> On 26 Nov 2017, at 19:31, Brent Hilpert via cctalk <cctalk@classiccmp.org> > wrote: > > > With a scope the objective would be to look at the levels when pixels are > 'on' in the video signal. > For white/on pixels, you need conductivity through TR102.E-C and TR103.E-C, > to allow electrons to flow from GND, through those transistors, into the > cathode of the CRT, > and be emitted from the cathode towards the screen. > > For a white pixel then: > - the video signal (and to a lesser level, the base of TR102) must be > going positive, to turn TR102 on. > - TR103 collector should be going 'lower' during the pixel period, the > black level is approx. 60 V, > the white level should drop well below that. > > You probably won't see much voltage variation at CRT PCB point H / TR102.C / > TR103.E > as it's just current-switching there with no collector pull-up. > > You could try connecting CRT PCB point H briefly to GND through a 150-300 ohm > R, or short TR102.C-E (not 103). > That should bring up a white screen. > If it doesn't it would suggest the problem is around TR103. > If it does the problem is likely around TR102.
I replied to this thread a while back but managed to only reply to Tony and not the list, oops! Rather than using my multi-transistor tester for TR103 I tested it with a DMM in diode mode and it’s almost a dead short from B-E, I get the same reading in both directions so I’m going to order a new one in a bit. > If your multimeter has an hfe mode, you could try pulling TR102 & 103 and see > if they show gain. It does but the legs on TR103 weren’t long enough to reach the contacts in the BCE holes which is why I went for diode mode instead :) Thanks for the explanation, if the new transistor doesn’t make a difference then I know where else I can look. Cheers! — Adrian/Witchy Binary Dinosaurs - Celebrating Computing History from 1972 onwards
