XBR3D815@DDATHD21.BITNET (08/10/88)
[line eaters breakfast ] Some days ago i sent a plea for help to all Atarians in netland: > Is someone out there in netland who knows a difference between older and the > newest SM124 B&W-monitors which could explain the following behavier: > Mega ST4 and old (Oct. 87) Monitor: > In reverse mode (light text on black background) is brightness dependent > on information on screen ! > Mega ST4 and new (Mar. 88) Monitor: > The symptom does not occur ! Our dealer didn't know of this problem, he told me all SM124 monitors are returned to Atari for repair because they get no parts from Atari. So they have no experience with these monitors. (Thanks to V.A.Brandt who gave me an important hint) I decided to take an oscilloscope and did some measurements. As far as i know at two places the brightness of a CRT is controlled: 1) the first gate (hope this is the right technical term) 2) the cathode voltage/current. The voltage between cathode and 1st gate should be constant, by adjusting this you can control brightness of the monitor. Therefore i measured the voltage of the first gate against ground: it was constant, so this was apparently not the cause of the trouble. Then i measured the cathode voltage, which should be +70V as the schematics say. I found these voltage to be screen-image dependant: with a light screen i found +70V, with completely dark screen i found +85V. The cathode current varied between 0mA (no bright, no contrast) and 17mA (full bright and full contrast). So i think the problem is a bad regulation of cathode voltage. If the load is too small (dark screen or only some light characters on a dark screen, the cathode current is small) cathode voltage goes up and voltage difference between cathode and 2nd gate goes down ---> screen fades. And now my fix: Beware --- I'm a simple minded chemist who ownes a soldering iron but is not an EE, so i will take no warranty for any side effects of my fix. It works with our monitor, but i fixed it today and so have no experience on the long run. It's a simple fix and should do no harm to your SM124. What do i do? I put a little extra load (1.5mA) at the high voltage transformer, so that the current never sinks under 1.5mA and cathode voltage never goes up again. With maximum brightness and contrast i measured 17mA, in normal operating condition about 8-13mA, so this extra current should never overload the transformer. But i give no warranty for this! Be carefull if you turn on full brightness and full contrast. The procedure is given for a monitor manufactured in oct '87, the newer ones have a different PCB layout. 1. Open the monitor by removing the 5 screws. 2. Look at the small board which is plugged on the end of the CRT. There you find the names of the parts printed. Look for C308 (47uF/100V) at the lower right edge of the board. If the board is covered with a shielding, you may have a newer monitor and this fix possibly does not apply. 3. Get a resistor 47k ohms 0.25 W and solder it across the pins of C308. Put the resistor at the right boarder of the board so that it does not touch the monitor box later. Pay attention to the small space between the monitor box and the board. Possible Damage of your monitor! 4. Close the monitor, cross fingers and switch on. Problem should have gone. PP: I also found some differences between the circuit drawings and the actual board, but this should be no problem. But i supplied R302 with 1k/0.25W, but this is not necessary for the fix part was should be --------------------------------------- R302 open 1k R311 68R 47R R312 270R 240R R313 820R 680R I hope this will help someone else, forgive the rather long posting. Werner W. Braun xbr3d815 @ ddathd21.bitnet
rwa@auvax.UUCP (Ross Alexander) (08/16/88)
In article <8808101333.AA23692@ucbvax.berkeley.edu>, XBR3D815@DDATHD21.BITNET writes: > [...] > As far as i know at two places the brightness of a CRT is controlled: > 1) the first gate (hope this is the right technical term) > 2) the cathode voltage/current. Let me say first, an excellent article! But to pick a nit (at your request), the control elements of thermionic emission devices (aka Fleming valves, or in some benighted venues "tubes" ;-) are called `grids'. The first grid is usually called the control grid; some common devices have up to five grids. In apology, let me say that I learned my basic electronic principles using this technology for my active devices (bar some 1N34's) :-). -- Ross Alexander @ Athabasca University <the-known-world>!alberta!auvax!rwa