Help with a Tig Welder.

[jwhmca]

I know, maybe I should be smacked for wanting help fixing my Tig welder... but you guys rule!

point 3 on the picture intermittently jumps up a particular voltage, 15vdc. Sometimes it locks at 6.3vdc and you can't adjust it. Point 1 will follow the locked voltage, but not the 15vdc. Point 2 always is the control voltage that it should be.

I've been racking my brain trying to get an understanding of this circuit. I can post the whole Service manual if that would help. IC A51 is an LM358...

Any pointers?

 

[dmp]

If the voltage at 1 and 2 are different, then current is passing through the resistor. There should be no place for current to go (assuming your multimeter is high impedance relative to the circuit).
Both the opamp and the capacitor should not sink any current.
Your rail voltage is probably 15vdc and the opamp is locking up to the rail. The opamp is trying to do it's job - make v(+) = v(-)
Maybe the feed back route from p7 to p6 is bad, or the opamp is bad, or the cap is bad?

 

[jwhmca]

A K L T B etc... are all pins on a card. The problem seem to come and go when I moved the card around. Could losing anyone of those connections cause pin 7 of A51 to go to the 15vdc rail?

 

[shabtek]

if wiggle makes intermittent it is possibly not clean/tight card contacts or bad solder joint--look first at mechanical and thermal stress points. maybe card socket solder joint

 

[dmp]

A50 and A51 are going to the rail?
A bad connection makes sense.

 

[jwhmca]

A50 and A51 are going to the rail?
A bad connection makes sense.

Sorry, A51 is not... just A50

if wiggle makes intermittent it is possibly not clean/tight card contacts or bad solder joint--look first at mechanical and thermal stress points. maybe card socket solder joint

Yeah, for sure... the more I pull the card to check things, the problem is going away! Which I hate, cause now it's harder to find. Hence, why I'm now trying to understand the circuit/ possible cause.

Here is an interesting note, could this be going on?

Application hint: The input impedance on some CMOS amplifiers is so high that without any input the
non-inverting input can float around to different voltages (i.e. the input pin picks up signals like an
antenna). In cases where the input could be disconnected (such as when coming from an external sensor)
it's a good idea to tie the input to ground via a high value resistor (1M-10MΩ). This keeps the input at
ground potential if the wire to the sensor becomes disconnected and still has high input impedance.

 

[JohnRoberts]

I know, maybe I should be smacked for wanting help fixing my Tig welder... but you guys rule!

Last time I fixed a welder there were nails behind the flywheel.  ;D

point 3 on the picture intermittently jumps up a particular voltage, 15vdc. Sometimes it locks at 6.3vdc and you can't adjust it. Point 1 will follow the locked voltage, but not the 15vdc. Point 2 always is the control voltage that it should be.

IIRC Lm358 is plain jane opamp  configured as simple unity gain non-inverting follower. So + input, - input, and ouput pin "should" all be within a couple mV of each other, when opamp is behaving.

If output (3 on your schemo) is going elsewhere investigate is opamp is getting good input, or opamp is kerfluckt (technical term).  Opamp output pegging to +15V suggests that perhaps negative feedback path is not well connected to opamp - input pin...

Simple voltmeter probing checks.

When opamp output is +15v what are input pins?
What are power pins,

Opamp output at +15v could be valid if + input pin is even a few mV higher than - input pin, or perhaps if -V supply is missing.

Opamps are dumb animals, they must do what their input pins tell them, as long as they have power, and can...

JR

I've been racking my brain trying to get an understanding of this circuit. I can post the whole Service manual if that would help. IC A51 is an LM358...

Any pointers?

[/quote]

 

[jwhmca]

IIRC Lm358 is plain jane opamp  configured as simple unity gain non-inverting follower. So + input, - input, and ouput pin "should" all be within a couple mV of each other, when opamp is behaving.

If output (3 on your schemo) is going elsewhere investigate is opamp is getting good input, or opamp is kerfluckt (technical term).  Opamp output pegging to +15V suggests that perhaps negative feedback path is not well connected to opamp - input pin...

Simple voltmeter probing checks.

When opamp output is +15v what are input pins?
What are power pins,

Opamp output at +15v could be valid if + input pin is even a few mV higher than - input pin, or perhaps if -V supply is missing.

Opamps are dumb animals, they must do what their input pins tell them, as long as they have power, and can...

Well, the real trick now is... the problem is gone. I didn't really do anything except lift the CARD a few times and clean the contacts. So, my question is : Could losing the contacts cause the opamp pin 7 to go high? losing ground side of R90?
This is a single power +V supply.
Pins 6&7 of the opamp are soldered together.
I wasn't able to ever check the inputs when the output was jumping to 15vdc.

 

[mjrippe]

Seems like a dirty contact was your culprit.  Funny because a friend told me I should get out of audio repair and do welder repair 'cuz nobody does it well.  I told him I preferred high voltage/low current tube gear to low voltage/high current welders ;-)  Sounds like I could make some $ with a can of DeOxit!

 

[CJ]

sometimes you run into problems measuring input pins to opamps as the Z is so high that the meter disrupts the normal voltages by putting a 10M or 100M resistor in parallel with an infinite input Z,