> On 23 Nov 2017, at 21:18, Brent Hilpert via cctalk <cctalk@classiccmp.org> > wrote: > > On 2017-Nov-23, at 11:34 AM, Adrian Graham via cctalk wrote: >> Hi folks, >> >> A random facebook post made me dig out my model 4P which has never worked as >> long as I’ve owned it, it’s always just sat on a shelf looking cute. >> >> Good old ASTEC PSU problem so I fixed that, reseated all the thankfully >> socketed chips and it burst into life. Sort of. I know it’s running because >> I can turn the brightness up to raster lines and see the pattern actually >> change when RESET is pressed. With contrast right up I can JUST see the boot >> logo appear while it reads the floppy. >> >> None of the pots on the video board deal with contrast so I’ve got it out on >> the bench to remove and test the capacitors. What else can I look at at the >> same time? Someone else has already mentioned the transistors Q101, Q102 and >> Q103 (on the yoke board). >> >> Screen pic: http://www.binarydinosaurs.co.uk/trs80Model4PScreen.jpg >> <http://www.binarydinosaurs.co.uk/trs80Model4PScreen.jpg> - you can JUST see >> the boot logo, I know it’s not screen burn because it does disappear on >> RESET and comes back again. >> >> Video board schematic: >> http://www.binarydinosaurs.co.uk/trs80Model4PVideoBoardSchematic.png >> <http://www.binarydinosaurs.co.uk/trs80Model4PVideoBoardSchematic.png> > > > 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. > Or get back to us with some voltage measurements at the BCE of TR101, 2 and > 3, and at the external connections to the little board on the end of the CRT. > Note that there will (should) be several hundred volts on some of those CRT > board connections. > > Does the "internal brightness" control on the video board have some effect? Yes, it works with the external control. It was the first thing I checked last night. Cheers A
