help reading circuit for welding machine (negative switching supply)

[coppercone2]

I am hoping that someone good at reading circuit pathes can answer a simple question for me

https://forum.millerwelds.com/filedata/fetch?id=615167&d=1622257766

In the middle of this circuit diagram, you can see the output for -15V . The description says its regulated from the 30V

If you just feed 30V into the board, should that -15V rail start up? The -15V is generated by Q21 and inductor L1 I think.

Does it have any other input requirements for starting to generate that rail?


I bought the pomona banana to round pin adapters (great purchase BTW, the kit of them allows you to connect banana cables to adapters that go into pin sockets or over pins, and they have necks that are flexible) so I can plug a supply into the board. When I add 30V, the 15V is made, but the -15V shows +0.5V.

I need to know if I am missing some input that enables the negative voltage rail to start working. I don't think so, but its a big circuit and I thought I might be missing something. 

I circled the area in red which I am confused about

[Andy Chee]

Your circled area appears to be an inverting buck-boost regulator, driven by an oscillator made from a schmitt trigger NAND gate.

But I can't work out how the regulation feedback functions, suffice to say, it looks like the designer has had some major difficulty in converting a negative voltage feedback into a positive voltage feedback signal.  The entire regulation circuit is non-intuitive!

[coppercone2]

yeah I loathe this blasted board

[coppercone2]

Around the area, all the 15, -15 and 28 points are connected to the pins labeled as such.

I ignore the Q11 and R13 area, because that is biased at 15V, so its just a 15V diff between30 and 15, aka its between two low impedance points, so that Q11 can't do anything to the +15 V rail, and it measures at +15V when its on.


Then you have the R64 tap and the R55 path, which go into some annoying bullshit that can mess up the gate some how. I don't know what that area is doing, it looks like its on because the difference is the 3 ohms on the 28v RAIL??? ?

[coppercone2]

what is supposed to oscillate this circuit.

The transistor driver is biased between +30V and +15V to get +15V full scale. That is correct because I measured it.

The transistor driver is controlled by Q25. All the oscillator signal must go through R55 to power the transistor base.
R55 is connected to the NAND gate.

THen it gets fucking confusing.

[coppercone2]

So every one of these circuits needs a feed back I think. I don't think they do open loop stuff.

So really your R19 resistor connected to -15V is the feedback from the output of Q21.

So the feedback path is mega weird to me.

[Andy Chee]

The oscillator is  the easy bit!  The 4093, C18 & R38 form a schmitt trigger oscillator.



Being a NAND gate allows the oscillator to be "enabled" and "disabled" by the signal on pin 1 of the 4093.

What I can't figure out is whether this pin 1 is the primary regulation mechanism (via Q16), or the regulation is through R22 & Q10 (which changes the feedback resistor, thus oscillator frequency).

[coppercone2]

is this what is happening?



I don't understand what is going on there

What the hell is R64. Is that current something?

waht in the shit is Q12 doing ?

[coppercone2]

and the main thing, did i miss something, because I don't see what could disable the -15V oscillator ? it has no other inputs there as far as I can tell?

I checked and there is continuity between all the +15, -15 and +28 pins in that diagram I re-drew. I thought maybe there was a missing external 15.

[Andy Chee]

What the hell is R64. Is that current something?

waht in the shit is R30 doing ?

R64 in conjunction with R53 & Q18, form some sort of current limiting circuit.

R30 looks is just a pull-up resistor for Q12 (to keep Q12 off when there's no signal on the base)

[Andy Chee]

and the main thing, did i miss something, because I don't see what could disable the -15V oscillator ? it has no other inputs there as far as I can tell?

When Q15 is on, this shorts out C18 and disables the oscillator.  This is probably related to the current limiting.

[coppercone2]

Ok but to step back from the complexities, the only "nodes" that might 'control' this thing are +15, -15V and +30V.


I measured every single node to the pins. All +15, -15 and +28V go to the pins on the bottom of the PCB. And the -15 is all feedback.

so I am thinking that something is wrong with the board that its not oscillating when I plug it into a power supply ?


**********
IT Is just a board, laying on a ESD mat, connected to a +28V power supply (linear). As far as the circuit shows, it should oscillate and make the -15V from just that ?

To me it looks like all the other pins are basically isolated from the function of the -15V rail.


I don't see any kind of lockout, disable, power down, etc functionality linked to the -15V rail.


ITS CONFUSING THE HELL OUT OF ME BECAUSE IT LOOKS LIKE IT HAS 4 FEEDBACK PATHS!!!!!!!!!!

[Andy Chee]

There are two ways I can see the oscillator is disabled;

a) Q15 is on

and/or

b) pin 1 of U2 is low

[coppercone2]

but there are no pin that control that, its all integrated on the PCB.

That makes me think its broken.

I thought one of the other pins might be related. But I don't see anything. 


I can't believe it has 4 feed back paths. !!!!! who the hell does this !!!

[Andy Chee]

To check for oscillator function, I would remove Q15.

For safety, I would also remove Q21.

Doing this should enable the oscillator, with or without -15V.

[Andy Chee]

Where does -15V go on receptacle RC1?

[coppercone2]

You mean where on the welder? I am not sure. It goes into the mother board. I think part of it  goes to the LEM current sensor. That schematic no one has made yet. It might power the other PCB that is the high voltage cascode looking mosfet shit

I re-drew it again I think

[Andy Chee]

You mean where on the welder? I am not sure. It goes into the mother board.

Knowing where the -15V goes on receptacle RC1 is vitally important!  The current limiting of Q21 is there for a purpose!

[coppercone2]

I thought it did not matter since the card is on a bench.

This card had a explosion on it, which was repaired,on R2. All semiconductors other then diodes were replaced, the resistor was replaced, but it won't work. It happened because some idiot on a nother forum wrote the wrong transistor and i lost my diagram and trusted or possibly misunderstood somehting

For me the usual procedure for checking confusing things is to make power supplies run first. If those are good you can isolate.

[coppercone2]

ok so the answer in this test case its not connected to anything and reading 0.5V

I thought about putting a load on it. I guess I can try. Maybe it needs a minimum load.

I need to order more banana to pin adapters because if one more grabber hook slips off of something I am going to throw this machine into the woods. So the load test will wait a week. Then I can get 2 banana connectors and a resistor and have a nice load properly hooked up without getting acidosis

i know its simple but i am at my limit. i can give you the results of a loaded run in about a week. 

[Andy Chee]

Ok.  Try my suggestion, remove Q15 (and Q21 for safety).

This should make U2 pin 3 oscillate.

You do have an oscilloscope to view the oscillation, yes?

[coppercone2]

Ok I can try that tomorrow

I got pogo pin probes for the scope recently just for reducing stress level

they should come with scopes it makes it actually useful for repair because doing it with the normal way is like doing things with tree branches instead of tools  . I can't believe that they actually expected that people would poke inflexible probes into things for so long. i am really starting to think that its completely insane to not have gold plated pogo pin probes even if you have a harbor freight meter only.  its completely medieval

really i can't believe that I was jousting with circuits for like 2 decades before I finally listen to dave jones and got the gold pogo probes. its like your doing it wrong buddy. its like refusing to buy suspension for a car. we are so backwards



if you have a nice meter and you don't have pogo pin probes its like not having a suspension for a car


if you have a cheap meter but you don't have pogo pin probes its like not having a suspension for your bicycle

[coppercone2]

OK I got excited because I realized there is a new banana connector to make, that is you can get a regular banana end plug (mating end), and put a loop on it instead of a wire, so you get a series adapter that lets you hook up a oscilloscope clip to a banana cable nicely! very simple but it gave me the positive energy to remove the transistors.




this is the invention


the green should be a bare stiff copper wire loop, instead of a wire, so you can clip a scope ground probe on it nicely! with a series splice

how good is that for probing circuits you are powering with external leads during repair? instead of trying to clip to some random inconveniant metal or having to change probe ground leads etc. for low precision repair work.


however


U2 pin 3 is constant DC output  with the two transistors you thought should be removed. 
looks like the gate oscillator is stuck on at +15V.


That makes me think something is broken!


So this might isolate the problem to
r53, r44, c24, q18, r45, r46, q15, c18, d13, r38, and U2 ?

[Andy Chee]

U2 pin 3 is constant DC output  with the two transistors you thought should be removed. 
looks like the gate oscillator is stuck on.

With the two transistors removed, can you check U2 pin 1?  Is it high or low?

The oscillator will only work if pin 1 is high.

[coppercone2]

Pin #1 of U#2 is 1.8V, #2 = 15V, #3 = 15V

mean voltage on scope with no sign of any pulse activity

You know its weird I thought i change all the transistors but some of these solder joints look factory, I think I only changed the ones near the 'blast'. Well its worth trying a shotgun approach on 04's and 06's . I did change all the IC's though (including optos)


But the transistors all measure fine on diode test. Maybe there is another problem.


But I did test all the transistors now in circuit with diode test, and all the diodes, it looks fine. The inductor too. I guess I can go through the resistors and then replace all the transistors again and maybe test the diodes with my new transistor tester.

Yeah, their transistors have the leads bent over in a certain way. I am sure now that I did not replace some of them because they tested fine and I was not thinking the damage can propagate there.

But holy crap could it really be one of those problems where they all look perfect on the diode test but one of em is off?  :'(


Unless you figure out what I missed.
I bet that maybe that oscillator has some other parameter that might be blocking its operation.



I keep thinking that in some of my repairs there is a bad transistor that measures good on the DMM. But then usually I find out its something else! its like paranoia.


Could it be a leaky transistor :O Could that day have finally come? Maybe it finally happened in a circuit because of the 800V power electronics nearby.


But if you have any more thoughts it would be welcome because i am extremely weary of working on this PCB