and moved to medium inductance. I was surprised at how different the weld sounded. More powerful and 'crackly' as opposed to the gentle buzz of the first weld.
Then I tried minimum inductance and sure enough it felt even more crackly and way more powerful.
Second pic shows the penetrations of the welds. These were all at power level 3 I think? with .8mm wire and at guesstimated settings of wire speed which I didn't spend any time tuning at all. Better welds would certainly be easily sorted with some adjustments.

Third pic is a piece of 3mm steel at higher settings (4 on the right then 5 on the left) again with no fiddling after guessing a wire speed and the same three inductances. Again a big difference in the resulting welds, their penetration and the amount of spatter produced. Clearly the lower inductance produces stronger results and more penetration for the same power settings.

I didn't try power 1 as I didn't think it would manage .8mm wire well and never went to power 6 as it had always tripped my house breaker when I'd tried it before.

I dare say many of you knew the effect of inductance settings but it was a surprise to me and I think makes the welder a rather nice machine for many different situations. I look forward to further experiments as I get to use it more.


I took voltage readings at each setting in turn and got the following:-

16.8
19.75
23.12
26.79
29.38 then tried no.6 and although the garage lights flickered momentarily the power stayed on and gave-
32.6

Overall I am pleased and impressed with the Oxford. I really don't think I will bother with adding capacitors as using the highest inductance it welds with the same lovely buzz I get from my Britweld 195 with it's capacitor and at low inductance it gives impressive 'oomph'.

I really don't want to sell either this or the Britweld and will keep my little Sealey Mightymig 100 as it's just so nice and portable so I now have three keeper machines. I will part with the SIP Topmig 150 and Migmate 130 as they just aren't going to get used by me now but not till lockdown is over as nobody will travel to pick either one up at the moment. Similarly I can't go get the Snapon (Smashweld) either so things are at a stop at the moment. Time to get going and make some of the things I've intended for the last few months. Wifey wants a garden cart to pull her bits and pieces around on so maybe get started on that first.

Thanks to eddie49 as usual for all the help he gives all of us on here and for all the other great guys who chip in with advice and suggestions and make this MW community such a great place to be.

y.

 

[carbon]

I used to use one of these oxford machines, possibly 20 years ago. Did blow the plug fuse if pushed hard when it was used with a 13amp plug.

 

[johnrev]

Yes I haven't tried welding at full power as it is touch and go if my breaker will trip. I'll maybe upgrade the supply at some time with a better breaker but not worried at the mo.
I actually found an original listing for this on ebay from middle of last year and it was said to be a 1980's machine. The guy I bought it from had just added £25 to the price he paid and relisted it at 180 amps instead of the original seller's 170 amps. Fair enough I'm happy with it. Strange that I can't seem to find any info so I reckon it didn't sell in great numbers.

 

[carbon]

I think the one I used had 180 on the front panel

My father bought it new but its unlikely that i have any paper work for it as it was over thirty years ago

 

[johnrev]

That's interesting as the original listing said 170amps but the seller didn't seem to be sure himself. Shame there's no sign of the usual data plate on this one. Unusual for a welder not to have one and especially a British made one of that era.
This one has been tarted up at some stage with a respray so no idea what colour it might have been originally as there's no sign of any different colours anywhere. There's also no sign of where a plate may have been removed from the usual place on the back panel although I think there is a 'patch' looking plate on one side panel. I'll have a closer look at that tomorrow.

 

[carbon]

Front panel was white and the case was green. I still have it but it is not easy to get at it to look, but I'll see what I can do

 

[johnrev]

That would be great thanks Chris. No hurry so don't make any special effort. I know what it's like moving piles of 'useful stuff' to get to the thing lurking at the back.

Take care
John

 

[eddie49]

The diodes arrived today. They are actually just about the same size as the originals and also have 18mm threads so the nuts went straight on

..and maybe I could look at altering it for a gasless option....

Sorry I guessed wrong about the stud thread size.

For a neat gasless option, as you probably already know: the best method is to join the positive output from the rectifier to a red Dinse socket on the front panel. Then bring the cable that used to link the Euro socket to the rectifier out through a grommeted hole in the front panel and fit a male Dinse plug. You then select if the torch is plugged in to Pos, or Neg ( i.e. one of the three Inductance choices ), and the work return cable and clamp goes into whichever polarity socket is not already used. It would be interesting to see what results you can get with decent gasless wire ( e.g. Lidl 0.9mm ) in this heavy-duty pro machine. I have found that .9 wire feeds fine in a 0.8mm roller and torch.

Your welding test results are very impressive, showing this machine was well worth fixing and fettling, and it is a keeper.

Regarding inductance, I recently wrote this in another thread:

"Having an inductor in the MIG welder output circuit helps with flow-out ( wetting ) of the weld pool. When the wire touches the workpiece there is a dead short. Since MIG welders use a constant-voltage power source, current rises very quickly. The inductor slows the rate of current rise, which also helps to protect the rectifier diodes. It alters the time ratio between the dead short, which melts the wire, and the arc, which heats the weld pool and the metal of the workpiece.
More explanation of inductance in MIG welding here:
https://www.twi-global.com/technical-knowledge/faqs/faq-what-does-inductance-do
https://www.weldingtipsandtricks.com/mig-welding-inductance.html
From the latter link, the basic advice is: use the low inductance setting for thin metal, and high inductance for thick metal."

Like a lot of what I say, I just made that up as I went along, but the bit about varying inductance altering the "bzzz" timing and the heat input does appear to correspond with what you have found.

 

[johnrev]

Better to have the nuts and not need them than need them and not have them so no probs at all (I hope nobody finds the preceding in any way a humorous inuendo??).

I think I will get around to setting it up for gasless later as it could be useful if I run out of gas but right now I'd like to get some practice with it. I wonder if the low inductance setting may prove extra useful with gasless as the two machines I've converted so far seem to produce a fair bit of spatter even though the welds seem really good with the cheapo gasless wire I bought from ebay. Maybe a high inductance will suit gasless wire? I'll show the results on here when I get around to trying it.

Once again Eddie thank you for your help with this and my other projects. It's good to know someone with your knowledge is prepared to spend the time not only offering advice but researching the sourcing of parts.

Take care and stay safe.

John

 

[carbon]

I made a mistake the one my father bought was a 170 model

 

[johnrev]

Thanks Chris that's the one alright. The data plate isn't very comprehensive but does at least tell me it's power output and that it's 125amps at 100% duty cycle which is a big step forward.
Really appreciate you looking it out for me so now I've actually found there are at least two in existance!
You've obviously got something you like more now but shame it's stuck away and not in use any longer. Reckon if ever you need it in emergency it will fire up for you so handy to have I guess.
Thanks again

John

 

[carbon]

It developed a fault and was only being used for one specific job at the time which was coming to an end so didn’t get repaired. Wouldn’t feed or energise the small relay on the control board from memory. Possibly a simple fix but as it had a bigger welder just continued with that

 

[johnrev]

Shame it could probably have been fixed (with Eddies help:-) but if you're not needing it I guess there's no real incentive to get stuck into it. I'd happily give you £100 for it but it's too far away to collect even if the virus allowed it and way too heavy to think about a courier. Maybe one day you'll get around to sorting it again.

All the best

John