Old 180 SD HF stuck on

[Tweekster]

5 years ago my 180 SD had the HF start stick on, and being in the middle of a project, I used that for justification to the Wife to buy a Dynasty 200 DX. The 180 SD has been in the corner ever since.

A friend now wants to buy the old 180 SD if I can fix it. I have the 180 SD Tech Manual, and I am in the process of tracing the circuit back from the HF coil. In the interest of saving a little time, I was wondering if anyone else has seen this problem, and if you could help narrow down my troubleshooting?

The HF start is on the minute I power the machine up, never turns off, and doesn't respond to the peddle. The peddle controls the amperage fine, and is the same peddle that I use with my 200 DX, which works fine. With the peddle unplugged, the HF is still on. HF Transformer T3 is getting constant voltage from the main PC1 circuit board. I haven'd taken measurements at the circuit board yet, but just looking at the circuit, it looks like it could be a shorted optoisolator (OC1) or shorted MOSFET (Q5).

SN KK184429

Any ideas?

Many thanks in advance!

[cruizer]

Looks more to me that Q3 (opto triac) has shorted causing RC1-8 (115Vac) to go direct to RC1-7 which is the control tor the HF transformer.

Could be wrong, but thats where I'd start looking. Just take a resitance off
RC1-7 to R1-8, you'd know for sure. (power off of course)

[Tweekster]

Thanks! I'll give it a shot.

I'm a self taught audio amp guy, so these control circuits are a little new to me.

[Tweekster]

Finally had a chance to get back on the troubleshooting.

Circuit Board #190772

Across RC1-7 and RC1-8 I got .5 Mohms/40+ Mohms forward and backwards, Q3 looks okay.

Across pins 4 and 6 of OC1 I get 85 Mohms forward and backwards, that looks okay.

Across pins 'S' and 'D' of Q5 I get 1.1 Mohms/infinity forwards and backwards, so that looks okay.

It looks like pin 4 of the quad gate IC U6 triggers Q5, but I'm not very familiar with checking linear IC's, so that's where I left off.

One thing I might add, is this occurred right after I moved into my new house, and the welder rode 700 miles in a U-Haul trailer. I'm not sure what components are vibration sensitive, or it was just a coincidence.

[ccawgc]

If 7 and 8 are not shorted then your problem is deep in the pcb.
I assume you are only getting HF and the welder out put is off.
If the welder output can be turned on by remote control and the HF does
not turn off in stick. this will help you narrow where to look.
most of the parts are simple gates and op amps. just follow the direction
of the arrows. the hardest part of this diagram is following the short cuts.
If you want to measure voltages on the pc board you must first kill the HF.
An easy place is to cut the but splice to the HF transformer. Replace it with an inline connector. The HF causes volt meters to act up. If you try to measure OCV with HF on it will kill your meter. Be careful and good luck.

[cruizer]

Well there is a way to get around this little quirk, and that is to get a 24VDC relay (you can use an automotive enclosed type) single pull single throw.

Splice into wire #4 to the new coil side of the relay

Splice into wire #21 to the other side of the coil of the relay

Cut wire #31, put each cut piece into the normally open sides of the relay.

Presto, the hf will only come on when you hit the foot pedal.

[Tweekster]

If 7 and 8 are not shorted then your problem is deep in the pcb.
I assume you are only getting HF and the welder out put is off.
If the welder output can be turned on by remote control and the HF does
not turn off in stick. this will help you narrow where to look.
most of the parts are simple gates and op amps. just follow the direction
of the arrows. the hardest part of this diagram is following the short cuts.
If you want to measure voltages on the pc board you must first kill the HF.
An easy place is to cut the but splice to the HF transformer. Replace it with an inline connector. The HF causes volt meters to act up. If you try to measure OCV with HF on it will kill your meter. Be careful and good luck.

I killed the AC to the HF transformer right at the start, then put a cheapo meter across it to monitor it for a change.

As soon as I turn the power on in either TIG mode the HF is on, the pedal still controls the welding output, but does nothing for the HF. The HF turns off in stick mode, the only time it ever goes off.

Looks like I need to trace out the gates and opamps, and figure out the logic.

[Tweekster]

Well there is a way to get around this little quirk, and that is to get a 24VDC relay (you can use an automotive enclosed type) single pull single throw.

Splice into wire #4 to the new coil side of the relay

Splice into wire #21 to the other side of the coil of the relay

Cut wire #31, put each cut piece into the normally open sides of the relay.

Presto, the hf will only come on when you hit the foot pedal.

I'm at work and need to check my schematic....wouldn't that let the HF turn on with the peddle, but will it ever turn off once the weld is started?

[ccawgc]

you can try crusers Idea. I haven't checked it out to see how it works.
Back to the pc board. The HF turns off in stick, this tells me all the parts you were testing are good. now look at RC 1 pins 6&9 mode switch closes this in ac.
also look at the weld voltage feed back. voltage feed back is used to turn off the HF after the arc is going. where these to signal paths meet on the circuit
board is a logical place for your failure. I will look more tonight and see if I can find it.

[cruizer]

No that'll be a draw back, otherwise you can put a switch inline with that new relay to turn the HF off.

Otherwise your looking at $600-800 bucks for a new board

[Tweekster]

you can try crusers Idea. I haven't checked it out to see how it works.
Back to the pc board. The HF turns off in stick, this tells me all the parts you were testing are good. now look at RC 1 pins 6&9 mode switch closes this in ac.
also look at the weld voltage feed back. voltage feed back is used to turn off the HF after the arc is going. where these to signal paths meet on the circuit
board is a logical place for your failure. I will look more tonight and see if I can find it.

Much appreciated! I'll work on it more tonight myself.

I have a good DMM and a scope, just to get my brain wrapped around gates and opamps. I understand the concepts, and have played with a couple of circuits before, just never had to troubleshoot one.

[Tweekster]

No that'll be a draw back, otherwise you can put a switch inline with that new relay to turn the HF off.

Otherwise your looking at $600-800 bucks for a new board

And that's why it's been sitting in the corner for 5 years, it cost more to fix than it's worth on the used market. Sad thing is, it's in perfect condition, looks like the day it came out of the box, inside and out.

I'm only selling it for $500 (just the box), if I can fix it.

Looks like I need to learn more about gates and opamps, I have a pretty good understanding, just never had to troubleshoot any. I'll figure it out if it kills me!

Thanks for all the help!

[Tweekster]

After a few of nights of learning and testing, it looks like it's U8, the CD4098.

What methods are people using to remove the conformal coating? Solvent or mechanical means? It seems to be polyurethane based, but I could be wrong.

TIA

[cruizer]

After a few of nights of learning and testing, it looks like it's U8, the CD4098.

What methods are people using to remove the conformal coating? Solvent or mechanical means? It seems to be polyurethane based, but I could be wrong.

TIA

Well, there are coating removers, but I just use a small flat blade screw driver and scape the coating away. A solder wick will remove most of the solder around the pins, but you will need a micro drill bit to clean out the pins holes, after you remove the linear.

It's very tedious as you don't want lifting of traces under that chip.

Found at most electronic supply houses or even radioshack carrys them with the drill bit holder.

[Tweekster]

Well, there are coating removers, but I just use a small flat blade screw driver and scape the coating away. A solder wick will remove most of the solder around the pins, but you will need a micro drill bit to clean out the pins holes, after you remove the linear.

It's very tedious as you don't want lifting of traces under that chip.

Found at most electronic supply houses or even radioshack carrys them with the drill bit holder.

Thanks, that's kind of what I thought. I have a micro drill I use for RC models, and have plenty of solder wick, so I better get to scraping.