CEMIG-106T gas valve buzzing also need schematic

[Stampeder]

I have an older HF CEMIG160T which up until the other day has been working like a champ. Oh, it has it's idiosyncrasies but on the whole does what I need around the place.
The other day when I turned it on it started buzzing like crazy. I looked inside and saw the relay (contactor) contacts vibrating. I disconnected it but the buzzing continued. I tracked the buzzing to the gas valve. I pulled the coil off it and using another 24v transformer (actually a door bell transformer) I checked the coil and it was operating just fine. I checked the output voltage on the 220VAC to 24vac transformer on the bottom of the welder and it was ouputting 27.3 volts which should be about right.
This brought me to the controller board.
I checked the internet and I don't see a schematic for the one I have. I know this welder was made by CEMONT.
I've attached a couple of pictures of the board in the hopes that someone my have the schematic of this welder and the controller board.
One final item is I am still puzzled what might be causing my problem, so any ideas would certainly be appreciated.

Thanks.
Glenn.

 

[Robotstar5]

What voltage are you getting on the contactor coil and valve solenoid? best using an analogue meter if you have one as digital can be too sensitive.

... and welcome to the forum.

 

[brightspark]

What voltage are you getting on the contactor coil and valve solenoid? best using an analogue meter if you have one as digital can be too sensitive.

... and welcome to the forum.

and also are u taking the voltage readings when the contactor and solenoid are energised under load in case theres a breakdown in the windings on the coils

 

[Stampeder]

Thanks for the replies. Never had a transformer behave quite this way. I'll have to look and see if I still have an old analog meter.....been a very long time since I had one of those.
I'll rummage around my parts box and see if I have another 24 volt transformer to sub in. It will have to be 110 not 220 though.
Any ideal on these transformers as to the VA rating?

 

[the snooper]

what did you test the output voltage on ac or dc, the circuit board will run on dc so it could be as simple as a small rectifier or diode gone hence the buzzing from the valve

 

[Stampeder]

Thanks Snooper.
I tested it with my digital meter on AC.
I took the board out as you can see and did ohmeter tests on all the diodes and they seem to be fine...about 450 ohms forward and infinity on reverse. Checked the 2N3055 as well, it seems to be fine also 450 ohms forward bias and infinity reverse. The caps seem to charge on high resistance so no shorts there.
Since the two coils (gas valve and main contactor) are AC I'm suspecting that the transformer is not up to the task.....
Your thoughts?
Thanks again.
Glenn

 

[Stampeder]

CEMIG-160T buzzing fixed.

Buzzing now stopped. Welder FIXED.
After going through the circuit board and checking the usual suspects, caps gone bad, diodes not working, I suspected that the LED inside IC1 was subject to aging.
I have seen this symptom many times in my past history of repairing electronics where the LED looses its ability to light to the same level as when it was new.
I had ordered two only IC1's (see my diagram). Note on the drawing that I did not draw out the complete welder. I was mostly focused on the controller board. Two because they had to come from Hong Kong so if I was getting one, I would get two just in case.
They arrived today.
Here's a tip for those who have to replace and through the board IC at any time. Cut the leads off of the IC that's in the board first then desolder the leads going through. This will save you from lifting pads off the board and makes the whole repair a lot easier.

After putting the new IC1 into the circuit and reinstalling the board, the welder is back working to perfection.

Now I've lost my excuse to go buy a new one.
One final note, the schematic does not guarantee any accuracy for liability for its use. USE AT YOUR OWN RISK.


Cheers.
Glenn.

 

[PTvor]

Thanks for the report. It's always nice to read a success story of fixing things.

I never realised that opto isolators could degrade, I thought they either worked or not. Definitely one to remember.