I have a Dialarc HF serial #JF 860958 from around 1985. I was in the middle of welding some 1/4" aluminum when the arc got unstable so I stopped and re ground the tungsten but when I tried to start welding again there was no High frequency A/C. It produces D/C OK and I can scratch start it but no high frequency for start or continuous use. I checked and reset the spark gaps but that did nothing. Any Suggestions or ideas would much appreciated .