On 07/15/2012 12:57 PM, keithostertag wrote:
Good News! We now have a working CRT and R7903!
The Short story: After replacing one resistor the screen came alive. After some adjustments there are two items remaining with this project that I need help with:
1. The readouts only gives one numeral. For instance, .5uS, 5uS, and 50uS all display as 5uS. Similarly, .5V, 5mV, and 50mV all say 5mV. Same for all settings, and regardless which plugins I use. The 10X multiplier makes no difference in the Readout display, though it does make it a little "wavy", like a slight vibration. Also, the Readouts are positioned a little low on the screen, so that the lower ones are half-obscured off the bottom of CRT frame.
2.) I'm going to need a source for some of the push-button switches on the front panel.
The longer story (for those interested):
I was following Albert's suggestion to check the continuity around P1675 when I found pin 3 to TP1883 measured around 5Meg ohms. When I went to touch one side of R1880 in order to lift it, it literally fell apart into two clean pieces. No burn or stress marks of any kind- and nothing else on that board was found bad.
After replacing R1880 the CRT came right up, and I went through both the PS cal as well as the z-axis cal. The are a few sections of the z-axis cal that I can't do (I guess) since I don't have a marker generator. (?)
I want to say that after all these weeks of working on this project I was immensely happy and relieved to see that first trace appear! My guess is that this R7903 had been sitting unused (due to its broken condition) for many years before it got to me. Many caps needed to be replaced on several boards, mostly the PS. Of course I would not have been able to do this without your help and instruction, and I am grateful for your patience and perseverance. Really a good example of group effort!
I know even _less_ about logic circuits than I do analog circuits, so fixing this Readout problem will be very interesting. I am reading the manual...
Your thoughts on how to approach the Readout problem?
Thanks,
Keith Ostertag
Keith,
Congrats on finally getting to the root of the problem. Now, if I could only have that "Ah Ha!" moment on this damn SC504 I've got. ;-)
Mark
