TRS80 Model 4P CRT contrast fail

[Adrian Graham]

> On 26 Nov 2017, at 19:31, Brent Hilpert via cctalk <cctalk at 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