Another UNILAB Educational Osciloscope - UK Vintage Radio Repair and Restoration Discussion Forum

daviddeakin · 25th Apr 2019, 9:50 pm

I turned on my Unilab 032.602 scope for the first time in about ten years and got.... no trace.
Helpfully I found this old thread where G4 Pete posted a schematic and noted that he fixed two with the same fault -a bad transistor holding the grid at -50V.
https://vintage-radio.net/forum/showthread.php?p=839054
(Maybe the mods want to add this post to that old thread? Seems relevant.)

Upon looking inside I went straight to the grid and, lo and behold, it was stuck at -50V. Aha it's bound to be the same fault as Pete's two! So I replaced the same transistor Pete did with an MPSA42 that I had and... still no trace!

You know what it was, don't you? One of the SKE1/06 rectifiers was dead, so I had no -9V rail. I probably should have measured the rails first I suppose!

Anyway I replaced all of them with some 1N4007s (overkill I know) and it works now.

However, on one particular timebase setting I get ripple on the trace (unaffected by Y gain), does anyone else get the same thing?

ajgriff · 25th Apr 2019, 10:32 pm

Do you still get the ripple with the case back in place and the input leads disconnected?

Alan

daviddeakin · 26th Apr 2019, 8:53 am

Quote:

Originally Posted by ajgriff

Do you still get the ripple with the case back in place and the input leads disconnected?

Yes. In fact I think it's there all the time, you just can't discern it at high or low scan rates. It looks like rectifier ripple so I guess it is simply the imperfect HT being derived from a simplistic voltage doubler.

ajgriff · 26th Apr 2019, 12:46 pm

Seems to be internally generated in any event. Might be worth checking out the ageing electrolytics in the power supply I guess.

Alan

daviddeakin · 26th Apr 2019, 6:35 pm

Hmm there seems to be 2Vpp of ripple on the HT. Not much

25th Apr 2019, 9:50 pm	#1
daviddeakin Hexode Join Date: Apr 2007 Location: W Yorks, UK. Posts: 407	Another UNILAB Educational Osciloscope I turned on my Unilab 032.602 scope for the first time in about ten years and got.... no trace. Helpfully I found this old thread where G4 Pete posted a schematic and noted that he fixed two with the same fault -a bad transistor holding the grid at -50V. https://vintage-radio.net/forum/showthread.php?p=839054 (Maybe the mods want to add this post to that old thread? Seems relevant.) Upon looking inside I went straight to the grid and, lo and behold, it was stuck at -50V. Aha it's bound to be the same fault as Pete's two! So I replaced the same transistor Pete did with an MPSA42 that I had and... still no trace! You know what it was, don't you? One of the SKE1/06 rectifiers was dead, so I had no -9V rail. I probably should have measured the rails first I suppose! Anyway I replaced all of them with some 1N4007s (overkill I know) and it works now. However, on one particular timebase setting I get ripple on the trace (unaffected by Y gain), does anyone else get the same thing? Attached Thumbnails

25th Apr 2019, 10:32 pm	#2
ajgriff Nonode Join Date: May 2004 Location: Halifax, West Yorkshire, UK. Posts: 2,587	Re: Another UNILAB Educational Osciloscope Do you still get the ripple with the case back in place and the input leads disconnected? Alan

26th Apr 2019, 12:46 pm	#4
ajgriff Nonode Join Date: May 2004 Location: Halifax, West Yorkshire, UK. Posts: 2,587	Re: Another UNILAB Educational Osciloscope Seems to be internally generated in any event. Might be worth checking out the ageing electrolytics in the power supply I guess. Alan

26th Apr 2019, 6:35 pm	#5
daviddeakin Hexode Join Date: Apr 2007 Location: W Yorks, UK. Posts: 407	Re: Another UNILAB Educational Osciloscope Hmm there seems to be 2Vpp of ripple on the HT. Not much