TRS80 Model 4P CRT contrast fail

[Adrian Graham]

> On 23 Nov 2017, at 21:18, Brent Hilpert via cctalk <cctalk at 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