> On 24 Nov 2017, at 17:23, Tony Duell via cctalk <cctalk@classiccmp.org> wrote: > > On Fri, Nov 24, 2017 at 8:41 AM, Brent Hilpert via cctalk > <cctalk@classiccmp.org> wrote: >> On 2017-Nov-23, at 5:07 PM, Adrian Graham via cctalk wrote: >>>> On 23 Nov 2017, at 21:18, Brent Hilpert via cctalk <cctalk@classiccmp.org> >>>> wrote: >>>> ... >>>> You might check the contrast control. >>>> I can't find the actual schematic for the contrast control connections, >>>> but I 'm guessing the ends are connected across 12V (video board pins 5 & >>>> 1/4/10) and the wiper to video board pin 8. >>>> The contrast control can be expected to take pin 8 somewhere + to get >>>> pixels fully on. >>> >>> You’re right with pin 8, tracing the circuit I can see why someone >>> recommended I check Q101 and Q102 which look OK in my tester. >>> The contrast control itself tests OK resistance wise, it goes from pretty >>> much 0ohms to 610M. >> >> 610M ? >> I hope that M is a typo. > > Indeed... > > I am working from the Model 4P service manual which I downloaded from > archive.org I > think. I can't find a schematic for the control panel, but the parts > list says the 2 > pots are 500 Ohms (contrast) and 500k brightness. One of diagrams at > the start of > the manual suggests the former has 3 wires (wired as a potentiometer), > the latter > 2 (wired as a variable resistor).
Yep, see my correction this morning. Just checked it again and it’s 0-641ohms. > I am assuming the fault is correctly described, a lack of contrast and not > brightness. Can you get a bright raster if you turn the brightness control up? Yep, that’s how I can *just* see the boot logo. > OK... Firstly, measure the voltage on pin 8 of the monitor PCB. Can you get > it to swing from 0 to around 12V? I’ll have to partially reassemble out of the case to test, but I can get to the other end of pin 8 with the control panel removed. > Scope the video output from the CPU board (on pin 2 of the monitor PCB). Is > it a good TTL level signal? Good point. > > Measure the voltage on pin A of the CRT base PCB. Expect around 60V here. > > Check that C101 is not shorted. You could try the machine with it removed > to see what happens. I removed it and tested it last night, looks OK. > Have you checked the TR103 on the CRT base PCB? Not yet, sleep was more important at that point :) I’ll report back tomorrow, cheers! — Adrian/Witchy Binary Dinosaurs - Celebrating Computing History from 1972 onwards