I don't have an LCR meter but I think it might be a good idea to get one. Regards
Rob > -----Original Message----- > From: cctalk <cctalk-boun...@classiccmp.org> On Behalf Of Aaron Jackson via > cctalk > Sent: 18 April 2020 15:13 > To: Peter Coghlan <cct...@beyondthepale.ie>; General Discussion: On-Topic > and Off-Topic Posts <cctalk@classiccmp.org> > Subject: Re: VAXmate PSU fixed, but no video > > >> 1. If the EHT lead was not properly connected to the CRT anode, could > >> that cause problems? > > > > Possibly. I have VT220 terminal which was making a smell of ozone > > when it was running which I should have done something about but never got > around to. > > This could have been due to corona discharge around the CRT anode > > connection or around the flyback transformer but I never found out. > > Eventually, it stopped working, drawing excess current from the 12V > > power supply. The flyback transformer appears to have been damaged. > > I recently decided to take another look at a VT220 I've got which appears to > have a bad FBT. Q202 switching transistor has been replaced and is outputting > a 14.7KHz signal but the flyback also seems to draw too much current and > causes the terminal to hiccup. > > It appears to be a problem with the primary winding, which has an inductance > of 5.6uH, although has now increased to 6.4uH after smoking again (I left it on > while hiccuping to check that the transistor was still switching)... If you have an > LCR meter I'd be curious to know what inductance you measure on the primary > winding. > > I did eventually notice a small crack in the plastic on the primary winding side. > I'm not sure if it is superficial but I suspect this is where the smoke escaped. > > >> 2. Is there anything I can safely do with a bench power supply to > >> isolate the problem? > >> 3. Any other suggestions for diagnosing the problem? > > > > One approach to testing flyback transformers seems to be to use a > > circuit that causes them to ring and observing whether the ringing is > > damped by shorted turns. I've never got around to trying this myself. > > If you want to check the secondary winding, there is a diode which has a high > forward bias voltage. If you pass 20v or so through the secondary and look at it > with a volt meter you should see the voltage drop. It won't conduct at all if the > voltage is too low. > > >> 4. There is an outline spec of the flyback transformer in the section > >> 4.4.3.2 of the VAXmate technical description, what chance of finding > >> a "modern" replacement? > > > > I wish you good luck with this. I never had any luck locating one for > > my > > VT220 :-( > > There were two VT220 designs (I think), using either an onboard or offboard > flyback. The part number of the onboard flyback is 16-26299-01, and there are > some available on eBay if you are feeling rich. > > Cheers, > Aaron > > (Sorry about the ridiculous footer that will appear below.) > > > This message and any attachment are intended solely for the addressee and > may contain confidential information. If you have received this message in > error, please contact the sender and delete the email and attachment. > > Any views or opinions expressed by the author of this email do not necessarily > reflect the views of the University of Nottingham. Email communications with > the University of Nottingham may be monitored where permitted by law. > >