Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Help with control board

Help with control board

Help with control board

I don't have a background in electronics, but have always been interested in learning more and I found this forum hoping there are some folks looking in here that can shed some light on the control board I'm working on.

The board is from a 1974 Lincoln welder, I'm restoring the machine and it has some electrical issues.
I have the control board out and removed most of the components, checked individually and on some individual parts ran into some issues with testing and theory, more about that later.
The schematic for the machine is L5208T and the schematic for the board is L5268T, both can be found here: Lincoln tig300/300

my first question, it appears to me to be both ac and dc control on this board, maybe that's not correct?
at #23 on L5268T I have 62vac along with 92vdc,both going to the control board, is this correct? and if so is it possible from just looking at the schematic to map out in one color the ac current paths and then map out in a different color the dc paths?
I don't have the experience to do this or know how difficult it would be, but assuming the diodes would determine the current flow it might not be too hard for someone with experience.
Thanks for entertaining my questions & quest for knowledge.

RE: Help with control board

having a tough time understanding the circuits.
The ac & dc share paths and then travel into separate circuits?
I have 2 boards, the original and a replacement, both suffered damage from a shorted remote cord.
If I could understand the how/where the current travels, troubleshooting would be much more precise.
thank you kindly, Doug

RE: Help with control board

The four diodes are in the form of a bridge rectifier. Their output can be described as pulsating DC. In a normal power supply, there would be a filter capacitor to filter out the pulses, thus making DC. In a welder, it's likely that the currents (and voltages) are so high that there's no feasible capacitor available. So the whole circuit probably works with the pulsating DC. ...Maybe.

FYI: This thread, being apparently hobbiest and thus potentially dangerous, is arguably on the edge of the approved scope of Eng-Tips.

RE: Help with control board

Hello Jeff,
Your reply is off the mark, The issue is not about DC circuit for welding.
The image I posted above is a small rectifying circuit for current control on L5268T and I believe there is an ac component along with the dc that powers the pulse transformer?

This is not the larger bridge rectifier that makes the DC for welding you mentioned, if you had looked at the complete welder schematic L5208T you would recognized both rectifying circuits.
there are 2 schematics, 1 covers the welder itself L5208T and the other L5268T is only the control circuit.
If you're not on the edge, you taking up too much space.

RE: Help with control board

The link you provided led to a Microsoft Skydrive page that doesn't seem to work with the devices I normally use to access the 'net. So, no, I've not seen the entire schematic; only the small extract that you posted inline. I apologize that my response was off the mark.

It is still "missing" an obvious filter capacitor.

And it's still normally called pulsating DC. It would be very unusual to base a design concept on it being DC and (separately) AC. But anything is possible.

Perhaps others will chime in. Good luck.

RE: Help with control board


Here are direct links to the .pdf schematics on another host if you cant browse the skydrive link.
control board
Thank you kindly- Doug

RE: Help with control board

Besides tracing the ac/dc to determine switching and control paths, Im having trouble identifying 2 components.
I've looked through my ge manuals and couldn't find these part #'s.
printed on the semiconductor is GED04-32 / GED04-34
Can you identify these parts?
Q2 & Q3 I have nos replacements, Q1, Q4 & Q5 having trouble matching them up.
Thank you kindly, Doug
control board with labeled components

RE: Help with control board

I can find no information on the GED04 transistors below. I scoured several GE manuals 1971-1977 and found no mention?
Hoping a savy chap can identify these and point me to a data sheet.
I found the T1006 crosses to NTE102A.
Regards, Doug

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close