Very old Migmaster help

[johnrev]

I bought this BOC Migmaster 180 the other day and wondered if anyone had access to a wiring/circuit diagram at all? I enclose some pics which may jog someones memory.

It is a non-runner mainly, I think, because one of the two massive rectifier diodes ( it's a centre tapped secondary transformer) is open circuit. The other one I'm not sure about as yet as I haven't had time to test properly. The wire feed sounds really odd too. Seems to sort of 'stutter' when running mainly noticeable at very lowspeed. Sounds a bit like a two stroke motorbike idling?? as if it's a PWM control switching at 5hz? Will look into that in time.

I only collected today so have had very little time to play- Christmas and all that. I removed both rec diodes and powered it up. I get an increasing AC voltage from the transformer with switch steps but my breaker tripped out at max setting. Not sure if that's a bad sign since it had no load at all at that point. First look at the wiring suggests it might be a simple step by step tapping selector power switch system but not sure of this yet. It would be nicer and a lot easier if I had a circuit diagram to work to.

It has been stripped and tarted up at some time with a quite nice spray job and tidy up. Just a bit of dust collection now which I'll clean up.
It has Spot timer and burnback as well as wire speed pots and three inductance choices.It's well built as you'd expect and bl...y heavy. Copper wound transformer and chunky auxi

liary supply to the electronics. Control board seems surprisingly simple at first glance with only one IC which I believe is a dual 555 type timer (I say that from a very quick glance earlier so could be a dual op-amp??) and a few disceet components. One puzzle is the pot mounted on the board just right of centre in the pic. Does anyone know what this is likely to do?


So if anyone has one of these or can point me in the right direction for info I would be grateful as I can't seem to find anything on here or elsewhere on the net about it.

Thank you
John

 

[johnrev]

Just noticed that one or two of the plugs on the board don't seem to be lined up with the pins? Look like they need to be moved over a bit or maybe turned 180 degrees. Can't see that something that simple has been overlooked during refurb. I'll take a look tomorrow.

 

[johnrev]

Thanks for the redirect- must have had a brain burp there!
Just tested the diodes and in fact, one is short circuit and the other seems OK. Seem to have trouble with my digital clamp meter getting sensible test readings sometimes. Used a battery and bulb for more certain results.
Neither of the diodes has a readable ID number as they are too tarnished. I would think they are probably difficult to find and damned expensive anyway so will find alternative rectifier. I have a 400amp diode rectifier package which looks puny in comparison but could be OK I suppose.

I'd feel way more confident playing with this machine if I had service details so if anyone can help I'd be really grateful.

John

 

[a111r]

I saw that on Ebay. Nicely made machine.
Do you like a soldering challenge? If so, parallel this lot up for a super cheap 250 Amp rectifier. And maybe upgrade the fan somewhat.

https://www.ebay.co.uk/itm/5PCS-KBP...40d397587dbec1272d53|ampid:PL_CLK|clp:2334524

 

[eddie49]

These diodes are 240 Amps 200 volt, £12.60 each, 2 in stock, £7.75 shipping:
https://www.radwell.co.uk/en-GB/Buy/SEMIKRON/SEMIKRON/SKR-240-DIV-02

 

[johnrev]

Thanks both.
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.
eddie thank you for the link. Just went on the site and went through the order process and worked out at £42 for some reason. Have left it and will go back and try again later. Still cheap for those at £21 I guess.
The 400amp unit I have is a two diode setup of schottky diodes mounted on a heatsink plate and is rated at 400amps but only 200volts. Loads from Chinese sellers on ebay at around the £10-£15 mark. Seems too good to be true but I've used the full wave bridge units before and they seem to work OK. They do look weedy little things beside these massive diodes from the welder though. Do you think it's worth a try eddie or would I just get another pop?

John

 

[eddie49]

That site has various options for used/new/original packaging etc. If I select 2 x "Radwell Certified pre-owned 2-yr warranty" and then go to Checkout, it shows a total of £25.20 for 2 diodes, plus shipping.
With a centre-tapped secondary you only need two diodes, a full 4-diode bridge will not work. The end-to-end voltage on the secondary will be 2 x the centre-tap voltage. For example, if the maximum welder power setting OCV is 30 volts DC, the secondary will be outputting 30-0-30 volts AC. You need to confirm that the remaining "good" diode is Normal rather than Reverse polarity, i.e. stud-cathode. I assume that the studs of both diodes are bolted to a common heatsink, which will become the Positive output to the torch, and the centre-tap will be Negative, via the inductor to the Dinse sockets.
No reason why these 240 Amp diodes should not work, or your existing 400A 200v 2-diode array should be fine. The 200 volt peak inverse volts ( PIV ) rating divided by sqr rt of 2 ( 1.414 ) equates to 141 volts, and you should only need about 60v end-to-end secondary AC voltage, shared across both of the diodes.

 

[johnrev]

Eddie you are great thank you.
When I spent a couple of minutes trying the trans output I used an old meter I keep in the garage and got near 100volts max out. May be a meter thing but the voltage did increase smoothly with each switch position. 100v across the total secondary did seem high but had no time to double check. I'll try again later with my Fluke and tell you the readings I get from each 'end' to

tapping. Hope the trans is not damaged- that would pretty much write off the machine I reckon. Didn't see any signs of heat in the windings so hope it's OK.

Here's a pic of the module I bought originally for 'stock' in case I got a SIP with blown rectifier. I guess the actual diode part of the BOC component may not be as big as it looks when you take away all that metal. Still prefer the idea of replacing with nice chunky original components so will go back to Radwell and try again.


Getting no time to play at the moment. We're not really having a Christmas with the covid stuff and wife is working for six days straight over the period (she's works in a carehome) but still seems we're spending silly amount of time getting stuff and preparing for the non-event!

So to all on this great site I wish you a good and safe Christmas and a less stressful 2021. Thanks for all your help and advice and if Santa doesn't bring me a circuit diagram for the BOC maybe one of you will find one for me???

John

 

[hermetic]

You will get the equivalent rectifiers from techarc, technical arc Ltd in York. If you send them a pic and the amperage required they will have them. They are manufacturers and repairers. I am lucky as I live close enough to call in on them, nice ppl!
Phil

 

[hermetic]

https://technicalarc.co.uk/

 

[johnrev]

Thanks for that Phil and I will ask for a price but suspect it will be expensive from them- they don't give prices on their site so must wait till after the holiday to contact them.

Radwell have a good basic price but their minimum order is £35 so with shipping it's £42 for the pair. I would happily buy three of the diodes but there are only two shown in the UK. Decided I'd order some other stuff to make up to the £35 but that proved harder than expected. Looked for other rectifier diode sets for stock but all those I found seemed to be available in USA and not UK. Eventually decided on four stud diodes to take me just over the £35 with three from USA and one from UK and order went to my basket OK. Total went to £60! They included a 'tax' charge on the imported ones.

So I'd be better to have the minimum charge added to my two diodes and pay the £42 unless I find a few odd ones stocked in the UK which I'll likely never use just to make up the £35. Penny pinching I know but I do hate wasting the £10 just to make up the minimum order charge.

John

 

[a111r]

Tec Arc have some of their stuff on Ebay.
https://www.ebay.co.uk/itm/400-AMP-...613295?hash=item1a7aeeb26f:g:ZLUAAOSwUzFakwwA

As you suspected they are not cheap. Considering it is probably from the East:
https://www.ebay.co.uk/itm/1PC-DQFI...225620?hash=item3b45451494:g:vMgAAOSwAJxe1bme

The old, small machines I mess with invariably have Italian made rectifiers; the month and year of build is marked on them. I'd imagine in new machines they would also be Chinese.

 

[eddie49]

Two of these would do the job, for a total of £28.50 inc. postage :
https://www.ebay.co.uk/itm/Sevcon-1...00V-300A-Huge-Stud-Diode-OM0295J/392279429576
but not sure if Normal or Reverse polarity, so in the worse case you would need to swap over the heatsink and centre-tap connections to the torch and the inductor.

 

[johnrev]

Yes that's what I thought. I've asked tec-arc for a price but sure it will be high for big beefy single diodes. Been trying to find a way of getting the order from Radwell sorted but in the end I've decided to go the ebay/chinese route. I've ordered a couple of the two-diode modules which are rated at 200amps per single diode. I'm going to parallel up each pair to effectively get a single 400amp diode and use the two in place of the original BOC diodes (I think I understood what I just wrote?)

I don't trust Chinese claims for their components even though I've used some before with success. Belt and braces overkill with these modules will, I hope, do the job for the grand price of just over £16 delivered (eventually).

Just finished fettling a little migmate 130. Did the usual mods including building a new wire feed unit, Euro torch, PWM speed control and converting to gas/gasless. Quite a nice little machine now but not worth much I suspect. The satisfaction of getting it running properly is nice though.

The Migmaster is really nice though. My kind of machine- old, big and chunky- a bit like me! At the moment it's looking like a keeper once done but then I've said that twice before.
I've also got a snapon/Esab 180 coming when the vrus allows me to fetch it and I expect I'll want to keep that one too once it's sorted. I need a bigger garage.

 

[johnrev]

Dammit how come I didn't find those? I searched Ebay and missed them completely.
Thanks once again Eddie that's perfect. I've just ordered a pair and will put the Chinese things in the 'storage' drawer. Sure they'll come in handy.
Still haven't had any time outside but I've been wondering about that 100volts reading from the transformer. If it's correct do you think it suggests there's something wrong or could it be an OK voltage?

 

[eddie49]

Dammit how come I didn't find those? I searched Ebay and missed them completely.

I did a search on eBay for "stud diode", with 400 results, then cut that down to 60 results by selecting "New Other".
I first found these diodes from that Seller:
https://www.ebay.co.uk/itm/Sevcon-932-62745-IR-840-54246-Huge-Stud-Diode-OM0295K/392283452854
which are smaller and only £10 each, but the specs are unknown.

I have never seen a stud diode with an axial threaded top connection. I think those connections should be tightened first, before the stud is fixed, so that there is no torque load on the glass/ceramic seal of the diode.

Due to use of a different type of junction ( metal-to-semiconductor rather than semiconductor-to-semiconductor ), the voltage drop across a Schottky diode is less than an ordinary diode ( 0.3v against 0.7v ), so the device dissipates less heat and therefore the packaging can be rather smaller and simpler.

[ The ultimate version of reducing device power dissipation is when MOSFET transistors, which can have an on-resistance as low as 2 milliOhms, are switched on & off in synchronisation with an AC waveform to act as diodes. See "Active Rectification".]

The pot on the control PCB may be a preset for minimum or maximum wirefeed speed.

The 100v AC end-to-end voltage on the centre-tapped secondary means a 50v DC OCV output, which is scarily high for a MIG welder. I have not seen more than 42v. With huge diodes and thick copper wire and strip windings on the transformer and the inductor, that OCV is not likely to drop much when on-load.
Perhaps the machine can be configured for 220/230/240v mains input, and it is set wrongly? If not, maybe the power range switch is faulty, so one section of the primary winding is getting bypassed or shorted? ( More wire on primary = less voltage output on secondary. )

 

[johnrev]

As usual I learn more from a few paragraphs you write than hours of research. Thanks for that and the good advice re tightening the diode connections.
I'll check for input voltage selection but didn't notice anything with a quick look when I got it home. I did wonder if the selector switch had been wired wrongly when it was reassembled after the refurb and I will check that too.

The control board pot seemed a bit overkill for a speed trimmer where you usually see a small trim pot but depending where in the cct it is sitting it maybe takes more current than a small trimmer could handle?? Wire feed roller seemed to be way too fast at full chat than could ever be useful so that's another thing I'll check.

I saw those other diodes that guy was selling and did try looking for the specs but as he says, there doesn't seem to be any info on them. His other listings didn't seem to offer anything at great prices that I saw but the diodes you pointed out seem good value to me.

I'll get out now and have another look over the machine and see what I can find.

Thanks Eddie

John

 

[johnrev]

Well I managed an hour outside today and things look slightly different now.

I removed the voltage switch and checked how it operates. It appears to have a common connection and six steps which bring in the tappings in order. All this works fine and each step is operating as it should.
I disconnected the transformer primaries and attempted to get a resistance reading from each to the common and then from each to it's neighbour. Well I tried with two different meters and got gobbledegook. Both the elderly Fluke and younger Chinese clamp meter just didn't have the accuracy to make any definitive readings at all. My impression was that the overall resistance of the primary was around an ohm or two but even that's a guess.

I then connected all back together and tested AC voltage out again and got:-

38.9
45.3
52.8
61.0
67.0
Position six tripped the fuse again as soon as I pulled the gun trigger. So seems 33-34volts at posituion 5 which looks a bit healthier than 50volts. I'd guess maybe 35-36volts at highest setting.
I'm assuming the breaker is tripping with the surge at position 6 even without a load? My Oxford 110amp oil cooled stick welder does this regularly and if I reset the trip it will usually then behave OK. For the time being I will ignore pos 6. I never use that high a welding power anyway and my garage runs from a standard 15amp domestic unit from the house which I haven't had uprated.

BTW the chip is a darlington array (relay switcher??) and the transistor on heatsink is a BT152 (motor power feed??)

 

[eddie49]

Those voltage readings look a lot better! Still going to be quite a punchy machine though.
With a 1-pole 6-way power selection switch, the transformer primary wiring sounds quite straighforward; some welders have switching that can be baffling.

As you have found, measuring the primary resistance using a typical DVM with a lowest Ohms range of 0 to 200 is difficult, and the best result you can get is "a couple of Ohms". I have a Beckman precision bench-top DVM, bought specifically for this kind of work, with a lowest range of 0 to 20 Ohms.
My Eland 175 MIG is a 200 Amp machine, with 8 power ranges. At the lowest power setting ( which is the highest transformer turns ratio, with all of the primary winding in circuit ), the primary resistance is 0.90 Ohm. Increasing the power setting decreases this resistance progressively, down to 0.14 Ohms at position 8. Measuring directly at the transformer primary tapping wires, avoiding the resistance of the input power cable, the contactor, and the power range switch, the readings are 0.90, 0.66, 0.44, 0.37, 0.30, 0.23, 0.17 and 0.14 Ohms.

I also cannot use the highest power setting. Pulling the trigger when set to position 8 blows the 16 Amp cartridge fuse in the garage CU. With a resettable trip, you can sometimes "build-up" magnetic flux in the core of a transformer Arc set, so that the back-emf limits the input surge.

The BT152 is a thyristor ( SCR ). They can be used to control the speed of a DC motor, when fed with unsmoothed rectified AC.
I have never come across a multiple Darlington pair array chip before. Seven of them in there, and powerful, at 0.5A and 50v each; the PCB tracks show only 3 are being used.

 

[johnrev]

Thanks Eddie. That explains why I couldn't get anything sensible when testing resistance of the primaries. At least the OP voltages now are less alarming.
The Darlington array was, I think, shown as no longer available when I looked at the data sheet. It was suggested as useful for relay control so I guess it's part of the spot timer/burnback cct which seemed to be working when I tried it.

The SCR being used for wire feed speed sounds a little odd. Does this mean it's effectively a PWM type output running at mains 50Hz frequency and would it be sort of rectified for a chopped DC output? Or would the output be smoothed somehow to give a variable DC or is it possible there's an AC motor for wire feed? I'm guessing it's giving a chopped output as the wirefeed motor doesn't sound at all smooth running at low speeds.

I didn't notice when I took the pics but looking at them afterwards the PCB soldering looks a bit suspect in places. May be an optical illusion and will have a close look but think I may go over all the soldering just in case. I was surprised how many unused spaces there are on the board. I wonder if it is also used for higher spec'd machines with more knobs to twiddle. I'm not a fan of 'bells and whistles' so the fewer the better I reckon.

I sent a message to Dave "notanormalcoder" to ask if he had cct diagram for this machine. Hope he doesn't mind being bothered for info all the time but he seems a nice guy and I can't find anything via internet searches so far. Even if I don't necessarily understand the info it's good to have it available to check what's connected to what!

I'll let you know how things go when the diodes arrive. Thanks again for your help and patience.

John